JP2014063873A - Compound thin-film solar cell and method of manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a CZTS-based compound thin-film solar cell excellent in photoelectric conversion efficiency by providing a buffer layer comprising a material suitable for CZTS-based solar cells.SOLUTION: A compound thin-film solar cell 1 comprises: a substrate 11; a lower electrode 12 provided on the substrate 11; a light absorbing layer 13 provided on the lower electrode 12; a buffer layer 14 provided on the light absorbing layer 13; and a transparent electrode 15 provided on the buffer layer 14. The light absorbing layer 13 comprises a compound containing Cu, Zn, Sn, and at least one of S and Se. The buffer layer 14 comprises an n-type semiconductor material in which a difference between an ionization potential and a band gap energy is 3.2 to 3.6 eV inclusive.

Description

  The present invention relates to a compound thin film solar cell and a method for producing the same.

  In recent years, renewable energy has attracted attention from the viewpoint of prevention of global warming or the finiteness of resource reserves of fossil fuels. Solar cells are attracting attention as a promising candidate for renewable energy, and many manufacturers have entered the solar cell industry.

  As materials for solar cells, Si (crystal, thin film) mainly occupied the market. However, CdTe-based compound thin-film solar cells have rapidly expanded their global share since the first solar company in the United States introduced CdTe-based compound thin-film solar cells to the market. Attracted attention.

  However, since CdTe-based compound thin film solar cells use Cd, which is harmful to the human body, it is necessary to construct a scheme that includes the collection of modules, which has been regarded as a problem. As the Japanese solar frontier company developed and put CIS solar cells that do not use Cd into the market, compound thin-film solar cells attracted more attention.

  Under such circumstances, as a compound thin film solar cell having a composition similar to a CIS solar cell, a CZTS solar cell in which a light absorption layer is composed of Cu, Zn, Sn, and S (or Se) has been reported ( For example, Non-Patent Document 1). In the CZTS solar cell, since the light absorption layer can be composed of abundant materials in the earth's crust, this CZTS solar cell has attracted a great deal of attention and has been actively studied (for example, Patent Document 1).

JP 2011-165839 A Japanese Patent No. 4264801 (JP 2004-47916 A) JP 2011-146595 A

K. Ito and T. Nakazawa: Jpn. J. Appl. Phys. 27 (1988) 2094

  However, the CZTS solar cell starts from replacing In in the light absorption layer of the CIS solar cell with Zn and Sn, and can basically be manufactured by the same method as the CIS solar cell. Therefore, in the CZTS solar cell, the investigation results of the CIS solar cell are diverted for the components other than the light absorption layer, and the method used in the CIS solar cell using the material used in the CIS solar cell. Thus, the components other than the light absorption layer are formed. Therefore, it cannot be said that materials of constituent elements (for example, a buffer layer) other than the light absorption layer in the CZTS solar cell are appropriately selected from the viewpoint of continuity of bands such as a valence level or a conduction band level. Is the situation.

  Although it is said that photoelectric conversion efficiency is improved by diffusing a part of the material contained in the buffer layer into the light absorption layer, it cannot be said that an appropriate buffer layer material is selected from this viewpoint. It is. For example, Patent Document 2 describes that a ZnS system is used as a buffer layer in a CIS solar cell. In a CIS solar cell, if a buffer layer composed of Zn and S is formed on a light absorption layer composed of Cu, In (or Ga), and S (or Se), the material contained in the buffer layer ( It is considered that a part of Zn or S) becomes a dopant by diffusing into the light absorption layer, and thus the photoelectric conversion efficiency is improved. However, in a CZTS solar cell, when a ZnS-based buffer layer is used, even if a part of the material (Zn or S) contained in the buffer layer diffuses into the light absorption layer, the material contained in the light absorption layer The material diffused into the light absorption layer is the same. Therefore, the material contained in the buffer layer cannot be a dopant in the light absorption layer, and thus it is difficult to improve the photoelectric conversion efficiency.

  Patent Document 3 describes a method of forming a buffer layer by a CBD method. However, the materials that can be formed by the CBD method are limited. In addition, since it is necessary to replace the raw material solution many times, the manufacturing cost of the buffer layer is increased.

  The present invention has been made in view of the above problems, and its object is to provide a buffer layer made of a material suitable for a CZTS-based solar cell, thereby providing a CZTS-based compound thin film solar cell having excellent photoelectric conversion efficiency. It is to provide a battery. Another object is to provide a CZTS-based compound thin-film solar cell with excellent photoelectric conversion efficiency at a low cost by forming a buffer layer by a low cost and high material selectivity forming method.

The compound thin film solar cell of the present invention includes a substrate, a lower electrode provided on the substrate, a light absorption layer provided on the lower electrode, a buffer layer provided on the light absorption layer, and a buffer layer. A transparent electrode provided. The light absorption layer is made of a compound containing Cu, Zn, Sn, S, and Se. The buffer layer is made of an n-type semiconductor material having a difference between the ionization potential and the band gap energy of 3.2 eV or more and 3.6 eV or less. Here, the “difference between ionization potential and band gap energy” is calculated using the following (formula 1). In addition, “difference between ionization potential and band gap energy” is sometimes referred to as “conduction band level”. “Difference between ionization potential and band gap energy”
= (Ionization potential)-(band gap energy) (Formula 1).

The buffer layer is preferably made of In ₂ S ₃ . The buffer layer is preferably formed by a spray coating method, preferably formed by setting the temperature of the substrate to 300 ° C. or more and 500 ° C. or less, and formed using a solution containing acetylacetone indium and thiourea. It is preferable. The thickness of the buffer layer is preferably 50 nm or more and 200 nm or less.

  It is preferable that an interface layer containing ZnO is provided between the buffer layer and the transparent electrode.

  In the method for producing a compound thin film solar cell of the present invention, a step of forming a lower electrode on a substrate and a light absorption layer made of a compound containing Cu, Zn, Sn, S and Se on the lower electrode are formed. Forming a buffer layer made of a material which is an n-type semiconductor material and has a difference between an ionization potential and a band gap energy of 3.2 eV or more and 3.6 eV or less on the light absorption layer, and on the buffer layer Forming a transparent electrode.

  The step of forming the buffer layer preferably includes a step of forming the buffer layer by a spray coating method, preferably including a step of setting the temperature of the substrate to 300 ° C. or higher and 500 ° C. or lower. It is preferable to include a step of applying a solution containing

  It is preferable to further include a step of forming an interface layer containing ZnO between the buffer layer and the transparent electrode. The step of forming the transparent electrode preferably includes a step of forming a transparent electrode made of ZnO containing a dopant material, and the transparent electrode and the interface layer are preferably formed in the same apparatus.

  Since the compound thin film solar cell according to the present invention includes a buffer layer made of a material suitable for a CZTS solar cell, it has excellent photoelectric conversion efficiency. In addition, in the method for producing a compound thin film solar cell according to the present invention, the buffer layer is formed by a method that is inexpensive and has high material selectivity. Therefore, it is possible to provide a compound thin film solar cell excellent in photoelectric conversion efficiency at a low cost. it can.

It is sectional drawing which shows an example of a structure of the compound thin film solar cell concerning this invention. It is sectional drawing which shows another example of a structure of the compound thin film solar cell which concerns on this invention. (A)-(i) is sectional drawing which shows an example of the manufacturing method of the integrated type compound thin film solar cell by which the compound thin film solar cell concerning this invention is connected in series in process order. It is a graph which shows the relationship between prebaking temperature, the band gap energy of a precursor layer, and S / (S + O) in a precursor layer. It is a graph which shows the relationship between a baking temperature and S / (Cu + Zn + Sn) and Cl / (Cu + Zn + Sn) in a light absorption layer. It is a graph which shows the relationship between the film-forming time of a buffer layer, and the thickness of a buffer layer. It is a graph which shows the relationship between the thickness of a buffer layer, an open circuit voltage, and a short circuit current. (A) is a graph which shows the measurement result of the ionization potential of the light absorption layer in Example 1, (b) is a graph which shows the measurement result of the ionization potential of the buffer layer in Example 1.

  Hereinafter, the compound thin film solar cell and the manufacturing method thereof of the present invention will be described with reference to the drawings. In the drawings of the present invention, the same reference numerals represent the same or corresponding parts. In addition, dimensional relationships such as length, width, thickness, and depth are changed as appropriate for clarity and simplification of the drawings, and do not represent actual dimensional relationships.

[Configuration of Compound Thin Film Solar Cell]
FIG. 1 is a cross-sectional view showing an example of the configuration of a compound thin film solar cell 1 according to the present invention. The compound thin film solar cell 1 shown in FIG. 1 includes a substrate 11, a lower electrode 12, a light absorption layer 13, a buffer layer 14, and a transparent electrode 15.

(substrate)
The substrate 11 is preferably made of a material having resistance to heat given when the compound thin film solar cell 1 is manufactured. For example, the substrate 11 is preferably made of soda lime glass or non-alkali glass, and a SUS plate or a polyimide film may be used as the substrate 11. The thickness of the board | substrate 11 is not specifically limited, For example, it is preferable that it is 100-4000 micrometers.

(Lower electrode)
The lower electrode 12 is provided on the substrate 11. The lower electrode 12 is preferably made of a conductive material capable of ohmic contact with the light absorption layer 13 formed on the lower electrode 12, and is preferably made of, for example, Mo, NiSi, Au, or Pt. The thickness of the lower electrode 12 is preferably a thickness that can ensure conductivity and can be scribed by scribe means such as laser scribe, and is preferably 0.2 to 1.5 μm, for example.

(Light absorption layer)
The light absorption layer 13 is provided on the lower electrode 12 and is made of a compound containing Cu, Zn, Sn, S, and Se. The element blending ratio (atomic ratio) of at least one of Cu, Zn, Sn, S, and Se in the light absorption layer 13 is not particularly limited. This element blending ratio can be adjusted by changing the blending amount of the solution used when forming the light absorption layer 13, and specifically includes at least one of Cu, Zn, Sn, S, and Se. It can be adjusted by changing the blending amount of the solution.

  The thickness of the light absorption layer 13 is preferably such that most of the sunlight incident on the light absorption layer 13 is absorbed before reaching the lower electrode 12 and the loss of carrier recombination is reduced. For example, it is preferable that it is 0.5-3 micrometers. The thickness of the light absorption layer 13 can be arbitrarily adjusted by the concentration of the coating liquid used when forming the light absorption layer 13 or the coating conditions when forming the light absorption layer 13.

(Buffer layer)
The buffer layer 14 is provided on the light absorption layer 13, and is preferably made of an n-type semiconductor material having a difference between the ionization potential and the band gap energy of 3.2 eV to 3.6 eV. As a result, the conduction band level of the buffer layer 14 becomes 0 to 0.4 eV higher than the conduction band level of the light absorption layer 13, and the band structure of the light absorption layer 13 and the buffer layer 14 is a so-called spike type band. It becomes a structure. Accordingly, hole-carrier recombination is prevented at the interface between the light absorption layer 13 and the buffer layer 14 due to the fact that the conduction band level of the buffer layer 14 is lower than the conduction band level of the light absorption layer 13. it can. In addition, it is possible to prevent the carrier from being taken out poorly due to an increase in resistance at the interface between the light absorption layer 13 and the buffer layer 14. From these things, since a carrier can be taken out from the light absorption layer 13 efficiently, the photoelectric conversion efficiency of the compound thin film solar cell 1 becomes high. Note that “the conduction band level of the buffer layer 14 is higher than the conduction band level of the light absorption layer 13” means that the conduction band level of the buffer layer 14 is lower than the conduction band level of the light absorption layer 13. It means that it is located at a position close to the position. Conversely, “the conduction band level of the buffer layer 14 is lower than the conduction band level of the light absorption layer 13” means that the conduction band level of the buffer layer 14 is lower than the conduction band level of the light absorption layer 13. It means that it is located far from the level.

In general, examples of the material constituting the buffer layer 14 include CdS, ZnS, ZnO, SnO ₂ , TiO _{2, and} In ₂ S ₃ . Among these, In ₂ S ₃ or the like can be given as an n-type semiconductor material in which the difference between the ionization potential and the band gap energy is 3.2 eV or more and 3.6 eV or less. Further, as the material constituting the buffer layer 14 preferably contains an element that is not included in the light absorbing layer 13, or the like is preferably used also for example an In ₂ S ₃ from this point. In this case, an element (for example, In) that is not included in the light absorption layer 13 among the elements included in the buffer layer 14 can diffuse into the light absorption layer 13 and function as a dopant. Therefore, the photoelectric conversion efficiency of the compound thin film solar cell 1 further increases.

  The thickness of the buffer layer 14 is not particularly limited, but is preferably 50 nm or more and 200 nm or less. Thereby, the buffer layer 14 is uniformly formed on the light absorption layer 13. Moreover, since the resistance increase of the buffer layer 14 can be prevented, the photoelectric conversion efficiency of the compound thin film solar cell 1 further increases.

The buffer layer 14 is preferably formed by a spray coating method, preferably in a state where the temperature of the substrate 11 is set to 300 ° C. or more and 500 ° C. or less, and a solution containing acetylacetone indium and thiourea is used. It is preferable to be formed. These will be described later. In addition, as a method for obtaining the ionization potential, a method of calculating the ionization potential by measuring the (number of photoelectrons) ^1/2 emitted when the buffer layer is irradiated with electromagnetic waves (for example, ultraviolet rays) can be mentioned. . As a method for obtaining the band gap energy, the following method can be cited. First, the transmission spectrum of the buffer layer is measured. The wavelength on the horizontal axis of the measured transmission spectrum is converted into light energy (unit of light energy is eV), and the transmittance on the vertical axis of the transmission spectrum is (αhν) ^1/2 (α is a light absorption coefficient, hv is converted into photon energy). In the converted graph, a portion where absorption rises is fitted with a straight line, and light energy (unit: eV) at a point where the fitted straight line intersects the base line is calculated. The calculated light energy is the band gap energy of the buffer layer.

(Transparent electrode)
The transparent electrode 15 is provided on the buffer layer 14. The transparent electrode 15 is preferably made of an n-type semiconductor material having a high carrier density and translucency. For example, the transparent electrode 15 is preferably made of ITO, IGZO, ZnO, SnO ₂ or the like. The thickness of the transparent electrode 15 is not particularly limited, and is preferably 10 nm or more and 1000 nm or less. Further, the sheet resistance of the transparent electrode 15 is not particularly limited, and is preferably 10,000 Ω / sq or less. The sheet resistance is preferably measured using, for example, a commercially available sheet resistance measuring device.

  As mentioned above, although an example of the structure of the compound thin film solar cell concerning this invention was demonstrated using FIG. 1, the compound thin film solar cell concerning this invention may have the structure shown in FIG. FIG. 2 is a cross-sectional view showing another example of the configuration of the compound thin-film solar cell according to the present invention.

  The compound thin film solar cell 2 shown in FIG. 2 includes an interface layer 19 between the buffer layer 14 and the transparent electrode 15. The interface layer 19 preferably contains ZnO, and more preferably consists of ZnO. Thereby, in the compound thin film solar cell 2 shown in FIG. 2, a buffer layer having a higher resistance than the buffer layer 14 is provided between the buffer layer 14 and the transparent electrode 15. The photoelectric conversion efficiency is further increased. The thickness of the interface layer 19 is not particularly limited, but is preferably 10 nm or more and 500 nm or less. Thereby, the effect obtained by providing the interface layer 19 can be obtained effectively.

  When the interface layer 19 containing ZnO is provided, the transparent electrode 15 is preferably made of ZnO containing a dopant material. Thereby, the transparent electrode 15 and the interface layer 19 can be formed with the same apparatus. Therefore, the compound thin film solar cell 2 shown in FIG. 2 can be manufactured without prolonging the manufacturing time and without increasing the manufacturing cost. Examples of the dopant material include aluminum, gallium, and boron. The content of the dopant material in the transparent electrode 15 is preferably 0.01% by mass or more and 10% by mass or less, for example.

[Method for producing compound thin-film solar cell]
3A to 3I are cross-sectional views showing an example of a manufacturing method of an integrated compound thin film solar cell in which the compound thin film solar cells shown in FIG. 1 are connected in series in the order of steps.

  First, as shown in FIG. 3A, the lower electrode 12 is formed on the substrate 11. For example, the lower electrode 12 made of Mo or the like is formed on the substrate 11 by sputtering or the like. Here, the thickness of the lower electrode 12 can be adjusted by adjusting the gas pressure, the temperature of the substrate 11, the film formation time, the bias voltage, or the like.

  Next, as shown in FIG. 3B, a part of the lower electrode 12 is removed. For example, a part of the lower electrode 12 is removed by laser ablation using nanosecond pulse laser light (for example, second harmonic of YAG laser). As a result, an opening 21 is formed in the lower electrode 12. At this time, it is preferable to secure a sufficient processing width (width of the opening 21) so that the adjacent lower electrodes 12 are not electrically connected to each other by a metal burr generated during ablation processing. On the other hand, if the processing width is too wide, it becomes difficult to extract the photocurrent from the light absorption layer 13 provided between the adjacent lower electrodes 12. The processing width is preferably set in consideration of these, and is preferably 100 nm or more and 500 nm or less, for example.

  When a part of the lower electrode 12 is removed using laser light, it is preferable to use light that is transmitted through the substrate 11 but absorbed by the lower electrode 12 as the laser light. Thereby, the damage to the board | substrate 11 resulting from irradiation of a laser beam can be suppressed to the minimum. Moreover, it is preferable to set the intensity | strength of a laser beam, the irradiation time of a laser beam, etc. according to the material of the lower electrode 12, the thickness of the lower electrode 12, processing width, etc.

  Next, the light absorption layer 13 is formed on the lower electrode 12. As a method for forming the light absorption layer 13, various methods such as a sputtering method, a liquid coating method, a spray coating method, or an MEB method can be used. Below, the method to form the light absorption layer 13 by the liquid coat method is demonstrated using FIG.3 (c)-(e).

  First, in the step shown in FIG. 3C, a precursor raw material liquid 22 containing Cu, Zn, Sn, S, and Se is prepared. For example, the precursor raw material liquid 22 can be obtained by mixing copper acetate, zinc acetate, tin chloride, and thiourea with water at a predetermined ratio. When sulfurization or selenization is performed in the firing step shown in FIG. 3 (e), the precursor raw material liquid 22 can be prepared without adding a raw material containing at least one of S and Se (for example, thiourea). it can. And the precursor raw material liquid 22 is apply | coated on the lower electrode 12 in which the opening part 21 was formed by the application means 23, such as a slit coater.

  Next, in the step shown in FIG. 3D, the solvent is evaporated from the precursor raw material liquid 22 applied on the lower electrode 12 (pre-baking). As a result, a precursor layer 24 is formed on the lower electrode 12.

  The pre-baking temperature is not particularly limited. However, if the prebaking temperature is too high, cracks may occur in the precursor layer 24, and the band gap energy of the formed light absorption layer 13 may deviate from 1.5 eV (ideal value). For example, the pre-bake temperature is preferably 125 ° C. or higher and 200 ° C. or lower, and more preferably 140 ° C. or higher and 175 ° C. or lower. The present inventors examined the relationship between the prebaking temperature, the band gap energy of the precursor layer 24, and S / (S + O) in the precursor layer 24 according to the method described below, and it was found that the prebaking temperature was within the above range. The result is preferable.

  Specifically, the precursor raw material liquid 22 was spin-coated on a soda lime glass substrate having an upper surface area of 50 mmsq. The rotation speed of the spin coat was 2000 rpm, and the time required for the spin coat was 30 seconds. Next, the soda lime glass substrate coated with the precursor raw material liquid 22 was heated in a nitrogen atmosphere (pre-baking). And the prebaking temperature was changed into the temperature shown to the horizontal axis | shaft of FIG. 4, and the several precursor layer 24 from which prebaking temperature differs was obtained. Then, the band gap energy of the precursor layer 24 was measured using a spectral sensitivity measuring device manufactured by Optel, and the contents of S and O in the precursor layer 24 were measured using EDX. The result is shown in FIG.

  FIG. 4 is a graph showing the relationship between the prebake temperature, the band gap energy of the precursor layer 24, and S / (S + O) in the precursor layer 24. From the result shown in FIG. 4 that the ideal band gap energy of the light absorption layer 13 of the CZTS solar cell is 1.5 eV, it can be seen that the prebake temperature is preferably 125 ° C. or higher and 200 ° C. or lower. However, as shown in FIG. 4, when the pre-baking temperature was 125 ° C. or 200 ° C., S / (S + O) in the precursor layer 24 was lowered. That is, if the pre-baking temperature is 125 ° C. or 200 ° C., oxygen may be mixed into the precursor layer 24, and as a result, the photoelectric conversion efficiency may be lowered. Therefore, it turns out that it is more preferable that prebaking temperature is 140 degreeC or more and 175 degrees C or less.

  In addition, prebaking conditions other than prebaking temperature are not specifically limited. For example, it is preferable to set the pre-bake time according to the thickness of the precursor raw material liquid 22 applied on the lower electrode 12. In order to prevent oxygen from being mixed into the precursor layer 24, it is preferable to perform pre-baking in an atmosphere that does not contain oxygen (for example, in a nitrogen atmosphere or a rare gas atmosphere).

  Subsequently, in the step shown in FIG. 3E, the precursor layer 24 is baked at a temperature higher than the pre-baking temperature. Thereby, components unnecessary for the light absorption layer are removed from the precursor layer 24, and thus the light absorption layer 13 made of CZTS is formed.

  Although a calcination temperature is not specifically limited, It is preferable that it is 350 to 500 degreeC, and it is more preferable that it is 400 to 500 degreeC. The present inventors examined the relationship between the firing temperature and S / (Cu + Zn + Sn) and Cl / (Cu + Zn + Sn) in the light absorption layer 13 according to the method described below, and the firing temperature is preferably within the above range. The result is obtained.

  Specifically, the soda lime glass substrate on which the precursor layer 24 was formed was baked in an atmosphere having an oxygen concentration of 0.1% by mass or less by nitrogen purge. The firing temperature was changed to the temperature shown on the horizontal axis of FIG. 5 to obtain a plurality of light absorption layers 13 having different firing temperatures. And each content of Cu, Zn, Sn, S, and Cl in the light absorption layer 13 was measured using EDX. In addition, since desorption of S is considered in this firing step, the ratio of the S content and the ratio of the Cl content with respect to the total content of Cu, Zn, and Sn considered to be difficult to desorb were determined.

  FIG. 5 is a graph showing the relationship between the firing temperature and S / (Cu + Zn + Sn) and Cl / (Cu + Zn + Sn) in the light absorption layer 13. As can be seen from FIG. 5, the higher the firing temperature, the lower the Cl content. However, when the firing temperature is 550 ° C., S is easily desorbed, and as a result, the photoelectric conversion efficiency may be lowered. Therefore, it can be seen that the firing temperature is preferably 350 ° C. or higher and 500 ° C. or lower, more preferably 400 ° C. or higher and 500 ° C. or lower.

  The firing conditions other than the firing temperature are not particularly limited. For example, the firing time is preferably set according to the thickness of the precursor layer 24 and the like. In order to prevent oxygen from being mixed into the light absorption layer 13, firing is preferably performed in an atmosphere that does not contain oxygen (for example, in a nitrogen atmosphere or a rare gas atmosphere).

  Further, at least one gas component of S and Se may be mixed in the baking oven to perform the sulfidation treatment or selenization treatment. In this case, it is preferable to set the firing temperature in consideration of the reaction between at least one gas component of S and Se mixed in the oven and the light absorption layer 13.

  Next, the buffer layer 14 is formed on the light absorption layer 13. A method for forming the buffer layer 14 may be, for example, a sputtering method, a CBD method, or a gas spray pyrolysis method, but is preferably a spray coating method. Thereby, it can prevent that the material which comprises the buffer layer 14 is limited to a specific material. In addition, the buffer layer 14 can be formed at a relatively low cost because it is not necessary to replace the raw material solution many times. Hereinafter, a method for forming the buffer layer 14 by spray coating will be described with reference to FIG. Here, the spray coating method includes a method of applying the raw material liquid by spraying the raw material liquid from a nozzle or the like, and a substance present in the raw material liquid by spraying the raw material liquid onto a heated substrate. And a method of depositing a solid phase of a target substance on a substrate by thermal decomposition or chemical reaction.

  First, the buffer layer raw material liquid 26 is prepared. The solvent contained in the buffer layer raw material liquid 26 is not particularly limited, but is preferably water. Thereby, since the solvent contained in the buffer layer raw material liquid 26 can be easily evaporated when the buffer layer raw material liquid 26 is sprayed, the buffer layer 14 can be easily formed. The solute contained in the buffer layer raw material liquid 26 is preferably selected according to the material constituting the buffer layer 14, and is an n-type semiconductor material, and the difference between the ionization potential and the band gap energy is 3.2 eV or more 3 The material is preferably 6 eV or less. As a result, the conduction band level of the buffer layer 14 becomes 0 to 0.4 eV higher than the conduction band level of the light absorption layer 13. Accordingly, hole-carrier recombination is prevented at the interface between the light absorption layer 13 and the buffer layer 14 due to the fact that the conduction band level of the buffer layer 14 is lower than the conduction band level of the light absorption layer 13. it can. Further, it is possible to prevent the carrier from being taken out poorly due to the increase in resistance at the interface between the light absorption layer 13 and the buffer layer 14. From these things, since a carrier can be taken out from the light absorption layer 13 efficiently, a compound thin film solar cell with high photoelectric conversion efficiency can be manufactured.

The solute contained in the buffer layer raw material liquid 26 is preferably acetylacetone indium and thiourea. Thereby, the buffer layer 14 made of In ₂ S ₃ can be formed. Therefore, In which is not contained in the light absorption layer 13 among the elements contained in the buffer layer 14 is diffused into the light absorption layer 13 and can function as a dopant. Therefore, a compound thin film solar cell with higher photoelectric conversion efficiency can be manufactured. The mixing ratio of acetylacetone indium and thiourea is preferably adjusted so that the atomic ratio of indium contained in acetylacetone indium and sulfur contained in thiourea is 1: 1 or more and 1: 2 or less.

  Next, the prepared buffer layer raw material liquid 26 is sprayed toward the upper surface of the light absorption layer 13 by the nozzle 25. At this time, the substrate 11 on which the light absorption layer 13 and the like are formed is placed on the heater 27, and the buffer layer raw material liquid 26 sprayed on the upper surface of the light absorption layer 13 is heated by the heat from the heater 27 to evaporate. And react. Thereby, the buffer layer 14 is formed on the light absorption layer 13.

  Although the temperature of the board | substrate 11, ie, the temperature of the heater 27, is not specifically limited, It is preferable that they are 300 degreeC or more and 500 degrees C or less. If the temperature of the substrate 11 is lower than 300 ° C., the buffer layer raw material liquid 26 sprayed on the upper surface of the light absorption layer 13 may hardly evaporate or react, and thus the buffer layer 14 may not be formed easily. . When the temperature of the substrate 11 exceeds 500 ° C., as shown in FIG. 5, desorption of S from the light absorption layer 13 may be caused, and thus the photoelectric conversion efficiency may be lowered.

  The thickness of the buffer layer 14 is preferably 50 nm or more and 200 nm or less. The present inventors examined the relationship between the thickness of the buffer layer 14 and the open-circuit voltage and short-circuit current according to the following method, and obtained the result that the thickness of the buffer layer 14 is preferably within the above range. ing. In order to form the buffer layer 14 having a thickness within the above range, the time required for forming the buffer layer 14 (deposition time for the buffer layer 14) is preferably 0.5 minutes or more and 2 minutes or less. The result is also obtained.

  Specifically, the deposition time of the buffer layer 14 was changed as shown on the vertical axis in FIG. 6 while the temperature of the heater 27 was set to 350 ° C. The thickness of the formed buffer layer 14 was measured using a contact step meter (DEKTAK) after ablation by the fourth harmonic of the YAG laser. Next, after forming the transparent electrode 15 on each buffer layer 14 and manufacturing a compound thin film solar cell, the open circuit voltage and the short circuit current were measured.

  FIG. 6 is a graph showing the relationship between the film formation time of the buffer layer 14 and the thickness of the buffer layer 14, and FIG. 7 is a graph showing the relationship between the thickness of the buffer layer 14, the open-circuit voltage, and the short-circuit current. The vertical axis in FIG. 7 shows the relative ratio when the value of the open circuit voltage and the value of the short circuit current are each 100% when the thickness of the buffer layer 14 is 100 nm.

  As can be seen from FIG. 7, when the thickness of the buffer layer 14 exceeded 200 nm, the short circuit current decreased. A possible reason for this is the occurrence of photocurrent extraction loss due to the resistance of the buffer layer 14. On the other hand, when the thickness of the buffer layer 14 was less than 50 nm, the open circuit voltage decreased. A possible reason for this is that the buffer layer 14 is only partially formed on the light absorption layer 13. From these, it is understood that the thickness of the buffer layer 14 is preferably 50 nm or more and 200 nm or less. In addition, it can be seen from FIG. 6 that the standard for the film formation time of the buffer layer 14 is preferably 0.5 minutes or more and 2 minutes or less.

  Thus, considering the appropriate range of the thickness of the buffer layer 14, it can be seen that when the buffer layer 14 is formed by a spray coating method, the time during which the substrate 11 can be heated is about 2 minutes. If the heating time is so short, for example, when a chloride such as indium chloride is used for the buffer layer raw material liquid 26, impurities such as chlorine remain in the buffer layer 14 and thus cause a decrease in photoelectric conversion efficiency. . For this reason, it is preferable that the buffer layer raw material liquid 26 does not contain chlorine, and it is more preferable that it is an aqueous solution containing acetylacetone indium and thiourea as described above.

  Next, as shown in FIG. 3G, a part of each of the buffer layer 14 and the light absorption layer 13 is physically removed by the scribe nozzle 28. Thereby, the opening part 29 is formed. The scribe nozzle 28 is preferably provided with a metal member at the tip, and is preferably made of a material softer than the lower electrode 12. Due to the hardness of each of the lower electrode 12, the light absorption layer 13 and the buffer layer 14, the adhesion between the lower electrode 12 and the light absorption layer 13, or the adhesion between the light absorption layer 13 and the buffer layer 14, the buffer layer 14 and each part of the light absorption layer 13 are physically removed. The width of the opening 29 depends on the size of the scribe nozzle 28 but is preferably about 200 μm.

  Next, the transparent electrode 15 is formed on the buffer layer 14. As a method for forming the transparent electrode 15, various methods such as a sputtering method, a spray coating method, or a liquid coating method can be used. Hereinafter, a method of forming the transparent electrode 15 by spray coating will be described with reference to FIG.

  First, the transparent electrode raw material liquid 31 is prepared. Although it does not specifically limit as the raw material liquid 31 for transparent electrodes, For example, what melt | dissolved zinc acetylacetonate and aluminum acetylacetonate in the mixed solvent of methanol and water can be used. Next, the prepared transparent electrode raw material liquid 31 is sprayed by the nozzle 30 toward the upper surface of the buffer layer 14. At this time, the substrate 11 on which the buffer layer 14 and the like are formed is placed on the heater 32, and the transparent electrode raw material liquid 31 dispersed on the upper surface of the buffer layer 14 is heated by the heat from the heater 32 to evaporate and react. Thereby, the transparent electrode 15 is formed on the buffer layer 14.

  Although the temperature of the board | substrate 11, ie, the temperature of the heater 32, is not specifically limited, It is preferable that it is 300 to 500 degreeC. If the temperature of the substrate 11 is less than 300 ° C., the transparent electrode raw material liquid 31 dispersed on the upper surface of the buffer layer 14 may hardly evaporate or react, and thus the transparent electrode 15 may not be formed easily. . When the temperature of the substrate 11 exceeds 500 ° C., as shown in FIG. 5, desorption of S from the light absorption layer 13 may be caused, and thus the photoelectric conversion efficiency may be lowered.

  Next, as shown in FIG. 3 (i), the transparent electrode 15, the buffer layer 14, and the light absorption layer 13 are each physically removed to form the opening 33. The opening 33 can be formed according to the method for forming the opening 29. In this way, an integrated compound thin film solar cell in which the compound thin film solar cells shown in FIG. 1 are connected in series can be manufactured. In addition, when manufacturing the compound thin film solar cell shown in FIG. 1, the scribe process shown in FIG.3 (b), FIG.3 (g), and FIG.3 (i) can be abbreviate | omitted.

  When manufacturing the compound thin film solar cell 2 shown in FIG. 2, after applying the transparent electrode material liquid which does not contain a dopant material on the buffer layer 14, the transparent electrode material liquid containing a dopant material is on it. What is necessary is just to apply. For example, when the interface layer 19 and the transparent electrode 15 are formed by a spray coating method, a transparent electrode material liquid containing no dopant material is sprayed on the buffer layer 14 from the nozzle 30 and then the transparent electrode material containing the dopant material. The liquid may be sprayed from the same nozzle 30. Thereby, since the interface layer 19 and the transparent electrode 15 are formed substantially simultaneously, the prolongation of the manufacturing time of the compound thin film solar cell 2 resulting from formation of the interface layer 19 can be prevented. Moreover, since it is not necessary to change the manufacturing apparatus of a compound thin film solar cell significantly in providing the interface layer 19, the compound thin film solar cell 2 can be manufactured at low cost. In addition, about content of dopant material in the dopant material and the transparent electrode 15, it is as having shown by the said [structure of a compound thin film solar cell].

  EXAMPLES Hereinafter, although an Example is given and this invention is demonstrated in detail, this invention is not limited to these.

<Example 1>
First, a soda lime glass plate (manufactured by AGC) having a thickness of 3.9 mm was prepared. Next, a lower electrode (thickness 1 μm) made of Mo was formed on a soda lime glass plate by sputtering. Subsequently, a part of the lower electrode was removed by laser ablation using nanosecond pulsed laser light. Thereby, an opening having a width of 100 nm was formed in the lower electrode.

  Next, copper acetate, zinc acetate, tin chloride, and thiourea were dissolved in water so that Cu: Zn: Sn: S = 2: 1: 1: 4 (atomic ratio). In this way, a precursor raw material liquid was obtained. The precursor material solution was applied onto the lower electrode using a slit coater, and then prebaked at 150 ° C. for 5 minutes. Then, it baked at 500 for 30 minutes. Thereby, a CZTS light absorption layer (thickness: 1.5 μm) was formed on the lower electrode.

Next, a soda lime glass plate on which a CZTS light absorption layer and the like were formed was placed on a heater set at 350 ° C. Thereafter, indium acetylacetone and thiourea were dissolved in water to prepare a buffer layer raw material solution. In the prepared buffer layer raw material liquid, the concentration of indium acetylacetone was 0.05M and the concentration of thiourea was 0.4M. This buffer layer raw material solution was sprayed from the nozzle onto the CZTS light absorption layer for 1 minute. Thereby, a buffer layer (thickness: 100 nm) made of In ₂ S ₃ was formed.

  Next, each part of the buffer layer and the CZTS light absorption layer was physically removed by a scribe nozzle. Thereby, an opening having a width of 200 nm was formed.

  Next, a soda lime glass plate on which a buffer layer and the like were formed was placed on a heater set at 350 ° C. Thereafter, zinc acetylacetonate and aluminum acetylacetonate were dissolved in a mixed solvent of methanol and water to prepare a raw material solution for a transparent electrode. In the prepared transparent electrode raw material liquid, the concentration of zinc acetylacetonate was 0.05M, and the concentration of aluminum acetylacetonate was 0.0015M. This raw material liquid for transparent electrode was sprayed on the buffer layer from the nozzle. Thereby, a transparent electrode (thickness 300 nm, sheet resistance 10 Ω / sq) made of ZnO was formed.

  Next, a part of each of the transparent electrode, the buffer layer, and the light absorption layer was physically removed by the scribe nozzle. Thereby, the compound thin film solar cell of Example 1 was manufactured.

  The ionization potential of each of the CZTS light absorption layer and the buffer layer was measured using an atmospheric photoelectron spectrometer AC-3 manufactured by Riken Keiki Co., Ltd. FIG. 8A shows the measurement result of the ionization potential of the CZTS light absorption layer, and FIG. 8B shows the measurement result of the ionization potential of the buffer layer. FIG. 8A shows that the ionization potential of the CZTS light absorption layer of this example is 5.1 eV, and FIG. 8B shows that the ionization potential of the buffer layer of this example is 5.9 eV. I understood that. Moreover, each band gap energy of a CZTS light absorption layer and a buffer layer was measured using the spectral sensitivity measuring apparatus by an OPTEL company. Using the ionization potential and band gap energy thus obtained, the conduction band levels of the CZTS light absorption layer and the buffer layer were determined. The results are shown in Table 1.

The manufactured compound thin-film solar cell was irradiated with light having an intensity of 1 kW / m ² (AM1.5 solar simulator) to measure the photoelectric conversion efficiency. The results are shown in Table 2. Table 2 shows the ratio of the photoelectric conversion efficiency in Example 1 when the photoelectric conversion efficiency in Comparative Example 1 is set to 100.

<Comparative Example 1>
A compound thin-film solar cell of Comparative Example 1 was manufactured according to the same method as in Example 1 except that a buffer layer made of ZnS was used. Specifically, a buffer layer raw material solution was prepared by dissolving ZnSO ₄ , ammonia, and thiourea in water. In the obtained buffer layer raw material liquid, the concentration of ZnSO ₄ was 0.16M, the concentration of ammonia was 7.5M, and the concentration of thiourea was 0.6M. A soda lime glass plate on which a CZTS light absorption layer or the like was formed was immersed in a buffer layer raw material liquid heated to 70 ° C. for 30 minutes (CBD method). Thus, a buffer layer made of ZnS was formed on the CZTS light absorption layer.

  When the ionization potential of the buffer layer of Comparative Example 1 was measured using an atmospheric photoelectron spectrometer AC-3 manufactured by Riken Keiki Co., Ltd., AC-3 was out of the measurement range. Therefore, the ionization potential of the buffer layer was measured by XPS. Further, the band gap energy of the buffer layer was measured according to the method described in Example 1 above. Using the ionization potential and the band gap energy thus obtained, the conduction band level of the buffer layer was obtained. The results are shown in Table 2. Moreover, the photoelectric conversion efficiency was measured according to the method described in Example 1 above. The results are shown in Table 2.

<Discussion>
As shown in Table 2, the photoelectric conversion efficiency in Example 1 was higher than that in Comparative Example 1. The reason is that in Example 1, the conduction band level of the buffer layer is 0.3 eV higher than the conduction band level of the CZTS light absorbing layer, whereas in Comparative Example 1, the conduction band level of the buffer layer is Is considered to be lower than the conduction band level of the CZTS light absorption layer. That is, in Example 1, In ₂ S ₃ , which is an n-type semiconductor material and has a difference between the ionization potential and the band gap energy of 3.2 eV or more and 3.6 eV or less, is used as the buffer layer material. The conduction band level of the buffer layer is 0 to 0.4 eV higher than the conduction band level of the CZTS light absorption layer. Therefore, it is possible to prevent recombination of hole-carriers at the interface between the light absorption layer and the buffer layer due to the fact that the conduction band level of the buffer layer is lower than the conduction band level of the light absorption layer. In addition, it is possible to prevent the carrier from being taken out poorly due to an increase in resistance at the interface between the light absorption layer and the buffer layer. From these things, since a carrier can be taken out from a light absorption layer efficiently, the compound thin film solar cell excellent in photoelectric conversion efficiency can be provided.

  It should be understood that the embodiments and examples disclosed herein are illustrative and non-restrictive in every respect. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

  1, 2 compound thin film solar cell, 11 substrate, 12 lower electrode, 13 light absorption layer, 14 buffer layer, 15 transparent electrode, 19 interface layer, 21, 29, 33 opening, 22 precursor raw material liquid, 23 coating means, 24 Precursor layer, 25, 30 nozzle, 26 buffer layer raw material liquid, 27, 32 heater, 28 scribe nozzle, 30 transparent electrode raw material liquid.

Claims (12)

A substrate,
A lower electrode provided on the substrate;
A light absorption layer provided on the lower electrode;
A buffer layer provided on the light absorption layer;
A transparent electrode provided on the buffer layer,
The light absorption layer is made of a compound containing Cu, Zn, Sn, S and Se,
The buffer layer is a compound thin-film solar cell made of an n-type semiconductor material and having a difference between an ionization potential and a band gap energy of 3.2 eV or more and 3.6 eV or less. The compound thin-film solar cell according to claim 1, wherein the buffer layer is made of In ₂ S ₃ .   The compound thin-film solar cell according to claim 1 or 2, wherein the buffer layer is formed by a spray coating method.   The said buffer layer is a compound thin film solar cell in any one of Claims 1-3 formed by setting the temperature of the said board | substrate to 300 to 500 degreeC.   The compound buffer solar cell according to claim 1, wherein the buffer layer is formed using a solution containing indium acetylacetone and thiourea.   The thickness of the said buffer layer is 50 nm or more and 200 nm or less, The compound thin film solar cell in any one of Claims 1-5.   The compound thin film solar cell according to any one of claims 1 to 6, wherein an interface layer containing ZnO is provided between the buffer layer and the transparent electrode. Forming a lower electrode on the substrate;
Forming a light absorption layer made of a compound containing at least one of Cu, Zn, Sn, S and Se on the lower electrode;
Forming a buffer layer made of an n-type semiconductor material having a difference between an ionization potential and a band gap energy of 3.2 eV or more and 3.6 eV or less on the light absorption layer;
The manufacturing method of a compound thin film solar cell provided with the process of forming a transparent electrode on the said buffer layer.   The method for producing a compound thin-film solar cell according to claim 8, wherein the step of forming the buffer layer includes a step of forming the buffer layer by a spray coating method.   10. The method of manufacturing a compound thin-film solar cell according to claim 8, wherein the step of forming the buffer layer includes a step of setting the temperature of the substrate to 300 ° C. or more and 500 ° C. or less.   11. The method for manufacturing a compound thin-film solar cell according to claim 8, wherein the step of forming the buffer layer includes a step of applying a solution containing indium acetylacetone and thiourea on the light absorption layer. A step of forming an interface layer containing ZnO between the buffer layer and the transparent electrode;
The step of forming the transparent electrode includes the step of forming a transparent electrode made of ZnO containing a dopant material,
The manufacturing method of the compound thin film solar cell in any one of Claims 8-11 which forms the said transparent electrode and the said interface layer with the same apparatus.

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