JP2013115235A - Photoelectric element and method for manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a photoelectric element which has a CZTS film having a large grain size and less voids and used as an optical absorption layer, and to provide a method for manufacturing the same.SOLUTION: The method for manufacturing a photoelectric element comprises: forming on a lower electrode a precursor (a) consisting of a laminated film laminated by a plurality of thin films including Cu, Zn or Sn, (b) without forming a Cu-Zn phase or a Cu-Zn-S phase during formation of the laminated film and in a sulfidizing process and (c) having a film thickness of 200-700 nm; and sulfidizing the precursor under a predetermined condition. By such a method, obtained is the photoelectric element including an optical absorption layer (a) consisting of a CZTS based compound, (b) including an area formed of only one crystal grain when viewing in a thickness direction of the film, (c) having a porosity of 10% or less and (d) a film thickness of 300-1000 nm.

Description

The present invention relates to a photoelectric device and a manufacturing method thereof, and more particularly to a photoelectric device using a Cu ₂ ZnSnS ₄ (CZTS) -based compound in a light absorption layer and a manufacturing method thereof.

  A photoelectric element refers to an element that can convert photon energy into an electrical signal (photoelectric conversion) through some physical phenomenon. A solar cell is a kind of photoelectric element, and can efficiently convert light energy of sunlight into electric energy.

Known semiconductors used in solar cells include single crystal Si, polycrystalline Si, amorphous Si, GaAs, InP, CdTe, CuIn _1-x Ga _x Se ₂ (CIGS), and CZTS.
Among these, chalcogenite-based compounds represented by CIGS and CZTS have a large light absorption coefficient, so that a thin film advantageous for cost reduction is possible. In particular, a solar cell using CIGS as a light absorption layer has high conversion efficiency in a thin film solar cell, and conversion efficiency exceeding that of a solar cell using polycrystalline Si is also obtained. However, CIGS has a problem that it contains an environmental load element and a rare element.
On the other hand, CZTS has a band gap energy (1.4 to 1.5 eV) suitable for a solar cell and is characterized by not containing an environmental load element or a rare element.

Various proposals have been made for CZTS compounds and their production methods.
For example, in Non-Patent Document 1, a Mo film is formed on a soda lime glass (SLG) substrate, and Cu / Sn / ZnS (film thickness: 570 to 610 nm), Sn / Cu / ZnS (film thickness) are formed on the Mo film. : 620 nm), or a precursor made of Cu / SnS ₂ / ZnS (film thickness: 950 nm), and a CZTS solar cell obtained by sulfiding at 510 to 550 ° C. × 1 to 3 h is disclosed. .
In this document, the conversion efficiency of a solar cell using Cu / Sn / ZnS laminated film or Cu / SnS ₂ / ZnS as a precursor is 3.80 to 3.93%, while Sn / Cu / is used as a precursor. It is described that the conversion efficiency of a solar cell using a ZnS laminated film is 4.53%. Further, it is described that when a Cu / Sn / ZnS laminated film is used as a precursor, many voids are contained in the CZTS film.
That is, in Non-Patent Document 1, a solar cell using a precursor in which a thin film containing Cu and a thin film containing Zn are adjacent has a higher conversion efficiency than a solar cell in which both are not adjacent. Have been described.

Non-Patent Document 2 discloses a CZTS solar cell obtained by forming a precursor made of Cu-Sn-Zn-S on a SLG substrate by simultaneous sputtering and sulfiding at 580 ° C. for 3 hours. ing.
This document describes that a solar cell having a CZTS film thickness of 2200 nm and a conversion efficiency of 6.77% can be obtained by such a method.

  As described in Non-Patent Document 2, when the manufacturing conditions are optimized, the conversion efficiency of the CZTS solar cell exceeds 6%. However, all of the CZTS films obtained by the conventional method have a small particle size and many voids. In order to improve the conversion efficiency of the CZTS solar cell, it is considered necessary to increase the particle size of the CZTS film and reduce voids. Moreover, there is no example in which a CZTS solar cell having a conversion efficiency of 7% or more has been proposed.

H. Katagiri, Thin Solid Films, 480-481 (2005), 426-432 H.Katagiri et al., Appl.Phys.express 1 (2008) 041201

The problem to be solved by the present invention is to provide a photoelectric device using a CZTS film having a large particle size and few voids as a light absorption layer, and a method for producing the photoelectric device.
Another problem to be solved by the present invention is to provide a photoelectric device using a CZTS film as a light absorption layer and having a conversion efficiency of 7% or more, and a method for manufacturing the photoelectric device.

In order to solve the above problems, the photoelectric device according to the present invention is
(A) a CZTS compound,
(B) includes a region composed of only one crystal grain when viewed in the film thickness direction;
(C) the porosity is 10% or less, and
(D) The gist is that a light absorption layer having a film thickness of 300 to 1000 nm is provided.

The gist of the method for manufacturing a photoelectric device according to the present invention is as follows.
(1) A lower electrode forming step of forming a lower electrode on the substrate.
(2) On the lower electrode,
(A) a laminated film in which a plurality of thin films containing Cu, Zn or Sn are laminated,
(B) without forming a Cu—Zn phase or a Cu—Zn—S phase during the formation of the laminated film and during the sulfiding process; and
(C) A precursor forming step of forming a precursor having a film thickness of 200 to 700 nm.
(3) Sulfurization step: sulfiding the precursor under the conditions of sulfidation temperature: 560 to 600 ° C., heating rate: 2.5 to 7.5 ° C./min, sulfidation time: 5 to 50 minutes.

For example, when a precursor having a three-layer structure composed of a Sn / Cu / Zn laminated film is sulfided, first, Cu diffuses into both the Sn phase and the Zn phase at the initial stage of heating, and the Cu-Sn alloy phase / Cu-Zn alloy phase. It changes to a two-phase structure. Next, sulfuration of the precursor occurs, and finally a CZTS film is obtained through a two-phase structure of Cu ₂ SnS ₃ phase / ZnS phase. That is, when the ZnS phase is generated from the Cu—Zn alloy phase, Cu contained in the Cu—Zn phase diffuses into the Cu—Sn phase. As a result, in the CZTS film finally obtained, the carkendar void generated by the difference in the diffusion coefficient of the element remains, and a porous CZTS film is obtained. Moreover, the grain growth is hindered by the void.
On the other hand, for example, when a precursor having a three-layer structure made of a Cu / Sn / ZnS laminated film is sulfided, the precursor does not generate a Cu—Zn alloy phase, and a two-phase structure of Cu ₂ SnS ₃ phase / ZnS phase. And finally becomes a CZTS film. As a result, element diffusion is minimized and voids are less likely to occur. Further, the grain growth is promoted, and a CZTS film including a large number of regions including only one crystal grain in the film thickness direction is obtained.

It is the result of the glow discharge emission spectroscopic analysis (GD-OES) of the film | membrane which sulfided the ZnS / Cu precursor at 600 degreeC. It is the result of the glow discharge emission spectroscopic analysis (GD-OES) of the film | membrane which sulfided the ZnS / SnS precursor at 600 degreeC. It is the result of the glow discharge emission spectroscopic analysis (GD-OES) of the film | membrane which sulfided SnS / Cu precursor at 600 degreeC. FIG. 4A shows the electron backscatter diffraction (EBSD) measurement result of the CZTS film obtained by sulfiding Mo / ZnS / SnS / Cu precursor. FIG. 4B is an electron backscatter diffraction (EBSD) measurement result of a CZTS film obtained by sulfiding a Mo / ZnS / Cu / Sn / ZnS precursor. It is a TEM image of the cross section of the CZTS solar cell obtained by the comparative example 1 (left figure) and Example 1 (right figure). It is the SEM image and EBIC image of the CZTS solar cell obtained by the comparative example 1 (left figure) and Example 1 (right figure). It is a figure which shows the IV characteristic (1sun, AM1.5) of the CZTS solar cell obtained in Example 1 and Comparative Example 1. FIG.

Hereinafter, an embodiment of the present invention will be described in detail.
[1. Photoelectric element]
The photoelectric element according to the present invention is
(A) a CZTS compound,
(B) includes a region composed of only one crystal grain when viewed in the film thickness direction;
(C) the porosity is 10% or less, and
(D) It has the light absorption layer whose film thickness is 300-1000 nm, It is characterized by the above-mentioned.

[1.1. Light absorption layer]
[1.1.1. CZTS compound]
In the present invention, the “CZTS compound” refers to a compound semiconductor based on Cu ₂ ZnSnS ₄ (CZTS).
In the present invention, the term “CZTS compound” includes not only stoichiometric compounds but also all non-stoichiometric compounds or all compounds containing Cu, Zn, Sn, and S as the main component. .
The CZTS-based compound may be composed only of Cu, Zn, Sn, and S, or may further contain other chalcogen elements, various dopants, unavoidable impurities, and the like.

[1.1.2. Average crystal grain size]
When a method described later is used, the light absorption layer includes a region where only one crystal grain exists when viewed in the film thickness direction. That is, the light absorption layer includes crystal grains having a grain size equal to the film thickness. When the manufacturing conditions are optimized, a light absorption layer in which most of the crystal grains appearing in the microscopic image of the polished cross section have a grain size equivalent to the film thickness can be obtained.
In the present invention, the “particle diameter” refers to the long diameter (minimum long diameter) of the rectangle that circumscribes the projected image of each crystal grain that appears in the microscopic image of the polished cross section of the light absorbing layer, with the smallest long diameter. When the minimum major axis exceeds the film thickness of the light absorption layer, the particle diameter means the film thickness of the light absorption layer.
“Average crystal grain size” refers to the average value of the top five particle sizes of all the particles measured in the field of view (magnification: 20,000 times). When the number of particles appearing in one field of view is less than 10, take a microscopic image of multiple fields so that the number of particles is 10 or more, and measure the particle size of all particles appearing in multiple fields of view. To do.
In order to obtain high conversion efficiency, the average crystal grain size is preferably 0.3 μm or more. The average crystal grain size is more preferably 0.5 μm or more.

[1.1.3. Porosity]
In the present invention, “porosity” refers to the ratio of the area of the pores to the total area of the light absorbing layer that appears in the microscopic image of the polished cross section of the light absorbing layer. The horizontal direction of the observation surface is about 10 times the film thickness.
In order to obtain high conversion efficiency, the porosity needs to be 10% or less. The porosity is more preferably 7% or less.
When a method described later is used, a CZTS film having almost no pores can be obtained.

[1.1.4. Film thickness]
The film thickness of the light absorption layer affects the conversion efficiency.
In general, when the thickness of the light absorption layer is too thin, the light absorption rate decreases. Therefore, the film thickness needs to be 300 nm or more. More preferably, the film thickness is 500 nm or more.
On the other hand, if the film thickness is too thick, the probability that carriers excited by light disappear due to recombination before reaching the electrode increases. Therefore, the film thickness needs to be 1000 nm or less. The film thickness is more preferably 800 nm or less, and still more preferably 750 nm or less.

[1.2. Conversion efficiency]
In the photoelectric element including the above-described light absorption layer, when the manufacturing conditions are optimized, the conversion efficiency is 7.0% or more, or 7.5% or more.

[1.3. Other components]
The photoelectric element according to the present invention may further include components other than the light absorption layer as necessary.
For example, a thin-film solar cell generally has a structure in which a substrate, a lower electrode, a light absorption layer, a buffer layer, a window layer, and an upper electrode are stacked in this order. Additional layers may be formed between the layers.
Specifically, as an additional layer,
(1) an adhesive layer for increasing the adhesion between the substrate and the lower electrode;
(2) a light scattering layer for reflecting incident light and increasing light absorption efficiency in the light absorption layer, which is formed on the upper electrode side from the light absorption layer;
(3) a light scattering layer provided on the substrate side from the light absorption layer;
(4) an antireflection layer for reducing the amount of reflection of incident light at the window layer and increasing the light absorption efficiency at the light absorption layer;
and so on.
In the present invention, the material of each layer other than the light absorption layer is not particularly limited, and various materials can be used according to the purpose.

When the photoelectric element is a solar cell, specific examples of the material of each layer other than the light absorption layer include the following.
Examples of the substrate material include glass (for example, SLG, low alkali glass, non-alkali glass, quartz glass, quartz glass into which Na ions are implanted, sapphire glass, etc.), ceramics (for example, silica, alumina, yttria, zirconia, etc.). Oxides, various ceramics containing Na), metals (for example, stainless steel, stainless steel containing Na, Au, Mo, Ti, etc.).

In particular, when glass is used as the substrate, the glass substrate is preferably low alkali glass. There is a correlation between the total content of Na ₂ O and K ₂ O contained in the glass substrate and the conversion efficiency, and the conversion efficiency is high both when the total content of Na ₂ O and K ₂ O is too low and too high. Decreases.
Therefore, the total content of Na ₂ O and K ₂ O in the glass substrate is preferably 3 wt% or more. The total content of Na ₂ O and K ₂ O is more preferably 5 wt% or more, more preferably 7 wt% or more.
The total content of Na ₂ O and K ₂ O in the glass substrate is preferably 14 wt% or less. The total content of Na ₂ O and K ₂ O is more preferably 12 wt% or less, and further preferably 10 wt% or less.

As a material of the lower electrode, for example, Mo, MoSi ₂ , stainless steel, In—Sn—O, In—Zn—O, ZnO: Al, ZnO: B, SnO ₂ : F, SnO ₂ : Sb, ZnO: Ga, TiO ₂ : Nb etc.
Examples of the material of the buffer layer include CdS.
Examples of the material of the window layer include ZnO: Al, ZnO: Ga, ZnO: B, In—Sn—O, In—Zn—O, SnO ₂ : Sb, and TiO ₂ : Nb.
Examples of the material of the upper electrode include Al, Cu, Ag, Au, and alloys containing any one or more of these. Specific examples of such an alloy include an Al—Ti alloy, an Al—Mg alloy, an Al—Ni alloy, a Cu—Ti alloy, a Cu—Sn alloy, a Cu—Zn alloy, a Cu—Au alloy, and Ag. -Ti alloy, Ag-Sn alloy, Ag-Zn alloy, Ag-Au alloy, and the like.

When a glass substrate is used as the substrate and Mo is used as the lower electrode, examples of the material for the adhesive layer include Ti, Cr, Ni, W, and alloys containing any one or more of these.
Examples of the material for the light scattering layer provided above the light absorption layer include oxides such as SiO ₂ and TiO _2, and nitrides such as Si—N.
Examples of the material for the light scattering layer provided on the substrate side with respect to the light absorption layer include a layer having an uneven surface.
As a material for the antireflection layer, for example, a transparent body having a refractive index smaller than that of the window layer, an aggregate composed of transparent particles having a diameter sufficiently smaller than the wavelength of sunlight, and the inside of which is smaller than the wavelength of sunlight. Some have a space with a sufficiently small diameter, others have an uneven structure with a period of a submicrometer on the surface. In particular,
(1) A thin film made of MgF ₂ , SiO ₂ or the like,
(2) multilayer films of oxides, sulfides, fluorides, nitrides, etc.
(3) Fine particles made of an oxide such as SiO ₂ ,
and so on.

[2. Photoelectric element manufacturing method]
The method for manufacturing a photoelectric device according to the present invention includes a lower electrode forming step, a precursor forming step, and a sulfiding step.

[2.1. Lower electrode formation process]
The lower electrode forming step is a step of forming the lower electrode on the substrate.
The materials for the substrate and the lower electrode are not particularly limited, and various materials can be used depending on the purpose. The details of the material of the substrate and the lower electrode are as described above, and thus the description thereof is omitted.
When a glass substrate is used as the substrate, it is preferable to use Mo having excellent adhesion to the glass substrate for the lower electrode.
In the case of using a glass substrate as the substrate, a glass substrate, as the total content of Na ₂ O and K ₂ O is less than 3 wt% 14 wt% (low alkali glass) is preferred.

  The method for forming the lower electrode is not particularly limited, and various methods can be used. Specifically, the lower electrode is formed by sputtering, vacuum deposition, pulsed laser deposition (PLD), plating, chemical solution deposition (CBD), electrophoretic deposition (EPD), chemical vapor, There are a phase film formation (CVD) method, a spray pyrolysis film formation (SPD) method, a screen printing method, a spin coating method, a fine particle deposition method, and the like.

[2.2. Precursor formation process]
The precursor forming step is a step of forming a precursor on the lower electrode.
In the present invention, the precursor is
(A) a laminated film in which a plurality of thin films containing Cu, Zn or Sn are laminated,
(B) without forming a Cu—Zn phase or a Cu—Zn—S phase during the formation of the laminated film and during the sulfiding process; and
(C) The film thickness is 200 to 700 nm.

[2.2.1. Laminated film]
A precursor containing Cu, Zn and Sn at a predetermined ratio can also be produced by, for example, a three-source simultaneous sputtering method. However, when a precursor produced by the three-source simultaneous sputtering method is sulfided, the crystal grains in the CZTS film become fine and voids increase.
Therefore, the precursor needs to be a thin film stack including any one or two kinds of elements selected from Cu, Zn, and Sn.
Each thin film constituting the laminated film may be a metal or a sulfide. Moreover, the thickness and composition of each thin film are not particularly limited as long as a CZTS film having a predetermined thickness can be produced.

[2.2.2. Cu-Zn phase or Cu-Zn-S phase]
Cu and Zn are easily alloyed by heating. For example, when the metal Cu and the metal Zn are adjacent to each other in the precursor, a Cu—Zn alloy phase is first generated during the sulfurization reaction, and then the Cu—Zn alloy phase and other phases are sulfided. Thus, when a precursor that forms a Cu—Zn alloy phase in the sulfidation process is sulfidized, the crystal grains in the CZTS film become fine and voids increase. This is because when a Cu—Zn phase is generated during the formation of the laminated film and when the precursor contains ZnS (that is, a Cu—Zn—S phase is produced during the formation of the laminated film or during the sulfidation process). The same applies to the case where
Therefore, the precursor must not generate a Cu—Zn phase or a Cu—Zn—S phase (a phase containing at least Cu and Zn) during the formation of the laminated film and during the sulfidation process.

As such a precursor, specifically,
(1) a laminated film in which a plurality of thin films containing Cu, Zn, or Sn are laminated so that a thin film containing Cu and a thin film containing Zn are not adjacent to each other;
(2) Cu ₂ SnS ₃ / ZnS laminated film,
and so on.
Examples of the former include ZnS / Sn / Cu, ZnS / SnS / Cu, Zn / Sn / Cu, and Zn / SnS / Cu.

[2.2.3. Film thickness]
When the precursor is sulfided, the film thickness increases. As described above, the film thickness of the CZTS film affects the conversion efficiency. Therefore, in order to obtain a CZTS film having a predetermined thickness, the thickness of the precursor needs to be 200 to 700 nm.

[2.2.4. Film formation method]
The method for forming the precursor is not particularly limited, and various methods can be used. The details of the method of forming the precursor are the same as those of the lower electrode, and thus description thereof is omitted.

[2.3. Sulfurization process]
The sulfiding step is a step of sulfiding the precursor under the conditions of sulfiding temperature: 560 to 600 ° C., heating rate: 2.5 to 7.5 ° C./min, and sulfiding time: 5 to 50 minutes.
Sulfurization is performed under an atmosphere containing hydrogen sulfide or sulfur vapor.

If the sulfiding temperature is too low, it is difficult to complete sulfiding in a short time. Therefore, the sulfurization temperature needs to be 560 ° C. or higher.
The higher the sulfurization temperature, the easier the precursor is sulfided. However, if the sulfiding temperature is too high, the resistance value of the lower electrode may increase excessively. In particular, when Mo is used as the lower electrode, sulfurization at a high temperature causes an increase in the resistance value of the lower electrode. Therefore, the sulfurization temperature needs to be 600 ° C. or less.

As will be described later, when the precursor is sulfided, a two-phase structure of Cu ₂ SnS ₃ phase / ZnS phase is generated first, and then these react to generate a CZTS phase. Therefore, in order to make such a reaction proceed smoothly, it is necessary to slowly pass the temperature range where the two-phase structure is generated to promote the generation of the two-phase structure. Therefore, the temperature rising rate needs to be 7.5 ° C./min or less.
On the other hand, even if the heating rate is made slower than necessary, there is no real benefit. Therefore, the temperature rising rate needs to be 2.5 ° C./min or more.

If the sulfurization time is too short, the precursor will be insufficiently sulfurized. Therefore, the sulfurization time needs to be 5 minutes or more.
The longer the sulfurization time, the easier the precursor is sulfided. However, if the sulfurization time is too long, the resistance value of the lower electrode may increase excessively. In particular, when Mo is used as the lower electrode, long-term sulfidation increases the resistance value of the lower electrode. Therefore, the sulfurization time needs to be 50 minutes or less.

[2.4. Other processes]
As described above, after forming the lower electrode and the light absorption layer on the substrate, other layers are formed on the light absorption layer as necessary. Examples of other layers include a buffer layer, a window layer, and an upper electrode.
The method for forming other layers is not particularly limited, and various methods can be used. The details of the method of forming the other layers are the same as those of the lower electrode, and thus description thereof is omitted.

[3. Action of photoelectric device and manufacturing method thereof]
As will be described later, as a result of analyzing the reaction behavior of the precursor, it was found that CZTS is generated according to the following equations (1) and (2) regardless of the initial state of the precursor.
2Cu + Sn + 3S → Cu ₂ SnS ₃ (1)
Cu ₂ SnS ₃ + ZnS → CZTS (2)

On the other hand, Cu and Zn are easily alloyed by heating. Therefore, for example, when a precursor having a three-layer structure composed of a Sn / Cu / Zn laminated film is sulfided, Cu is first diffused into both the Sn phase and the Zn phase at the initial stage of heating, and a Cu—Sn alloy phase / Cu—Zn alloy The phase changes to a two-phase structure. Next, sulfuration of the precursor occurs, and finally a CZTS film is obtained through a two-phase structure of Cu ₂ SnS ₃ phase / ZnS phase. That is, when the ZnS phase is generated from the Cu—Zn alloy phase, Cu contained in the Cu—Zn phase diffuses into the Cu—Sn phase. As a result, in the CZTS film finally obtained, the carkendar void generated by the difference in the diffusion coefficient of the element remains, and a porous CZTS film is obtained. Moreover, the grain growth is hindered by the void. This is the same when a ZnS phase is present in the precursor.

Non-Patent Document 2 describes a method in which a precursor in which Cu—Zn—Sn—S is mixed almost uniformly is produced by a three-source simultaneous sputtering apparatus, and this is sulfided to produce a CZTS film. However, such a precursor structure is also the same as described above, and when the CZTS film is formed, complicated diffusion of elements and complicated reaction processes occur. As a result, a film with fine crystal grains and a relatively large amount of voids is likely to occur.
Further, since the sulfiding conditions are also high temperature and long time, when Mo is used as the lower electrode, the Mo sulfide layer becomes thick. As a result, the fill factor (FF) value of the photoelectric element decreases. On the other hand, if the sulfidation conditions are moderated to avoid this, it becomes difficult to produce a sufficiently high performance CZTS film.

On the other hand, for example, when a precursor having a three-layer structure made of a Cu / Sn / ZnS laminated film is sulfided, the precursor does not generate a Cu—Zn alloy phase, and a two-phase structure of Cu ₂ SnS ₃ phase / ZnS phase. And finally becomes a CZTS film. As a result, element diffusion is minimized and voids are less likely to occur. Further, the grain growth is promoted, and a CZTS film having only one crystal grain in the film thickness direction is obtained. That is, when the method according to the present invention is used, a dense and uniform CZTS film can be obtained.

Furthermore, when Mo is used as the lower electrode, if the precursor on the lower electrode is sulfided, a Mo sulfide layer is generated at the CZTS / Mo interface. Since the Mo sulfide layer becomes a high electrical resistance layer, it should be reduced as much as possible in the photoelectric element.
On the other hand, in the precursor as described above, since the diffusion of the element is easy, the sulfurization temperature can be reduced and the time can be shortened. As a result, even when Mo is used as the lower electrode, the Mo sulfide layer generated at the CZTS / Mo interface can be reduced, and the DC resistance component of the photoelectric element characteristics can be reduced. As a result, the conversion efficiency of the photoelectric element increases due to the improvement of the FF value.

Non-Patent Document 1 discloses a CZTS film having a film thickness of 1300 nm by forming a Cu / Sn / ZnS precursor having a film thickness of 570 to 610 nm on an SLG substrate and sulfidizing it at 550 ° C. × 3 h. The points obtained are described. Moreover, the obtained CZTS film | membrane has many voids and the point that the conversion efficiency of a solar cell is 3.8% is described.
This is because (1) Sn was evaporated by sulfidation for a long time of 3 h, and many voids were formed.
(2) Since the Mo film of the lower electrode is sulfided and the Mo-S film of the high resistance layer is formed thick,
it is conceivable that.

(Example A, Comparative Example A)
[1. Investigation of reaction behavior]
[1.1. Preparation of sample]
The precursor was composed of three types of materials, ZnS, SnS and Cu.
First, two arbitrary types were selected from these. That is, ZnS / Cu (sample 1), ZnS / SnS (sample 2), and SnS / Cu (sample 3) were selected as precursors. Each selected film was formed by sputtering on a PV200 glass substrate (Na ₂ O content: 5 wt%) with a Mo film (film thickness: 1 μm). The film thickness was adjusted so that each layer was 300 nm. The film forming conditions are ZnS: 200W × 0.5Pa, SnS: 200W × 0.5Pa, Cu: 300W × 0.5Pa.
The thin film sample was heat-treated in two kinds of atmospheres (H ₂ S, S vapor) and three kinds of temperatures (200, 400, 600 ° C.).

[1.2. Test method]
About the prepared sample,
(1) Identification of a production phase by XRD (SmartLab: manufactured by Rigaku),
(2) Evaluation of diffusion state by GD-OES, and
(3) The microstructure was observed by FE-SEM (S-4800: manufactured by HITACHI), and the reaction behavior was investigated.

[1.3. result]
In the heat treatment in the H ₂ S and S vapor atmospheres, basically the same reaction and diffusion tendency were observed.
[1.3.1. ZnS / Cu (Sample 1)]
In the case of Sample 1, no reaction phase of ZnS and Cu was observed in all atmospheres and temperature ranges. Cu exhibited a sulfurization reaction at 200 ° C., and formed CuS.
FIG. 1 shows the results of GD-OES of a film obtained by sulfurizing a ZnS / Cu precursor at 600 ° C. In FIG. 1, the sputtering time on the horizontal axis corresponds to the depth from the surface. From FIG. 1, it was confirmed that Cu and Zn were distributed almost uniformly in the depth direction, and that ZnS and Cu were interdiffused.

[1.3.2. ZnS / SnS (Sample 2)]
In the case of Sample 2, no reaction phase of ZnS and SnS was observed in all atmospheres and temperature ranges. In addition, it was confirmed that SnS was sulfided and SnS ₂ was formed as the temperature increased.
FIG. 2 shows the result of GD-OES of a film obtained by sulfiding a ZnS / SnS precursor at 600 ° C. Although no diffusion was observed up to 400 ° C., it was confirmed that diffusion occurred slightly at 600 ° C. as shown in FIG. In the case of ZnS / Sn, Sn was first sulfided to form SnS. After that, it is the same as ZnS / SnS.

[1.3.3. SnS / Cu (Sample 3)]
In the case of sample 3, Cu was first sulfided at 200 ° C. to form CuS. Subsequently, it was confirmed that SnS and CuS reacted at 400 ° C. to form Cu ₂ SnS ₃ .
FIG. 3 shows the result of GD-OES of a film obtained by sulfiding SnS / Cu precursor at 600 ° C. FIG. 3 shows that, unlike the other two, SnS and Cu easily react to form Cu ₂ SnS ₃ . In the case of Sn / Cu, it was first confirmed that a Cu—Sn alloy was formed at 200 ° C. and Cu ₂ SnS ₃ was formed at 400 ° C.

[2. Preparation and evaluation of CZTS film]
[2.1. Preparation of sample]
From the above experimental results and literature, the reaction route was estimated as a two-step reaction described below.
(1) 2CuS + SnS → Cu ₂ SnS ₃ (when SnS is used), or
Cu—Sn alloy + S → Cu ₂ SnS ₃ (when Sn is used),
(2) Cu ₂ SnS ₃ + ZnS → Cu ₂ ZnSnS ₄
Note that Cu diffused in ZnS is considered to react with SnS to form Cu ₂ SnS ₃ once being discharged from ZnS.

Based on both this reaction and diffusion, the following conditions were considered necessary for the layer structure.
As a first condition, SnS or Sn and Cu must be adjacent to form Cu ₂ SnS ₃ .
As a second condition, ZnS and Cu are not allowed to be adjacent to each other because mutual diffusion occurs and becomes an inhibiting factor for the formation of Cu ₂ SnS ₃ .
As a third condition, since the reaction of SnS or Sn and Cu is completed by the formation of Cu ₂ SnS ₃ before the diffusion of ZnS and SnS or Sn occurs, ZnS and SnS or Sn may be adjacent to each other.

The layer configuration derived from these three conditions is Mo / ZnS / SnS / Cu, Mo / ZnS / Sn / Cu, Mo / Cu / SnS / ZnS, or Mo / Cu / Sn / ZnS. Judging from the adhesion of Mo, Mo / ZnS / SnS / Cu precursor or Mo / ZnS / Sn / Cu precursor was sulfided to produce a CZTS film (Example A).
For comparison, a CZTS film was prepared by sulfiding Mo / ZnS / Cu / Sn / ZnS precursor, which is a standard configuration so far (Comparative Example A).

[2.2. Test method]
About the produced sample, the microstructure and the crystal state were observed by SEM and EBSD.
[2.3. result]
FIG. 4A shows an EBSD measurement result of a CZTS film obtained by sulfiding a Mo / ZnS / SnS / Cu precursor. FIG. 4B shows an EBSD measurement result of a CZTS film obtained by sulfiding a Mo / ZnS / Cu / Sn / ZnS precursor.
FIG. 4 shows that the CZTS film obtained in Example A has larger crystal grains and fewer voids than Comparative Example A. Further, the CZTS film obtained in Example A had coarse crystal grains, and most of the crystal grains had a grain size equivalent to the film thickness.

(Example 1, Comparative Example 1)
[1. Preparation of sample]
A Mo film having a thickness of 1 μm was formed on a low alkali glass substrate by a sputtering apparatus. Subsequently, a precursor having the following structure was produced on the Mo film by a multi-source sputtering apparatus or a vacuum deposition apparatus.
Example 1: Cu / Sn / ZnS / Mo (precursor film thickness: 410 nm).
Comparative Example 1: ZnS / Sn / Cu / ZnS / Mo (precursor film thickness: 410 nm).

These precursors were sulfided. The sulfiding conditions were as follows: sulfiding gas: 20% H ₂ S (balance gas: N ₂ ), sulfiding temperature: 580 ° C., sulfiding time: 20 min, heating rate: 5 ° C./min.
After sulfidation, CdS was deposited to a thickness of about 100 nm as a buffer layer by the CBD method, and subsequently Ga: ZnO was deposited as a window layer to a thickness of about 100 nm by sputtering. Finally, as an upper electrode, a comb-shaped Al film was formed by vacuum deposition to produce a solar battery cell.

[2. Test method]
A TEM image of a cross section of the CZTS solar cell and an SEM & EBIC image were taken. Moreover, the solar cell characteristic of the CZTS solar cell was measured.

[3. result]
In FIG. 5, the TEM image of the cross section of the CZTS solar cell obtained by the comparative example 1 (left figure) and Example 1 (right figure) is shown. FIG. 6 shows SEM images and EBIC images of CZTS solar cells obtained in Comparative Example 1 (left diagram) and Example 1 (right diagram). FIG. 7 shows the IV characteristics (1 sun, AM1.5) of the CZTS solar cells obtained in Example 1 and Comparative Example 1.
5 and 6, the CZTS film of Comparative Example 1 (film thickness: 800 nm) is a porous film with many voids, whereas the CZTS film of Example 1 (film thickness: 700 nm) is dense. It can be seen that a uniform film is obtained. In Example 1, it was found from the STEM-EDS analysis that each element was uniformly distributed and no impurities were generated.
Further, in Example 1, carriers (electrons) generated from the entire film can be extracted from EBIC, whereas in Comparative Example 1, only electrons excited on the CZTS surface can be extracted. As a result, the conversion efficiency as the solar cell characteristic was improved to 7.6% in Example 1 as compared to 6.8% in Comparative Example 1 (FIG. 7).

(Example 2)
[1. Preparation of sample]
A Mo film having a thickness of 1 μm was formed on a low alkali glass substrate by a sputtering apparatus. Next, a Cu / Sn / ZnS precursor (film thickness: 410 nm) was formed on the Mo film. This precursor was sulfidized at various heating rates and sulfidation times. The sulfurization temperature was 580 ° C., and N ₂ + 20% H ₂ S gas was used as the sulfur gas. The CZTS film thickness after sulfidation was 700 nm, respectively. Hereinafter, solar cells were produced in the same manner as in Example 1.

[2. result]
The conversion efficiencies of solar cells that were sulfurized under various conditions were measured. Table 1 shows the results. From Table 1, it was found that the conversion efficiency depends on the sulfiding time and the heating rate. It can also be seen that high conversion efficiency is obtained when the sulfurization time is 15 to 50 minutes and the heating rate is 2.5 to 7.5 ° C./min.

(Examples 3.1 to 3.2, Comparative examples 3.1 to 3.5)
[1. Preparation of sample]
A Mo film having a thickness of 1 μm was formed on a low alkali glass substrate by a sputtering apparatus. Next, precursors (thickness: 410 nm) having various laminated structures were formed on the Mo film. This precursor was sulfided under the conditions of sulfurization gas: N ₂ + 20% H ₂ S, sulfurization temperature: 580 ° C., sulfurization time: 20 min, and heating rate: 5 ° C./min. The CZTS film thickness after sulfidation was 700 nm, respectively. Hereinafter, solar cells were produced in the same manner as in Example 1.

[2. result]
TEM observation was performed on the solar cells that had been sulfided under various conditions, and the average crystal grain size and porosity were measured. Moreover, the conversion efficiency was measured about each photovoltaic cell. Table 2 shows the results.
In Comparative Examples 3.1 to 3.5 using a precursor in which a layer containing Cu and a layer containing Zn are adjacent, the average crystal grain size is 0.2 to 0.3 μm, and the porosity is 10 % Exceeded. The conversion efficiency was 0.5 to 6.8% depending on the type of the precursor.
On the other hand, in Examples 3.1 to 3.2, all had an average crystal grain size equivalent to the film thickness and a porosity of 10% or less. Moreover, all the conversion efficiencies exceeded 7%.

Example 4
[1. Preparation of sample]
A Mo film having a thickness of 1 μm was formed on a low alkali glass substrate by a sputtering apparatus. Next, precursors having different film thicknesses were formed on the Mo film. The film configuration of the precursor was Cu / SnS / ZnS, and the film thickness ratio was Cu: 19%, SnS: 44%, and ZnS: 36%. This precursor was sulfided under the conditions of sulfurization gas: N ₂ + 20% H ₂ S, sulfurization temperature: 580 ° C., sulfurization time: 20 min, and heating rate: 5 ° C./min.

[2. result]
TEM observation of the obtained photovoltaic cell was performed, and the film thickness of the CZTS film was measured. Moreover, the conversion efficiency was measured about each photovoltaic cell. Table 3 shows the results.
Table 3 shows the following.
(1) The conversion efficiency depends on the thickness of the CZTS film and the thickness of the precursor.
(2) When the thickness of the CZTS film is 600 to 850 nm, the conversion efficiency is about 5.0% or more.
(3) When the thickness of the CZTS film is 650 to 750 nm, the conversion efficiency is about 6.0% or more.

(Examples 5.1 to 5.3)
[1. Preparation of sample]
A Mo film having a thickness of 1 μm was formed on the substrate by a sputtering apparatus. As the substrate, various glass substrates having different Na ₂ O contents were used. Next, a Cu / SnS / ZnS precursor (film thickness: 410 nm) was formed on the Mo film. The film thickness ratio was Cu: 19%, SnS: 44%, and ZnS: 36%. This precursor was sulfided under the conditions of sulfurization gas: N ₂ + 20% H ₂ S, sulfurization temperature: 580 ° C., sulfurization time: 20 min, and heating rate: 5 ° C./min. The film thickness of the CZTS film after sulfidation was 700 nm, respectively.

[2. result]
The conversion efficiency was measured about each obtained photovoltaic cell. Table 4 shows the results.
Table 4 shows the following.
(1) The conversion efficiency depends on the Na ₂ O content contained in the glass substrate.
(2) When the Na ₂ O content is ₂ to 9 wt%, the conversion efficiency is about 6% or more.
(3) When the Na ₂ O content is 4 to 7 wt%, the conversion efficiency is about 7% or more.

  Although the embodiments of the present invention have been described in detail above, the present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the gist of the present invention.

  The photoelectric element according to the present invention can be used for thin film solar cells, photoconductive cells, photodiodes, phototransistors, and the like.

Claims (6)

(A) a CZTS compound,
(B) includes a region composed of only one crystal grain when viewed in the film thickness direction;
(C) the porosity is 10% or less, and
(D) A photoelectric device including a light absorption layer having a film thickness of 300 to 1000 nm.   The photoelectric device according to claim 1, wherein the light absorption layer has an average crystal grain size of 0.3 μm or more.   The photoelectric device according to claim 1, wherein the conversion efficiency is 7% or more. The manufacturing method of the photoelectric element provided with the following structures.
(1) A lower electrode forming step of forming a lower electrode on the substrate.
(2) On the lower electrode,
(A) a laminated film in which a plurality of thin films containing Cu, Zn or Sn are laminated,
(B) without forming a Cu—Zn phase or a Cu—Zn—S phase during the formation of the laminated film and during the sulfiding process; and
(C) A precursor forming step of forming a precursor having a film thickness of 200 to 700 nm.
(3) Sulfurization step: sulfiding the precursor under the conditions of sulfidation temperature: 560 to 600 ° C., heating rate: 2.5 to 7.5 ° C./min, sulfidation time: 5 to 50 minutes.   5. The method of manufacturing a photoelectric element according to claim 4, wherein the precursor is formed by laminating the thin films so that a thin film containing Cu and a thin film containing Zn are not adjacent to each other. The method for producing a photoelectric device according to claim 4, wherein the substrate is a low alkali glass substrate having a total content of Na ₂ O and K ₂ O of 3 wt% or more and 14 wt% or less.

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