WO2016013670A1 - Compound thin film solar cell, method for manufacturing compound thin film solar cell, and light absorption layer - Google Patents

Abstract

This light absorption layer of a compound thin film solar cell contains a polycrystalline body that contains S and/or Se and three elements, namely Cu, Zn and Sn. This polycrystalline body comprises a crystal grain boundary part that has a composition satisfying Cu/(Zn + Sn) ≤ 1.11.

Description

Compound thin film solar cell, method for producing compound thin film solar cell, and light absorption layer

The present invention relates to a compound thin-film solar cell including a light absorption layer that is a CZTS-based compound semiconductor layer, a manufacturing method thereof, and a light absorption layer.

The CZTS-based compound thin film solar cell includes a CZTS-based compound semiconductor layer (Cu ₂ ZnSnS ₄ , Cu ₂ ZnSnSe ₄ , Cu ₂ ZnSn (S, Se) _4, etc.) as a light absorption layer. Such CZTS-based compound thin film solar cells do not use rare metals in the light absorption layer compared to CIGS-based (CuInGaSe ₂ etc.) compound thin-film solar cells that are being industrially produced. Is possible. Therefore, research and development of CZTS-based compound thin film solar cells are being activated as next-generation solar cells.

Improvement of photoelectric conversion efficiency in CZTS compound thin film solar cells is the most important issue. For example, Patent Document 1 discloses that the photoelectric conversion efficiency can be improved by controlling the composition ratio of the entire light absorption layer.

JP 2010-215497 A

However, the mechanism for changing the photoelectric conversion efficiency of the CZTS-based compound thin film solar cell has not been completely elucidated, and it is difficult to say that sufficient knowledge about factors affecting the photoelectric conversion efficiency has been obtained. Therefore, there is still room for improvement in photoelectric conversion efficiency.

An object of the present invention is to provide a compound thin film solar cell, a method for manufacturing a compound thin film solar cell, and a light absorption layer that can increase the photoelectric conversion efficiency.

The compound thin film solar cell that solves the above problem is a compound thin film solar cell including a light absorption layer, and the light absorption layer includes at least one element of S and Se, and Cu, Zn, and Sn. A polycrystalline body including three elements, and the polycrystalline body includes a crystal grain boundary portion having a composition satisfying Cu / (Zn + Sn) ≦ 1.11.

According to the above configuration, it is possible to suppress a leak path from being formed in the light absorption layer due to a decrease in resistance at the crystal grain boundary. As a result, the photoelectric conversion efficiency is increased.
In the compound thin-film solar cell, the inside of the crystal grains included in the polycrystal preferably has a composition satisfying Zn / Sn ≦ 1.

According to the above configuration, ZnS _x Se _1-x (0 ≦ x ≦ 1) that becomes a different phase is difficult to be precipitated in the light absorption layer. As a result, the photoelectric conversion efficiency is increased.
In the compound thin-film solar cell, the inside of the crystal grains included in the polycrystal preferably has a composition satisfying Cu / (Zn + Sn) ≦ 1.

According to the above configuration, it is possible to suppress the carrier concentration from becoming too high inside the crystal grains. As a result, the photoelectric conversion efficiency is increased.
In the compound thin-film solar cell, it is preferable that each of the two or more crystal grains adjacent to each other at the crystal grain boundary portion has a particle diameter of 0.1 μm or more.

The grain boundary part composed of crystal grains having a particle size of 0.1 μm or more is larger than the crystal grain boundary part composed of crystal grains having a particle diameter of less than 0.1 μm, and therefore, the photoelectric conversion efficiency The impact on the Therefore, according to the above configuration, since the composition of the crystal grain boundary portion having a high correlation with the photoelectric conversion efficiency satisfies Cu / (Zn + Sn) ≦ 1.11, the photoelectric conversion efficiency can be accurately increased.

In the compound thin film solar cell, the rate of change of the Zn / Sn ratio in the film thickness direction of the light absorption layer, the maximum value of the Zn / Sn ratio in the film thickness direction of the light absorption layer is Rmax, and the film of the light absorption layer When the minimum value of the Zn / Sn ratio in the thickness direction is Rmin and the rate of change is Rvar, the rate of change is preferably 16% or less when Rvar = (Rmax ÷ Rmin−1) × 100.

According to the above configuration, since the deviation of the Zn / Sn ratio in the light absorption layer can be suppressed, the uneven distribution of Zn in the light absorption layer and the formation of defects in the light absorption layer can be suppressed. As a result, the photoelectric conversion efficiency is increased.

In the compound thin-film solar cell, the composition of the entire light absorption layer preferably satisfies 0.9 ≦ Zn / Sn ≦ 1.2.
According to the above arrangement, since Zn / Sn ratio of the entire light absorbing layer is 0.9 or more, formation of defects due to _{Cu Zn} is suppressed. On the other hand, since the Zn / Sn ratio of the entire light absorption layer is 1.2 or less, uneven distribution of Zn can be suppressed. As a result, the photoelectric conversion efficiency is increased.

The compound thin-film solar cell further includes a front electrode and a back electrode, and the light absorption layer is located between the front electrode and the back electrode and is close to the front electrode in the light absorption layer The Zn / Sn ratio of the part is preferably lower than the Zn / Sn ratio of the part close to the back electrode.

According to the said structure, since the movement of a carrier is suppressed by being unevenly distributed in the part near the surface electrode of a light absorption layer, a photoelectric conversion efficiency is raised further.
A method for manufacturing a compound thin-film solar cell that solves the above-described problem is a precursor of a light absorption layer, and includes at least one element of S and Se, and three elements of Cu, Zn, and Sn. A precursor forming step for forming the precursor, and a crystal grain boundary part having a composition satisfying Cu / (Zn + Sn) ≦ 1.11 by crystallizing the precursor by heat-treating the precursor. A heat treatment step of forming the light absorption layer.

According to the above method, a light absorption layer including a crystal grain boundary part having a composition satisfying Cu / (Zn + Sn) ≦ 1.11 is formed, and a compound thin film solar cell including this light absorption layer is obtained. Therefore, the photoelectric conversion efficiency is increased in the compound thin film solar cell.

In the method for manufacturing a compound thin film solar cell, the precursor forming step includes an ink containing fine particles made of a compound containing at least one element of S and Se and three elements of Cu, Zn, and Sn. It is preferable to form a coating film which is the precursor by coating.

According to the said method, a light absorption layer can be formed easily.
In the method for manufacturing the compound thin-film solar cell, it is preferable that the heat treatment is performed in an atmosphere containing sulfur and selenium.

In the method for manufacturing a compound thin film solar cell, in the heat treatment step, the heat treatment is preferably performed in an atmosphere containing hydrogen and selenium, and the temperature of the heat treatment is preferably 523 ° C. or lower.
In the method for manufacturing a compound thin-film solar cell, in the heat treatment step, heat treatment is performed in an atmosphere containing sulfur and nitrogen, the heat treatment temperature is 580 ° C., and the heat treatment time is preferably 20 minutes or more. .

In the method for manufacturing a compound thin-film solar cell, the heat treatment step includes a first step heat treatment step and a second step heat treatment step. In the first step heat treatment step, the heat treatment is performed in an atmosphere containing sulfur. In the second heat treatment step, the heat treatment is preferably performed in an atmosphere containing selenium.

According to the heat treatment step in each of the above methods, since the Cu / (Zn + Sn) ratio is suppressed from increasing at the crystal grain boundary portion of the light absorption layer, a crystal having a composition satisfying Cu / (Zn + Sn) ≦ 1.11. The light absorption layer including the grain boundary part can be suitably formed.

In the method for manufacturing the compound thin-film solar cell, the rate of change of the Zn / Sn ratio in the film thickness direction of the light absorption layer formed in the heat treatment step is expressed as When the maximum value is Rmax, the minimum value of the Zn / Sn ratio in the film thickness direction of the light absorption layer is Rmin, and the change rate is Rvar, the change rate is expressed as Rvar = (Rmax ÷ Rmin−1) × 100. 16% or less is preferable.

According to the above method, a compound thin film solar cell including a light absorption layer having a Zn / Sn ratio change rate of 16% or less is obtained. Therefore, the photoelectric conversion efficiency is increased in the compound thin film solar cell.

In the method for manufacturing a compound thin film solar cell, the precursor forming step includes an ink containing fine particles made of a compound containing at least one element of S and Se and three elements of Cu, Zn, and Sn. It is preferable to form a coating film which is the precursor by coating.

According to the said method, a light absorption layer can be formed easily.
In the method for manufacturing the compound thin-film solar cell, it is preferable that the heat treatment is performed in an atmosphere containing sulfur and selenium.

In the method for manufacturing a compound thin-film solar cell, the heat treatment step includes a first step heat treatment step and a second step heat treatment step. In the first step heat treatment step, the heat treatment is performed in an atmosphere containing sulfur. In the second heat treatment step, the heat treatment is preferably performed in an atmosphere containing selenium.

Since the change rate of the Zn / Sn ratio can be suppressed according to the heat treatment step in each of the above methods, the light absorption layer having the change rate of 16% or less can be suitably formed.

In the method for manufacturing a compound thin film solar cell, the fine particles are preferably amorphous particles.
According to the above method, the denseness of the formed light absorption layer can be improved.

The light absorption layer that solves the above problem includes a polycrystal including at least one element of S and Se and three elements of Cu, Zn, and Sn, and the polycrystal includes Cu / A crystal grain boundary portion having a composition satisfying (Zn + Sn) ≦ 1.11 is included.

According to the above configuration, it is possible to suppress the formation of a leak path in the light absorption layer due to a decrease in resistance at the crystal grain boundary. As a result, the photoelectric conversion efficiency in the compound thin film solar cell including the light absorption layer is increased.

According to the present invention, photoelectric conversion efficiency can be increased in a compound thin film solar cell.

It is sectional drawing which shows the whole structure of the compound thin film solar cell of 1st and 2nd embodiment. It is a figure which shows the STEM image of the cross section in the compound thin film solar cell of Example 1 with the target location of a composition analysis. It is a figure which shows the STEM image of the cross section in the compound thin film solar cell of Example 2 with the object location of a composition analysis. It is a figure which shows the STEM image of the cross section in the compound thin film solar cell of Example 3 with the object location of a composition analysis. It is a figure which shows the STEM image of the cross section in the compound thin film solar cell of the comparative example 1 with the object location of a composition analysis. It is a figure which shows the SEM image of the cross section in the compound thin film solar cell of Example 4 with a composition analysis area | region. It is a figure which shows the SEM image of the cross section in the compound thin film solar cell of Example 5 with a composition analysis area | region. It is a figure which shows the SEM image of the cross section in the compound thin film solar cell of Example 6 with a composition analysis area | region. It is a figure which shows the SEM image of the cross section in the compound thin film solar cell of the comparative example 2 with the object area | region of a composition analysis. It is a figure which shows the SEM image of the cross section in the compound thin film solar cell of the comparative example 3 with the object area | region of a composition analysis.

(First embodiment)
With reference to FIG. 1, 1st Embodiment of a compound thin film solar cell, its manufacturing method, and a light absorption layer is described.

[Configuration of Compound Thin Film Solar Cell]
As shown in FIG. 1, the compound thin film solar cell includes a substrate 10. On the substrate 10, the back electrode 11, the light absorption layer 12, the buffer layer 13, the semi-insulating layer 14, the window layer 15, and the surface electrode 16 are positioned in this order.

For example, a glass plate, a metal plate, or a resin film such as plastic is used as the substrate 10.
As the material of the back electrode 11, for example, a metal such as Mo, Ni, Cu, Ti, Fe, Al, titania, stainless steel, or the like is used. Alternatively, as the back electrode 11, a carbon-based electrode such as carbon or graphene, or a transparent conductive film such as ITO (Indium Tin Oxide) or ZnO may be used.

The light absorption layer 12 is a p-type compound semiconductor. The light absorption layer 12 is composed of a CZTS-based compound semiconductor, that is, a CZTS semiconductor. The CZTS compound semiconductor comprises at least one element of group VIB (group 16) S and group VIB (group 16) Se, group IB (group 11) Cu, group IIB (group 12) Zn, and , And a compound semiconductor containing three elements of Sn of group IVB (group 14). That is, the CZTS semiconductor is a IB _2- (IIB-IVB) -VIB Group ₄ compound semiconductor.

The light absorption layer 12 is a thin film made of a polycrystal. The crystal grain boundary part in the light absorption layer 12, that is, the crystal grain boundary part included in the polycrystal has a composition satisfying Cu / (Zn + Sn) ≦ 1.11. Note that the crystal grain boundary part is a region including a crystal grain boundary that is an interface between two or more adjacent crystal grains, and is a region included in a range of a radius of 10 nm or less centering on the crystal grain boundary. . For example, when the crystal grain boundary portion is subjected to composition analysis by energy dispersive X-ray spectroscopy (EDS) using a scanning transmission electron microscope (STEM) with respect to the light absorption layer 12 cut into a flaky shape, This is a region to be analyzed by irradiating the grain boundary with an electron beam. The composition analysis is performed, for example, under the following conditions.

Scanning transmission electron microscope: HD-2700 manufactured by Hitachi High-Technologies
Accelerating voltage: 200kV
Beam diameter: about 1nmφ
Elemental analyzer: VOYAGER III M3100 manufactured by NORAN
X-ray detector: Si / Li semiconductor detector Energy resolution: 137 eV (specification value)
X-ray extraction angle: 22 ° (Side take off method)
Solid angle: 0.12 sr
Capture time: 30 seconds A transmission electron microscope (TEM) may be used instead of the scanning transmission electron microscope.

In addition, the crystal grain boundary part whose composition ratio is to be measured is a region that is 70 nm or more away from the surface in contact with the light absorption layer 12 in the buffer layer 13 and 70 nm or more away from the surface in contact with the light absorption layer 12 in the back electrode 11. It is preferable that it is a crystal grain boundary part contained in. When another layer is present between the back electrode 11 and the light absorption layer 12, the crystal grain boundary part whose composition ratio is to be measured is the light in the layer located between the back electrode 11 and the light absorption layer 12. Any crystal grain boundary portion included in a region separated by 70 nm or more from the surface in contact with the absorption layer 12 may be used. For example, when the back electrode 11 is made of Mo, and a layer made of MoSe ₂ is sandwiched between the back electrode 11 and the light absorption layer 12, the crystal grain boundary part that is the measurement target of the composition ratio is Any crystal grain boundary portion included in a region separated by 70 nm or more from the surface in contact with the light absorption layer 12 in the layer made of MoSe ₂ may be used.

Note that the composition of the crystal grain boundary portion satisfies Cu / (Zn + Sn) ≦ 1.11, in other words, in the portion having the highest value of the Cu / (Zn + Sn) ratio in the region including the crystal grain boundary. , Cu / (Zn + Sn) ≦ 1.11 is satisfied.

In the crystal grains subjected to heat treatment for obtaining the crystallinity of CZTS, the concentration of Cu contained in the crystal grains is higher as the surface of the crystal grains, and the Cu of the portion constituting the grain boundary portion in the crystal grains is higher. The concentration is higher than the inside of the crystal grain. When the Cu / (Zn + Sn) ratio exceeds 1.11 in the crystal grain boundary part, the resistance of the crystal grain boundary part decreases, and as a result, a leak path is easily formed in the light absorption layer 12. This leads to a significant decrease in fill factor and a decrease in photoelectric conversion efficiency. On the other hand, if the Cu / (Zn + Sn) ratio is 1.11 or less at the crystal grain boundary part, such a phenomenon can be suppressed, so that the photoelectric conversion efficiency can be improved.

Moreover, it is preferable that the inside of the crystal grain in the light absorption layer 12, that is, the inside of the crystal grain included in the polycrystal has a composition satisfying Zn / Sn ≦ 1. The inside of the crystal grain is a region that is different from the crystal grain boundary portion and does not include the crystal grain boundary. If the Zn / Sn ratio is 1 or less inside the crystal grains, ZnS _x Se _1-x (0 ≦ x ≦ 1) is difficult to precipitate in the light absorption layer 12. When ZnS _x Se _1-x is precipitated, ZnS _x Se _1-x exists as a heterogeneous phase in the polycrystal, resulting in a decrease in photoelectric conversion efficiency. However, if the Zn / Sn ratio is 1 or less in the crystal grains, This phenomenon is suppressed.

In addition, the inside of the crystal grains in the light absorption layer 12 preferably has a composition satisfying Cu / (Zn + Sn) ≦ 1. If the Cu / (Zn + Sn) ratio is 1 or less inside the crystal grains, the carrier concentration inside the crystal grains can be prevented from becoming too high, so that a decrease in photoelectric conversion efficiency can be suppressed.

In the above configuration, the Cu / (Zn + Sn) ratio and the Zn / Sn ratio are both atomic ratios, and the atomic percentage of Cu with respect to the sum of atomic percentage of Zn and atomic percentage of Sn is Cu / (Zn + Sn). ), And the atomic% of Zn with respect to the atomic% of Sn is Zn / Sn.

Here, the crystal grain boundary included in the crystal grain boundary part having a composition satisfying Cu / (Zn + Sn) ≦ 1.11 is an interface between two or more crystal grains having a grain size of 0.1 μm or more. preferable. The particle size indicates the maximum diameter of crystal grains. Since the crystal grain having a grain size of less than 0.1 μm does not progress sufficiently, the analysis of Cu / (Zn + Sn) is performed at the crystal grain boundary portion formed by the crystal grain having a grain size of 0.1 μm or more. The result is highly correlated with the photoelectric conversion efficiency. That is, the photoelectric conversion efficiency is accurately increased in the configuration in which the Cu / (Zn + Sn) ratio is 1.11 or less in the crystal grain boundary portion formed by the crystal grains having a grain size of 0.1 μm or more.

Even if the crystal grain boundary part included in the light absorbing layer 12 includes a crystal grain boundary part with a Cu / (Zn + Sn) ratio exceeding 1.11, the Cu / (Zn + Sn) ratio is 1.11. Since the presence of the following crystal grain boundary part prevents the leakage path from being connected, the photoelectric conversion efficiency is improved as compared with the configuration in which the Cu / (Zn + Sn) ratio exceeds 1.11 in all the crystal grain boundary parts. Is possible.

In the light absorption layer 12, the Zn / Sn ratio in the portion close to the front electrode 16 is preferably lower than the Zn / Sn ratio in the portion close to the back electrode 11. According to such a configuration, the photoelectric conversion efficiency is further increased. The thickness of the light absorption layer 12 is preferably 0.3 μm or more and 10 μm or less, and preferably 2 μm, for example.

The buffer layer 13 is an n-type compound semiconductor. As the buffer layer 13, for example, CdS, Zn (S, O, OH), ZnS, ZnSe, In ₂ S ₃ or the like is used. The semi-insulating layer 14 is an i-type compound semiconductor. As the semi-insulating layer 14, for example, a metal oxide such as ZnO is used. The window layer 15 is an n-type compound semiconductor. As the window layer 15, for example, ZnO or ITO to which Al, Ga, B or the like is added is used.

The surface electrode 16 is laminated on a part of the upper surface of the window layer 15. As a material of the surface electrode 16, for example, a metal such as Al or Ag is used. Alternatively, as the surface electrode 16, a carbon-based electrode such as carbon or graphene, or a transparent conductive film such as ITO or ZnO may be used.

Note that at least one of the buffer layer 13, the semi-insulating layer 14, and the window layer 15 may be omitted. Moreover, the compound thin film solar cell may have layers other than said each layer. For example, an antireflection film may be laminated on the window layer 15. Since the antireflection film has a function of suppressing light reflection, the light absorption layer 12 can absorb more light by providing the antireflection film. The antireflection film is made of MgF ₂ to a thickness of about 100 nm, for example. Further, for example, an intermediate layer having a composition for increasing the photoelectric conversion efficiency may be provided between the back electrode 11 and the light absorption layer 12.

[Method for producing compound thin-film solar cell]
As an example of a method for producing a compound thin film solar cell, a method for producing a light absorption layer by applying ink containing fine particles will be described.

The compound thin film solar cell is formed by laminating a back electrode 11, a light absorption layer 12, a buffer layer 13, a semi-insulating layer 14, a window layer 15, and a surface electrode 16 in this order on a substrate 10.

The back electrode 11 is formed on the upper surface of the substrate 10 by using, for example, sputtering, vapor deposition, CVD (Chemical Vapor Deposition), or the like.
The light absorption layer 12 is formed using a light absorption layer forming ink containing fine particles composed of the constituent elements of the light absorption layer 12.

The fine particles are nano-sized particles. The fine particles are generated by a reaction between a solution containing a metal salt or a metal complex and a solution containing a chalcogenide salt. The fine particles are, for example, particles made of a compound represented by a composition formula of Cu _2-x Zn _{1 + y} SnS _z Se _4-Z (0 ≦ x ≦ 1, 0 ≦ y ≦ 1, 0 ≦ z ≦ 4). In addition, the ink for forming the light absorption layer is formed of Cu _2-x S _y Se _2-y (0 ≦ x ≦ 1, 0 ≦ y ≦ 2), Zn _2-x as fine particles in addition to the particles composed of the above compound. S _y Se _2-y (0 ≦ x ≦ 1, 0 ≦ y ≦ 2), Sn _2-x S _y Se _2-y (0 ≦ x ≦ 1, 0 ≦ y ≦ 2) Particles composed of at least one compound selected from the group consisting of compounds may be included. Alternatively, the light absorption layer forming ink is represented by a composition formula of Cu _2−x Zn _{1 + y} SnS _z Se _4−Z (0 ≦ x ≦ 1, 0 ≦ y ≦ 1, 0 ≦ z ≦ 4) as fine particles. Instead of the particles made of the compound, the particles made of three kinds of compounds included in the group may be included.

As the fine particles, it is preferable to use amorphous particles. Since crystalline particles are in a stable state with respect to energy, each atom contained in the particles is likely to be fixed at a certain position, that is, difficult to diffuse. Easy to diffuse. Therefore, the use of amorphous particles improves the density of the light absorption layer 12.

The solvent for dispersing the fine particles in the light absorbing layer forming ink is not particularly limited as long as it is an organic solvent. The organic solvent can be selected from, for example, alcohols, ethers, esters, aliphatic hydrocarbons, alicyclic hydrocarbons, or aromatic hydrocarbons. As the organic solvent, alcohols having less than 10 carbon atoms such as methanol, ethanol, butanol, pyridine, diethyl ether, pentane, hexane, cyclohexane, and toluene are preferable, and methanol, pyridine, toluene, and hexane are particularly preferable.

The light absorbing layer forming ink preferably contains a binder in order to improve leveling properties during coating. The content of the binder is preferably 5% by mass or more and 70% by mass or less, and more preferably 20% by mass or more and 60% by mass or less of the fine particles contained in the light absorbing layer forming ink. When the binder content is 5% by mass or more, formation of cavities in the coating film after heat treatment is suppressed. When the binder content is 70% by mass or less, an increase in the surface roughness of the coating film after the heat treatment is suppressed.

As the binder, a thiol organic substance, a selenol organic substance, or an alcohol having 10 or more carbon atoms can be used. Further, Se particles, S particles, Se compounds, S compounds, or the like may be used as the binder. Furthermore, sodium sulfide, sodium selenide, potassium selenide, sodium selenate, thiosulfate, or the like may be used as the binder. Among these substances, it is preferable to use a thiol organic substance as the binder, and it is more preferable to use thiourea. When an organic compound is used as the binder, the light absorption layer 12 formed from the light absorption layer forming ink has a trace amount of about 0.3 atomic% to 0.5 atomic% with respect to the entire light absorption layer 12. Of carbon remains.

In the formation process of the light absorption layer 12, first, a precursor formation process of the light absorption layer 12 is performed. In the precursor forming step, the light absorbing layer forming ink is applied to the upper surface of the back electrode 11 to form a film containing fine particles. And the solvent contained in a film | membrane is removed by drying the coated film | membrane, and the coating film which is a precursor of the light absorption layer 12 is formed. Next, a heat treatment step is performed, and the coating film is heat treated in the heat treatment step, whereby sintering and crystallization of the fine particles proceed to form the light absorption layer 12.

Examples of the coating method of the light absorbing layer forming ink include a doctor blade method, a spin coating method, a slit coating method, a coating method such as a spray method, a gravure printing method, a screen printing method, a reverse offset printing method, Or printing methods, such as a relief printing method, are mentioned.

The heat treatment is performed, for example, by annealing using a heating furnace or rapid thermal annealing (RTA). The atmosphere of the heat treatment is at least selected from the group consisting of H ₂ S gas, H ₂ Se gas, nitrogen gas, Ar gas, Se vapor, S vapor, hydrogen gas, and a mixed gas of hydrogen and an inert gas. Preferably one is included. The heat treatment temperature is preferably 250 ° C. or higher. When glass is used as the substrate 10, the temperature of the heat treatment is a temperature that the glass can withstand, specifically, 650 ° C. or less is preferable, and 600 ° C. or less is more preferable.

In order to obtain the light absorption layer 12 in which the composition of the crystal grain boundary portion satisfies Cu / (Zn + Sn) ≦ 1.11 in the heat treatment step performed under such conditions, in particular, any one of the following conditions 1 to 4 It is preferred that one is satisfied.

Condition 1: The atmosphere of heat treatment contains sulfur and selenium.
Condition 2: The atmosphere of the heat treatment includes hydrogen and selenium, and the temperature of the heat treatment is 523 ° C. or lower.

Condition 3: The atmosphere of the heat treatment includes sulfur and nitrogen, the temperature of the heat treatment is 580 ° C., and the time of the heat treatment is 20 minutes or more.
Condition 4: The heat treatment step includes a first-stage heat treatment step performed in an atmosphere containing sulfur and a second-stage heat treatment step performed in an atmosphere containing selenium.

In an example of a conventional heat treatment process, heat treatment is performed in an atmosphere in which only selenium is added to nitrogen gas. Under such conventional conditions, the present inventor tends to precipitate CuSe at the crystal grain boundary of the light absorption layer 12, and as a result, the Cu / (Zn + Sn) ratio at the crystal grain boundary part becomes too high to form a leak path. It has been found that this phenomenon is easily performed, and that this phenomenon leads to a decrease in fill factor and photoelectric conversion efficiency.

On the other hand, in condition 1, by adding sulfur to the heat treatment atmosphere in addition to selenium, the Cu / (Zn + Sn) ratio is suppressed from becoming too high at the crystal grain boundary, and the photoelectric conversion efficiency is increased. It is done.

In addition, when heat treatment is performed in an atmosphere containing only selenium, the Cu / (Zn + Sn) ratio at the crystal grain boundary can be lowered by lowering the heat treatment temperature, but if the heat treatment temperature is low Crystal growth is difficult to proceed.

On the other hand, under condition 2, the temperature of the heat treatment is set to 523 ° C. or less at which the precipitation of CuSe is suppressed, and about 3% of hydrogen gas is added to the atmosphere of the heat treatment. Thereby, the progress of crystal growth can be promoted while suppressing the Cu / (Zn + Sn) ratio from becoming too high at the crystal grain boundary.

In addition, when heat treatment is performed in an atmosphere in which only sulfur is added to nitrogen gas, precipitation of CuSe at the grain boundary is suppressed, and the Cu / (Zn + Sn) ratio at the crystal grain boundary is prevented from becoming too high. However, crystal growth is difficult to proceed under these conditions.

On the other hand, under the condition 3, the progress of crystal growth is promoted by performing the heat treatment at 580 ° C. for 20 minutes or more. As a result, the progress of crystal growth can be promoted while suppressing the Cu / (Zn + Sn) ratio from becoming too high at the crystal grain boundary.

In the condition 1, the atmosphere of the heat treatment contains selenium and sulfur at the same time. However, as in the condition 4, the heat treatment is performed in an atmosphere containing sulfur by the first heat treatment step, Even if the heat treatment is performed in an atmosphere containing selenium by the two-stage heat treatment process, it is possible to prevent the Cu / (Zn + Sn) value from becoming too high at the crystal grain boundary part.

The light absorption layer 12 formed from the light absorption layer forming ink is preferably subjected to a surface cleaning treatment with an acid such as hydrochloric acid or pure water. As a result, Zn atoms in the vicinity of the upper surface of the light absorption layer 12 are dissolved, so that the Zn / Sn ratio in the portion near the front electrode 16 in the light absorption layer 12 is higher than the Zn / Sn ratio in the portion near the back electrode 11. Can also be actively lowered. In addition, as an acid, a well-known inorganic acid and organic acid can be used.

The buffer layer 13 is formed on the upper surface of the light absorption layer 12 using, for example, a CBD (Chemical Bath Deposition) method, a MOCVD (Metal Organic Chemical Vapor Deposition) method, or an ALD (Atomic Layer Deposition) method.

The semi-insulating layer 14 is formed on the upper surface of the buffer layer 13 by using, for example, MOCVD or sputtering. The window layer 15 is formed on the upper surface of the semi-insulating layer 14 by using, for example, MOCVD or sputtering.

The surface electrode 16 is formed on a part of the upper surface of the window layer 15 by using, for example, sputtering, vapor deposition, CVD, or the like.
In addition, the light absorption layer 12 can also be formed by heat-processing with respect to the film | membrane which is the precursor formed using sputtering. However, when the light absorption layer is formed from a film formed by sputtering and when the light absorption layer is formed from a coating film containing fine particles as in this embodiment, the crystallization of the film during heat treatment The process is different. That is, when sputtering is used, first, as a precursor, a layer for each metal constituting the light absorption layer or one layer containing these metals is formed by sputtering. In the heat treatment step, after these metals form an alloy of these metals, they react with elements contained in the atmosphere of the heat treatment, and crystallization proceeds. On the other hand, when a light absorption layer is formed from a coating film containing fine particles, crystallization proceeds with little formation of such an alloy.

Due to the difference in the progress of crystallization, the distribution of each composition component in the light absorption layer between the light absorption layer formed by sputtering and the light absorption layer formed from a coating film containing fine particles. The tendency is different. Therefore, the light absorption layer having a crystal grain boundary part having a composition satisfying Cu / (Zn + Sn) ≦ 1.11 is particularly high in photoelectric conversion efficiency when the light absorption layer is formed from a coating film containing fine particles. Bring.

Specifically, when a light absorption layer is formed from a coating film containing fine particles, diffusion of atoms contained in the fine particles occurs for each fine particle, so that the concentration of Cu increases as the surface of the crystal grains increases, Cu is easily segregated. On the other hand, when the layer for each metal constituting the light absorption layer is formed by sputtering, Cu segregation at the crystal grain boundary portion hardly occurs. Since the segregation of Cu is a factor in reducing the photoelectric conversion efficiency as described above, paying attention to the Cu / (Zn + Sn) ratio of the crystal grain boundary portion in the light absorption layer formed from the coating film containing fine particles, Suppressing the Cu / (Zn + Sn) ratio in the crystal grain boundary part greatly contributes to improvement in photoelectric conversion efficiency.

However, even when the light absorption layer is formed from a film formed by sputtering, the polycrystalline body constituting the light absorption layer is the same as when the light absorption layer is formed from a coating film containing fine particles. And having a structure including a crystal grain boundary part. As long as the light absorption layer has a structure including a crystal grain boundary part, the photoelectric conversion efficiency can be improved if the light absorption layer has a crystal grain boundary part having a composition satisfying Cu / (Zn + Sn) ≦ 1.11. is there.

In short, the photoelectric conversion efficiency can be improved if the light absorption layer 12 includes a crystal grain boundary portion having a composition satisfying Cu / (Zn + Sn) ≦ 1.11 regardless of the manufacturing method. Then, such a compound thin film solar cell has a precursor forming step of forming a precursor of the light absorption layer 12 and crystallizing the precursor by heat-treating the precursor, so that Cu / (Zn + Sn) ≦ 1.11. And a heat treatment step for forming the light absorption layer 12 including the crystal grain boundary portion having the composition to be satisfied.

(Example)
The compound thin film solar cell, the manufacturing method thereof, and the light absorption layer of the first embodiment will be described using specific examples and comparative examples.

[Example 1]
(Adjustment of light absorbing layer forming ink)
CuI, ZnI ₂ , and SnI ₄ were dissolved in pyridine such that the molar ratio of Cu: Zn: Sn was 1.8: 1.1: 1 to prepare a first solution. In addition, a _second solution was prepared by dissolving Na ₂ Se in methanol.

The first solution and the second solution were mixed to obtain a mixed solution having a Cu: Zn: Sn: Se molar ratio of 1.8: 1.1: 1: 4. This mixed solution was reacted at 0 ° C. in an inert gas atmosphere to produce Cu—Zn—Sn—Se fine particles. The solution after the reaction is filtered and the residue is washed with methanol, and then the washed Cu—Zn—Sn—Se fine particles and thiourea are mixed so that the mass ratio of the fine particles to thiourea is 3: 2. Then, pyridine and methanol were further added to the mixture to prepare a light absorption layer forming ink containing Cu—Zn—Sn—Se fine particles. The solid content contained in the light absorbing layer forming ink is 2% by mass.

(Formation of back electrode)
Using soda lime glass as a substrate, a back electrode composed of Mo was formed by sputtering.

(Formation of light absorption layer)
A light absorbing layer forming ink was applied to the upper surface of the back electrode by a spray method, and the solvent was evaporated in an oven at 250 ° C., followed by a heat treatment step. In the heat treatment step, a coating film, which is a precursor formed on the upper surface of the back electrode, is placed in a quartz case containing 48 mg of sulfur and 10 mg of selenium, and is placed at 580 ° C. for 20 minutes in an atmosphere introduced with nitrogen gas. The heat treatment was performed. Thereby, the light absorption layer comprised from the CZTS thin film with a film thickness of about 2 micrometers was obtained.

(Formation of buffer layer)
A mixture containing 0.0015 M cadmium sulfate (CdSO ₄ ), 0.0075 M thiourea (NH ₂ CSNH ₂ ), and 1.5 M aqueous ammonia (NH ₄ OH) was heated to 67 ° C. A buffer layer made of CdS having a film thickness of 100 nm was formed on the light absorption layer by immersing the structure in which the above light absorption layer was formed in this mixed solution.

(Formation of semi-insulating layer)
A semi-insulating layer made of ZnO having a thickness of 50 nm was formed on the buffer layer by MOCVD using diethyl zinc and water as raw materials.

(Formation of window layer)
A window layer made of ZnO: B having a thickness of 1 μm was formed on the semi-insulating layer by MOCVD using diethyl zinc, water, and diborane as raw materials.

(Formation of surface electrode)
A surface electrode made of Al having a film thickness of 0.3 μm was formed on the window layer by vapor deposition. Thereby, the compound thin film solar cell of Example 1 was obtained.

[Example 2]
A compound thin-film solar cell of Example 2 was obtained by the same process as Example 1 except that the conditions in the heat treatment step when forming the light absorption layer were changed as follows. In the heat treatment step in Example 2, a coating film, which is a precursor formed on the upper surface of the back electrode, is placed in a quartz case containing 40 mg of selenium, and an atmosphere in which about 3% of hydrogen gas is added to nitrogen gas is used. A heat treatment was performed at 500 ° C. for 30 minutes.

[Example 3]
A compound thin-film solar cell of Example 3 was obtained by the same process as Example 1 except that the conditions in the heat treatment step when forming the light absorption layer were changed as follows. In the heat treatment step in Example 3, the coating film, which is a precursor formed on the upper surface of the back electrode, is put in a quartz case containing 48 mg of sulfur, and at 580 ° C. for 20 minutes in an atmosphere introduced with nitrogen gas. The heat treatment was performed.

[Comparative Example 1]
A compound thin-film solar cell of Comparative Example 1 was obtained by the same process as Example 1 except that the conditions in the heat treatment step when forming the light absorption layer were changed as follows. In the heat treatment step in the comparative example, the coating film, which is a precursor formed on the upper surface of the back electrode, is put in a quartz case containing 40 mg of selenium, and the atmosphere is introduced at 580 ° C. for 20 minutes in an atmosphere introduced with nitrogen gas. Heat treatment was performed.

[Evaluation methods]
(Photoelectric conversion efficiency)
About the compound thin film solar cell of each Example and the comparative example, photoelectric conversion efficiency was evaluated using the standard sunlight simulator (light intensity: 100 mW / cm < ² >, air mass: 1.5).

(Composition ratio)
For each of the examples and comparative examples, the light absorbing layer was cut into a thin piece by the FIB method (μ-sampling method), and then energy dispersive X-ray spectroscopy using a scanning transmission electron microscope (Hitachi High-Technologies HD-2700). Composition analysis was performed by the method (EDS). Detailed conditions of the composition analysis are shown below.

Accelerating voltage: 200kV
Beam diameter: about 1nmφ
Elemental analyzer: VOYAGER III M3100 manufactured by NORAN
X-ray detector: Si / Li semiconductor detector Energy resolution: 137 eV (specification value)
X-ray extraction angle: 22 ° (Side take off method)
Solid angle: 0.12 sr
Acquisition time: 30 seconds [Result]
FIG. 2 shows a cross-sectional STEM image of the compound thin film solar cell of Example 1. P1 in FIG. 2 shows the measurement location of the composition of a crystal grain boundary part, and P2 shows the measurement location of the composition inside a crystal grain. Similarly, FIG. 3 shows a STEM image of a cross section of the compound thin film solar cell of Example 2, FIG. 4 Example 3 and FIG. P3 in FIG. 3, P5 in FIG. 4, and P7 in FIG. 5 indicate the measurement points of the composition of the crystal grain boundaries. P4 in FIG. 3, P6 in FIG. 4, and P8 in FIG. Indicates the measurement location. 2, 4, and 5 are bright field STEM images, and FIG. 3 is a dark field STEM image.

Table 1 shows the calculation result of the composition ratio of the crystal grain boundary and the crystal grain and the calculation result of the photoelectric conversion efficiency for each example and comparative example.

As shown in Table 1, in the grain boundary part, in comparison example 1 in which the Cu / (Zn + Sn) ratio exceeds 1.11, the Cu / (Zn + Sn) ratio is 1.11 or less. 3 showed that high photoelectric conversion efficiency was obtained. In addition, in the grain boundary part included in Comparative Example 1, the Cu / (Zn + Sn) ratio exceeded 1.11 in other parts other than P7 as well as P7.

Thus, the present inventor has obtained the knowledge that the concentration of Cu tends to be high at the crystal grain boundaries in the light absorption layer, and this is one of the factors that lower the photoelectric conversion efficiency. Focusing on the Cu / (Zn + Sn) ratio in the crystal grain boundary, the inventors have found a configuration that can increase the photoelectric conversion efficiency.

As described above using the examples, according to the first embodiment, the effects listed below can be obtained.
(1) Since the light absorption layer 12 after the heat treatment includes a crystal grain boundary part having a composition satisfying Cu / (Zn + Sn) ≦ 1.11, in the light absorption layer due to a decrease in resistance at the crystal grain boundary part. The formation of a leak path is suppressed. As a result, the photoelectric conversion efficiency is increased.

(2) Since the inside of the crystal grains included in the light absorption layer 12 after the heat treatment has a composition satisfying Zn / Sn ≦ 1, ZnS _x Se _1-x (0 ≦ x) that becomes a different phase in the light absorption layer 12 ≦ 1) is difficult to precipitate. As a result, the photoelectric conversion efficiency is increased.

(3) Since the inside of the crystal grains included in the light absorption layer 12 after the heat treatment has a composition satisfying Cu / (Zn + Sn) ≦ 1, it is possible to suppress the carrier concentration from becoming too high inside the crystal grains. As a result, the photoelectric conversion efficiency is increased.

(4) When the grain size of the crystal grain constituting the crystal grain boundary part is 0.1 μm or more, the influence of the composition of the crystal grain boundary part on the photoelectric conversion efficiency is great. Since the composition of the crystal grain boundary portion satisfies Cu / (Zn + Sn) ≦ 1.11, the photoelectric conversion efficiency can be accurately increased.

(5) By a manufacturing method including a precursor forming step of forming a precursor of the light absorbing layer 12 and a heat treatment step of crystallizing the precursor by heat treating the precursor to form the light absorbing layer 12, A compound thin film solar cell including the light absorption layer 12 including the crystal grain boundary part having the above composition can be formed. In particular, when a coating film that is a precursor is formed by applying ink containing fine particles, the light absorption layer 12 can be easily formed as compared with the case where the precursor is formed using sputtering or the like. In addition, the effect of increasing the photoelectric conversion efficiency is high.

(6) In the heat treatment step, the Cu / (Zn + Sn) ratio is increased at the crystal grain boundary portion of the light absorption layer 12 by performing the heat treatment under the condition according to any one of the above conditions 1 to 4. Can be suppressed. As a result, the light absorption layer 12 including the crystal grain boundary part having the above composition can be suitably formed.

(7) Since the fine particles contained in the light absorbing layer forming ink used in the precursor forming step are amorphous particles, the denseness of the formed light absorbing layer 12 can be improved.
(Second Embodiment)
A compound thin-film solar cell, a manufacturing method thereof, and a second embodiment of the light absorption layer will be described. In addition, in the compound thin film solar cell of 2nd Embodiment, the structures other than the light absorption layer 12 are the same as the structure of 1st Embodiment. Below, it demonstrates centering around the structure of the light absorption layer 12, and its manufacturing method, and abbreviate | omits description about the structure similar to 1st Embodiment.

[Configuration of Compound Thin Film Solar Cell]
Similar to the first embodiment, the light absorption layer 12 of the compound thin film solar cell of the second embodiment is a thin film made of a polycrystalline body and is made of a CZTS semiconductor. Similar to the first embodiment, the crystal grain boundary portion in the light absorption layer 12 has a composition satisfying Cu / (Zn + Sn) ≦ 1.11, and the light absorption layer 12 has a composition ratio in the film thickness direction. Features include the following features.

The rate of change Rver of the Zn / Sn ratio in the film thickness direction of the light absorption layer 12, the maximum value of the Zn / Sn ratio in the film thickness direction of the light absorption layer 12 is Rmax, and the Zn / Sn in the film thickness direction of the light absorption layer 12. When the minimum value of the ratio is represented by Rmin and expressed by the following formula (1), the rate of change Rver is 16% or less. The Zn / Sn ratio is an atomic ratio, and the atomic% of Zn with respect to the atomic% of Sn is Zn / Sn.

Rver = (Rmax ÷ Rmin−1) × 100 (1)
The maximum value Rmax and the minimum value Rmin are obtained by performing composition analysis on a plurality of regions having different positions in the film thickness direction in the light absorption layer 12. As a result of composition analysis, the maximum value of the obtained Zn / Sn ratios in each region is the maximum value Rmax, and the minimum value is the minimum value Rmin.

The composition analysis is performed on the polished cross section of the light absorption layer 12 by energy dispersive X-ray spectroscopy (EDS) using a scanning electron microscope (SEM).
The composition analysis region, which is a region to be subjected to composition analysis, is all or part of each region obtained by equally dividing the cross section of the light absorption layer 12 in the film thickness direction. For example, the cross section of the light absorption layer 12 is divided into three, and each of these three regions is a composition analysis region, or a part of each of these three regions is a composition analysis region, and the three regions One composition analysis region is set for each.

Specifically, the composition analysis region may be a rectangular region having a size of 100 nm or more in the film thickness direction when viewed from the direction facing the cross section of the light absorption layer 12. The composition analysis region includes a plurality of crystal grains and a plurality of crystal grain boundaries.

When the change rate Rver is 16% or less, the deviation of the Zn / Sn ratio in the light absorption layer 12 is suppressed, so that ZnS or the like segregates in the light absorption layer 12 to form a heterogeneous phase, or light absorption. The formation of defects in the layer 12 is suppressed. As a result, the photoelectric conversion efficiency is increased.

The direction of change of the Zn / Sn ratio in the film thickness direction of the light absorption layer 12 is not particularly limited, and the Zn / Sn ratio may decrease from a portion close to the back electrode 11 toward a portion close to the front electrode 16. And it may become large toward the part near the surface electrode 16 from the part close to the back surface electrode 11, and may become the maximum or the minimum at the center part in the film thickness direction.

However, when ZnS or the like segregates in the portion of the light absorption layer 12 that is particularly close to the surface electrode 16, the movement of carriers is likely to be limited due to the difference in the band gap. Therefore, it is preferable that the Zn / Sn ratio in the part close to the front electrode 16 is lower than the Zn / Sn ratio in the part close to the back electrode 11, and the Zn / Sn ratio is changed from the part close to the back electrode 11 to the front electrode 16. It is more preferable that it becomes smaller toward the near part.

Moreover, it is preferable that the composition of the light absorption layer 12 as a whole satisfies 0.9 ≦ Zn / Sn ≦ 1.2. When the Zn / Sn ratio is 0.9 or more in the entire light absorption layer 12, the formation of defects due to Cu _Zn is suppressed. On the other hand, when the Zn / Sn ratio is 1.2 or less in the entire light absorption layer 12, the uneven distribution of Zn is suppressed. As a result, the photoelectric conversion efficiency is increased. The Zn / Sn ratio in the entire light absorption layer 12 is calculated from data obtained by mapping by EDS on the entire cross-section of the polished light absorption layer.

Furthermore, when the composition of each composition analysis region satisfies 0.9 ≦ Zn / Sn ≦ 1.2, the above effect is enhanced.
Note that when the rate of change Rver is 16% or less, it is difficult to completely prevent the segregation of Zn even though the segregation of Zn as ZnS or the like in the light absorption layer 12 is suppressed. Segregates between crystal grains in the light absorption layer 12. Since the composition analysis region includes a plurality of crystal grains and a region between them, the Zn / Sn ratio of the composition analysis region and the Zn / Sn ratio of the entire light absorption layer 12 are the Zn / Sn ratio inside the crystal grains. Can be larger. That is, even if the Zn / Sn ratio is 1 or less inside the crystal grains, the Zn / Sn ratio in the composition analysis region and the Zn / Sn ratio in the entire light absorption layer 12 may be larger than 1, If the composition of the entire absorption layer 12 satisfies 0.9 ≦ Zn / Sn ≦ 1.2, the above-described effect can be obtained.

Further, the thickness of the light absorption layer 12 is preferably 0.3 μm or more and 10 μm or less, and for example, preferably 2 μm.
[Method for producing compound thin-film solar cell]
The heat treatment step in the method for producing the compound thin film solar cell of the second embodiment will be described. The manufacturing method other than the heat treatment step is the same as the manufacturing method of the first embodiment.

In order to obtain the light absorption layer 12 with a change rate Rver of Zn / Sn ratio of 16% or less, it is particularly preferable that either of the following conditions 5 or 6 is satisfied in the heat treatment step.

Condition 5: The atmosphere of heat treatment contains sulfur and selenium.
Condition 6: The heat treatment step includes a first-stage heat treatment step performed in an atmosphere containing sulfur and a second-stage heat treatment step performed in an atmosphere containing selenium.

In the case where the heat treatment is performed in an atmosphere in which only sulfur is added to nitrogen gas, the present inventor has found that the Cu / (Zn + Sn) ratio at the crystal grain boundary portion becomes too high as described in the first embodiment. Although it can be suppressed, in particular, Zn is unevenly distributed near the surface electrode 16 in the light absorption layer 12 and the change rate Rver of the Zn / Sn ratio is increased. As a result, the short-circuit current is reduced and the photoelectric conversion efficiency is reduced. Found that the decline.

On the other hand, under condition 5, by adding selenium to the atmosphere of the heat treatment in addition to sulfur, the change rate Rver of the Zn / Sn ratio is suppressed, and the photoelectric conversion efficiency is increased.

In addition, when the back surface electrode 11 contains Mo, it is preferable that Se / S ratio (molar ratio) in the atmosphere of heat processing is 4 or less. If the ratio of selenium to sulfur is 4 or less, selenium in the heat treatment atmosphere reacts with Mo of the back electrode 11 to suppress the formation of a high-resistance MoSe ₂ layer. As a result, the photoelectric conversion efficiency is increased.

Further, in condition 5 above, sulfur and selenium are simultaneously contained in the heat treatment atmosphere, but as in condition 6, the heat treatment is performed in an atmosphere containing sulfur by the first stage heat treatment process, The heat treatment may be performed in an atmosphere containing selenium by a two-step heat treatment step. In this case, even if the uneven distribution of Zn occurs in the first stage heat treatment process, the unevenly distributed Zn is rediffused in the light absorption layer 12 in the second stage heat treatment process. An increase in the change rate Rver of the / Sn ratio can be suppressed.

Condition 5 corresponds to condition 1 of the first embodiment, and condition 6 corresponds to condition 4 of the first embodiment. That is, when any one of the conditions 5 and 6 is satisfied, the composition of the crystal grain boundary portion satisfies Cu / (Zn + Sn) ≦ 1.11, and the change rate Rver of the Zn / Sn ratio is 16% or less. A certain light absorption layer 12 can be obtained.

Note that the light absorbing layer 12 may be subjected to a surface cleaning treatment with an acid such as hydrochloric acid or pure water. As a result, Zn atoms in the vicinity of the upper surface of the light absorption layer 12 are dissolved, so that the Zn / Sn ratio in the portion near the front electrode 16 in the light absorption layer 12 is higher than the Zn / Sn ratio in the portion near the back electrode 11. Can also be actively lowered. In addition, as an acid, a well-known inorganic acid and organic acid can be used.

As described in the first embodiment, in the light absorption layer formed using sputtering and the light absorption layer formed from a coating film containing fine particles, the distribution of each composition component in the light absorption layer The trend is different. Therefore, the change rate Rver of the Zn / Sn ratio being 16% or less brings particularly high photoelectric conversion efficiency when the light absorption layer is formed from the coating film containing fine particles.

However, irrespective of the manufacturing method, if the change rate Rver of the Zn / Sn ratio in the film thickness direction of the light absorption layer 12 is 16% or less, the photoelectric conversion efficiency can be improved. Such a compound thin-film solar cell includes a precursor forming step for forming a precursor of the light absorption layer 12, and a heat treatment step for forming the light absorption layer 12 satisfying the change rate Rver by heat-treating the precursor. May be manufactured by a manufacturing method including:

(Example)
The compound thin film solar cell, the manufacturing method thereof, and the light absorption layer of the second embodiment will be described using specific examples and comparative examples.

[Example 4]
(Adjustment of light absorbing layer forming ink)
CuI, ZnI ₂ , and SnI ₄ were dissolved in pyridine such that the molar ratio of Cu: Zn: Sn was 1.8: 1.1: 1 to prepare a first solution. In addition, a _second solution was prepared by dissolving Na ₂ Se in methanol.

The first solution and the second solution were mixed to obtain a mixed solution having a Cu: Zn: Sn: Se molar ratio of 1.8: 1.1: 1: 4. This mixed solution was reacted at 0 ° C. in an inert gas atmosphere to produce Cu—Zn—Sn—Se fine particles. The solution after the reaction is filtered and the residue is washed with methanol, and then the washed Cu—Zn—Sn—Se fine particles and thiourea are mixed so that the mass ratio of the fine particles to thiourea is 3: 2. Then, pyridine and methanol were further added to the mixture to prepare a light absorption layer forming ink containing Cu—Zn—Sn—Se fine particles. The solid content contained in the light absorbing layer forming ink is 2% by mass.

(Formation of back electrode)
Using soda lime glass as a substrate, a back electrode composed of Mo was formed by sputtering.

(Formation of light absorption layer)
As the precursor forming step, a light absorbing layer forming ink was applied to the upper surface of the back electrode by a spray method, and the solvent was evaporated in an oven at 250 ° C., followed by a heat treatment step. In the heat treatment step, a coating film, which is a precursor formed on the upper surface of the back electrode, is placed in a quartz case containing 48 mg of sulfur and 10 mg of selenium, and is placed at 580 ° C. for 20 minutes in an atmosphere introduced with nitrogen gas. The heat treatment was performed. Thereby, the light absorption layer comprised from the CZTS thin film with a film thickness of about 2 micrometers was obtained.

(Formation of buffer layer)
A mixture containing 0.0015 M cadmium sulfate (CdSO ₄ ), 0.0075 M thiourea (NH ₂ CSNH ₂ ), and 1.5 M aqueous ammonia (NH ₄ OH) was heated to 67 ° C. A buffer layer made of CdS having a film thickness of 100 nm was formed on the light absorption layer by immersing the structure in which the above light absorption layer was formed in this mixed solution.

(Formation of semi-insulating layer)
A semi-insulating layer made of ZnO having a thickness of 50 nm was formed on the buffer layer by MOCVD using diethyl zinc and water as raw materials.

(Formation of window layer)
A window layer made of ZnO: B having a thickness of 1 μm was formed on the semi-insulating layer by MOCVD using diethyl zinc, water, and diborane as raw materials.

(Formation of surface electrode)
A surface electrode made of Al having a film thickness of 0.3 μm was formed on the window layer by vapor deposition. Thereby, the compound thin film solar cell of Example 4 was obtained.

[Example 5]
A compound thin-film solar cell of Example 5 was obtained by the same process as Example 4 except that the conditions in the heat treatment step when forming the light absorption layer were changed as follows. In the heat treatment step in Example 5, as a first heat treatment step, a coating film, which is a precursor formed on the upper surface of the back electrode, was placed in a quartz case containing 48 mg of sulfur, and nitrogen gas was introduced. Heat treatment was performed at 500 ° C. for 60 minutes in an atmosphere. Thereafter, as a second heat treatment step, the precursor that has undergone the first heat treatment step is placed in a quartz case containing 40 mg of selenium, and heat treatment is performed at 520 ° C. for 20 minutes in an atmosphere introduced with nitrogen gas. Went.

[Example 6]
A compound thin-film solar cell of Example 6 was obtained by the same process as Example 4 except that the conditions in the heat treatment step for forming the light absorption layer were changed as follows. In the heat treatment step in Example 6, a coating film, which is a precursor formed on the upper surface of the back electrode, is placed in a quartz case containing 8 mg of sulfur and 79 mg of selenium, and in an atmosphere where nitrogen gas is introduced, 580 Heat treatment was carried out at 20 ° C. for 20 minutes.

[Comparative Example 2]
A compound thin-film solar cell of Comparative Example 2 was obtained by the same process as in Example 4 except that the conditions in the heat treatment step when forming the light absorption layer were changed as follows. In the heat treatment step in Comparative Example 2, the coating film, which is a precursor formed on the upper surface of the back electrode, is placed in a quartz case containing 48 mg of sulfur, and at 600 ° C. for 20 minutes in an atmosphere introduced with nitrogen gas. The heat treatment was performed.

[Comparative Example 3]
A compound thin-film solar cell of Comparative Example 3 was obtained by the same process as Example 4 except that the conditions in the heat treatment step when forming the light absorption layer were changed as follows. In the heat treatment step in Comparative Example 3, the coating film, which is a precursor formed on the upper surface of the back electrode, is placed in a quartz case containing 48 mg of sulfur, and at 540 ° C. for 20 minutes in an atmosphere introduced with nitrogen gas. The heat treatment was performed.

[Evaluation methods]
(Photoelectric conversion efficiency)
About the compound thin film solar cell of each Example and each comparative example, photoelectric conversion efficiency was evaluated using the standard sunlight simulator (light intensity: 100 mW / cm < ² >, air mass: 1.5).

(Composition ratio)
For the compound thin film solar cells of each example and each comparative example, after the cross section of the light absorption layer was polished flat with Ar ions, evaluation by SEM-EDS (energy dispersive X-ray analysis for scanning electron microscope) was performed, The Zn / Sn ratio in the three regions of the upper, middle, and lower portions of the light absorption layer was calculated. The upper part is a part directed in the direction in which light is incident, that is, a part close to the surface electrode in the light absorption layer. A lower part is a part close | similar to a back surface electrode in a light absorption layer. The middle part is a central part of the light absorption layer in the film thickness direction, and is a part between the upper part and the lower part. Then, the change rate Rver of the Zn / Sn ratio was calculated based on the Zn / Sn ratios of the upper, middle, and lower parts.

Moreover, based on the mapping with respect to the whole surface of the said cross section of a light absorption layer, Zn / Sn ratio of the whole light absorption layer was computed.
[result]
6 shows a scanning electron microscope (SEM) image of a cross section of the compound thin film solar cell of Example 4. FIG. The area indicated by A1 in FIG. 6 indicates the upper composition analysis area, the area indicated by B1 indicates the middle composition analysis area, and the area indicated by C1 indicates the lower composition analysis area. In addition, a region indicated by D1 in FIG. 6 indicates a region used for calculating the Zn / Sn ratio of the entire light absorption layer.

Similarly, FIG. 7 shows an SEM image of a cross section of the compound thin film solar cell of Example 5, FIG. 8 shows Example 6, FIG. 9 shows Comparative Example 2, FIG. 10 shows Comparative Example 3, and FIG. Each of the area indicated by A2 in FIG. 7, the area indicated by A3 in FIG. 8, the area indicated by A4 in FIG. 9, and the area indicated by A5 in FIG. 10 is the upper part in each example or each comparative example. The composition analysis region is shown. Each of the area indicated by B2 in FIG. 7, the area indicated by B3 in FIG. 8, the area indicated by B4 in FIG. 9, and the area indicated by B5 in FIG. 10 is the middle part in each example or each comparative example. The composition analysis region is shown. Each of the area indicated by C2 in FIG. 7, the area indicated by C3 in FIG. 8, the area indicated by C4 in FIG. 9, and the area indicated by C5 in FIG. 10 is a lower part in each example or each comparative example. The composition analysis region is shown. Each of the area indicated by D2 in FIG. 7, the area indicated by D3 in FIG. 8, the area indicated by D4 in FIG. 9, and the area indicated by D5 in FIG. 10 is the light in each example or each comparative example. The area | region used for calculation of Zn / Sn ratio of the whole absorption layer is shown.

Table 2 shows the proportion of Sn atoms and the proportion of Zn atoms in the composition analysis regions in the upper, middle, and lower portions, and the upper, middle, and lower portions calculated from these values for each example and each comparative example. The Zn / Sn ratio in the composition analysis region is shown. In Table 2, the Zn / Sn ratio indicates a value obtained by rounding off the third decimal place. Table 3 shows the regions where the Zn / Sn ratio in Table 2 is the maximum and the change rates calculated using the Sn atom ratio and the Zn atom ratio in the minimum region in Table 2 for each Example and each Comparative Example. Rver, the ratio of Sn atoms and the ratio of Zn atoms in the entire light absorption layer, the Zn / Sn ratio of the entire light absorption layer calculated from these values, and the calculation result of the photoelectric conversion efficiency are shown. In Table 3, the Zn / Sn ratio of the entire light absorption layer shows a value obtained by rounding off the third decimal place.

As shown in Table 3, in Example 4 and Example 6 manufactured according to Condition 5 in the heat treatment step of the manufacturing method of the second embodiment, and in Example 5 manufactured according to Condition 6, the Zn / Sn ratio The minimum value Rmin is controlled to 0.867 or more of the maximum value Rmax, and the difference between the minimum value Rmin and the maximum value Rmax is suppressed from increasing. Further, in comparison examples 2 and 3 in which the change rate Rver of the Zn / Sn ratio exceeds 16%, in Examples 4 to 6 in which the change rate Rver is 16% or less, high photoelectric conversion efficiency is obtained. It was done.

Moreover, it was shown that in Example 5 where the Zn / Sn ratio near the front electrode is lower than the Zn / Sn ratio near the back electrode, particularly high photoelectric conversion efficiency can be obtained.
Further, the composition of the upper composition analysis region in Example 6 does not satisfy 0.9 ≦ Zn / Sn ≦ 1.2, and such Example 6 has a higher photoelectric conversion efficiency than Comparative Examples 2 and 3. However, it was shown that the photoelectric conversion efficiency was low as compared with Examples 4 and 5.

Then, due to the high Zn / Sn ratio in the upper composition analysis region in Example 6, the Zn / Sn ratio of the entire light absorption layer in Example 6 is the same as that of the entire light absorption layer in Examples 4 and 5. It is higher than the Zn / Sn ratio. Specifically, the composition of the entire light absorption layer in each of Examples 4 to 6 satisfies 0.9 ≦ Zn / Sn ≦ 1.2. When Examples 4 and 5 are compared with Example 6, In Examples 4 and 5 in which the Zn / Sn ratio of the entire light absorbing layer is 1.1 or less, the photoelectric conversion efficiency is higher than that of Example 6 in which the Zn / Sn ratio of the entire light absorbing layer exceeds 1.1. can get. That is, when the composition of the whole light absorption layer satisfy | fills Zn / Sn <= 1.1, it was shown that especially photoelectric conversion efficiency is improved.

Conventionally, the relationship between the composition of the entire light absorption layer and the photoelectric conversion efficiency has attracted attention, but no useful knowledge has been obtained about the relationship between the state of uneven distribution of Zn in the light absorption layer and the photoelectric conversion efficiency. It was.

On the other hand, the inventor pays attention to the change rate Rver of the Zn / Sn ratio in the film thickness direction of the light absorption layer as a parameter deeply related to the uneven distribution of Zn inside the light absorption layer, and can increase the photoelectric conversion efficiency. I came to find the composition.

In addition, Examples 4 to 6 are manufactured according to Condition 5 or Condition 6 in the heat treatment step of the manufacturing method of the second embodiment. That is, since Examples 4 to 6 are manufactured according to Condition 1 or Condition 4 in the heat treatment step of the manufacturing method of the first embodiment, the light absorption layers of Examples 4 to 6 have Cu / (Zn + Sn) ≦ 1. .11 including a grain boundary portion having a composition satisfying .11. On the other hand, Example 3 described as an example of the first embodiment is manufactured according to the condition that the change rate Rver of the Zn / Sn ratio tends to be large as described in the second embodiment. However, the photoelectric conversion efficiency of Example 3 is lower than the photoelectric conversion efficiency of Examples 4 to 6, but is higher than the photoelectric conversion efficiency of Comparative Examples 1 to 3. That is, even if the change rate Rver of the Zn / Sn ratio exceeds 16%, if the crystal grain boundary portion in the light absorption layer has a composition satisfying Cu / (Zn + Sn) ≦ 1.11, the photoelectric conversion efficiency Can be improved.

Each of Comparative Examples 1 to 3 is manufactured by a manufacturing method that does not satisfy both the conditions 1 to 4 of the first embodiment and the conditions 5 and 6 of the second embodiment. Therefore, it is suggested that Comparative Examples 1 to 3 do not satisfy the condition of the numerical range described in the above embodiment with respect to both the composition of the crystal grain boundary part and the change rate Rver of the Zn / Sn ratio.

As described above, according to the second embodiment, the effects listed below can be obtained as described using the examples.
(8) Since the change rate Rver of the Zn / Sn ratio in the film thickness direction of the light absorption layer 12 after the heat treatment is 16% or less, Zn is unevenly distributed in the light absorption layer 12, The formation of defects is suppressed. As a result, the photoelectric conversion efficiency is increased.

(9) The composition of the entire light absorption layer 12 after the heat treatment satisfies 0.9 ≦ Zn / Sn ≦ 1.2. Since the Zn / Sn ratio is 0.9 or more in the entire light absorption layer 12, the formation of defects due to Cu 2 _Zn is suppressed. On the other hand, since the Zn / Sn ratio is 1.2 or less in the entire light absorption layer 12, uneven distribution of Zn can be suppressed. As a result, the photoelectric conversion efficiency is increased.

(10) In the light absorption layer 12 after the heat treatment, the Zn / Sn ratio in the portion close to the front electrode 16 is lower than the Zn / Sn ratio in the portion close to the back electrode 11. According to such a configuration, it is possible to suppress the movement of carriers due to the uneven distribution of Zn in the portion near the surface electrode 16 of the light absorption layer, thereby further improving the photoelectric conversion efficiency.

(11) The rate of change Rver is 16% by a manufacturing method including a precursor forming step of forming a precursor of the light absorption layer 12 and a heat treatment step of forming the light absorption layer 12 by heat-treating the precursor. A compound thin film solar cell including the light absorption layer 12 as described below can be formed. In particular, when a coating film that is a precursor is formed by applying ink containing fine particles, the light absorption layer 12 can be easily formed as compared with the case where the precursor is formed using sputtering or the like. In addition, the effect of increasing the photoelectric conversion efficiency is high.

(12) In the heat treatment step, it is possible to suppress the Zn / Sn ratio change rate Rver from increasing by performing the heat treatment under the condition according to either of the above condition 5 or condition 6. As a result, the light absorption layer 12 having the change rate Rver of 16% or less can be suitably formed.

DESCRIPTION OF SYMBOLS 10 ... Board | substrate, 11 ... Back electrode, 12 ... Light absorption layer, 13 ... Buffer layer, 14 ... Semi-insulating layer, 15 ... Window layer, 16 ... Surface electrode.

Claims (19)

A compound thin film solar cell comprising a light absorption layer,
The light absorption layer includes a polycrystal including at least one element of S and Se and three elements of Cu, Zn, and Sn;
The polycrystalline body includes a crystal grain boundary portion having a composition satisfying Cu / (Zn + Sn) ≦ 1.11. The compound thin film solar cell according to claim 1, wherein the inside of the crystal grains included in the polycrystal has a composition satisfying Zn / Sn ≦ 1. The compound thin-film solar cell according to claim 1 or 2, wherein the inside of the crystal grains included in the polycrystal has a composition satisfying Cu / (Zn + Sn) ≤1. The compound thin-film solar cell according to any one of claims 1 to 3, wherein each of the two or more crystal grains adjacent to each other at the crystal grain boundary part has a grain size of 0.1 µm or more. The rate of change of the Zn / Sn ratio in the film thickness direction of the light absorption layer, the maximum value of the Zn / Sn ratio in the film thickness direction of the light absorption layer is Rmax, and the Zn / Sn ratio in the film thickness direction of the light absorption layer Assuming that the minimum value of Rmin is Rmin and the rate of change is Rvar,
Rvar = (Rmax ÷ Rmin−1) × 100 (1)
The compound thin-film solar cell according to any one of claims 1 to 4, wherein the rate of change is 16% or less. The compound thin film solar cell according to claim 5, wherein a composition of the entire light absorption layer satisfies 0.9 ≦ Zn / Sn ≦ 1.2. It further comprises a front electrode and a back electrode,
The light absorption layer is located between the front electrode and the back electrode,
7. The compound thin-film solar cell according to claim 5, wherein the Zn / Sn ratio in a portion near the front electrode in the light absorption layer is lower than the Zn / Sn ratio in a portion close to the back electrode. A precursor forming step of forming a precursor of a light absorption layer, the precursor including at least one element of S and Se, and three elements of Cu, Zn, and Sn;
Heat-treating the precursor to crystallize the precursor to form the light absorption layer including a crystal grain boundary portion having a composition satisfying Cu / (Zn + Sn) ≦ 1.11;
The manufacturing method of the compound thin film solar cell containing this. In the precursor forming step, the precursor is formed by applying an ink containing fine particles made of a compound containing at least one element of S and Se and three elements of Cu, Zn, and Sn. The manufacturing method of the compound thin film solar cell of Claim 8 which forms a coating film. The method for manufacturing a compound thin-film solar cell according to claim 9, wherein the heat treatment is performed in an atmosphere containing sulfur and selenium. The method for producing a compound thin-film solar cell according to claim 9, wherein in the heat treatment step, heat treatment is performed in an atmosphere containing hydrogen and selenium, and the temperature of the heat treatment is 523 ° C or lower. The method for producing a compound thin-film solar cell according to claim 9, wherein in the heat treatment step, heat treatment is performed in an atmosphere containing sulfur and nitrogen, the temperature of the heat treatment is 580 ° C, and the time of the heat treatment is 20 minutes or more. . The heat treatment process includes a first stage heat treatment process and a second stage heat treatment process,
In the first stage heat treatment step, heat treatment is performed in an atmosphere containing sulfur,
The method for manufacturing a compound thin-film solar cell according to claim 9, wherein in the second heat treatment step, heat treatment is performed in an atmosphere containing selenium. The rate of change of the Zn / Sn ratio in the film thickness direction of the light absorption layer formed in the heat treatment step is represented by Rmax, the maximum value of the Zn / Sn ratio in the film thickness direction of the light absorption layer is represented by Rmax. When the minimum value of the Zn / Sn ratio in the film thickness direction is represented by Rmin and the rate of change is represented by Rvar, the following formula (1):
Rvar = (Rmax ÷ Rmin−1) × 100 (1)
The said change rate is 16% or less. The manufacturing method of the compound thin film solar cell of Claim 8. In the precursor forming step, the precursor is formed by applying an ink containing fine particles made of a compound containing at least one element of S and Se and three elements of Cu, Zn, and Sn. The manufacturing method of the compound thin film solar cell of Claim 14 which forms a coating film into a film. The method for manufacturing a compound thin-film solar cell according to claim 15, wherein in the heat treatment step, heat treatment is performed in an atmosphere containing sulfur and selenium. The heat treatment process includes a first stage heat treatment process and a second stage heat treatment process,
In the first stage heat treatment step, heat treatment is performed in an atmosphere containing sulfur,
The method for manufacturing a compound thin-film solar cell according to claim 15, wherein in the second heat treatment step, heat treatment is performed in an atmosphere containing selenium. The method for producing a compound thin-film solar cell according to any one of claims 9 to 13 and 15 to 17, wherein the fine particles are amorphous particles. A light absorbing layer,
The light absorption layer includes a polycrystal including at least one element of S and Se and three elements of Cu, Zn, and Sn;
The polycrystal is a light absorption layer including a crystal grain boundary having a composition satisfying Cu / (Zn + Sn) ≦ 1.11.

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