JP2017183549A - Light absorption layer, compound thin film solar cell, manufacturing method of light absorption layer, and manufacturing method of compound thin film solar cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a light absorption layer capable of improving photoelectric conversion efficiency, a compound thin film solar cell, a manufacturing method of the light absorption layer, and a manufacturing method of the compound thin film solar cell.SOLUTION: The present invention relates to a light absorption layer 13 containing five elements of Cu, Zn, Sn, S and Se and used for a compound thin film solar cell 10. A rate of change of an S concentration in a film thickness direction of the light absorption layer 13 is 10% or less, and a rate of change of an Se concentration in the film thickness direction of the light absorption layer 13 is 10% or more and 30% or less. The Se concentration is increased from a surface directed to a back electrode 12 of the compound thin film solar cell 10 among surfaces that the light absorption layer 13 has, to a surface directed to a surface electrode 17 of the compound thin film solar cell 10, and a composition of the entire light absorption layer 13 satisfies 0.98≤(S+Se)/(Cu+Zn+Sn).SELECTED DRAWING: Figure 1

Description

  The present invention relates to a light absorption layer made of a CZTS-based compound semiconductor, a compound thin film solar cell including the light absorption layer, a method for manufacturing the light absorption layer, and a method for manufacturing the compound thin film solar cell.

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. The light absorption layer is formed, for example, by subjecting a precursor layer containing a constituent element of the light absorption layer to a heat treatment involving sulfidation or selenization. The precursor layer is formed by, for example, sputtering or coating of ink containing nano-sized fine particles made of a CZTS-based compound (see Patent Documents 1 and 2).

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. Especially, improvement of the photoelectric conversion efficiency in a CZTS type compound thin film solar cell is the most important subject. For example, Patent Document 1 discloses that the photoelectric conversion efficiency can be improved by controlling the Cu / (Zn + Sn) ratio and the Zn / Sn ratio in the light absorption layer.

JP 2010-215497 A International Publication No. 2013/047461

  However, the mechanism of 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. For example, the light absorption layer contains a chalcogen element such as S or Se in addition to a metal element such as Cu, Zn, or Sn. The influence of the composition distribution of these chalcogen elements on the photoelectric conversion efficiency is also one of the items that are desired to be analyzed.

  An object of this invention is to provide the manufacturing method of the light absorption layer which can improve a photoelectric conversion efficiency, a compound thin film solar cell, a light absorption layer, and a compound thin film solar cell.

The light absorption layer that solves the above-mentioned problem is a light absorption layer that includes five elements of Cu, Zn, Sn, S, and Se, and is used in a compound thin film solar cell, and the film thickness direction of the light absorption layer The change rate of each atomic concentration of S concentration and Se concentration in the film is represented by Rmax as the maximum value of the atomic concentration in the film thickness direction, Rmin as the minimum value of the atomic concentration in the film thickness direction, and Rr as the change rate of the atomic concentration. When expressed by the following formula (1):
Rr = (Rmax ÷ Rmin−1) × 100 (1)
The change rate of the S concentration is 10% or less, the change rate of the Se concentration is 10% or more and 30% or less, and the Se concentration is the compound thin film among the surfaces of the light absorption layer. From the surface directed to the back electrode of the solar cell, the surface increases to the surface directed to the surface electrode of the compound thin film solar cell, and the composition of the entire light absorption layer is 0.98 ≦ (S + Se) / (Cu + Zn + Sn). Fulfill.

The compound thin film solar cell that solves the above problems is a back electrode, a front electrode, and a light absorption layer located between the back electrode and the front electrode, and includes Cu, Zn, Sn, S, and Se. A light absorption layer containing the five elements, and a change rate of each atomic concentration of the S concentration and Se concentration in the film thickness direction of the light absorption layer, and a maximum value of the atomic concentration in the film thickness direction as Rmax. When the minimum value of atomic concentration in the film thickness direction is represented by Rmin and the change rate of the atomic concentration is represented by Rr,
Rr = (Rmax ÷ Rmin−1) × 100 (1)
The change rate of the S concentration is 10% or less, the change rate of the Se concentration is 10% or more and 30% or less, and the Se concentration is along the direction from the back electrode to the front electrode. The composition of the entire light absorbing layer satisfies 0.98 ≦ (S + Se) / (Cu + Zn + Sn).

The manufacturing method of the light absorption layer which solves the said subject is a light absorption layer containing five elements, Cu, Zn, Sn, S, and Se, Comprising: The precursor of the said light absorption layer used for a compound thin film solar cell A precursor layer forming step of forming a body layer by applying an ink containing fine particles comprising a compound containing an element contained in the light absorbing layer; and heating the precursor layer to form the light absorbing layer. A change rate of each atomic concentration of S concentration and Se concentration in the film thickness direction of the light absorption layer formed in the heat treatment step, and a maximum value of the atomic concentration in the film thickness direction as Rmax. When the minimum value of atomic concentration in the film thickness direction is represented by Rmin and the change rate of the atomic concentration is represented by Rr,
Rr = (Rmax ÷ Rmin−1) × 100 (1)
The change rate of the S concentration is 10% or less, the change rate of the Se concentration is 10% or more and 30% or less, and the Se concentration is the compound thin film among the surfaces of the light absorption layer. From the surface directed to the back electrode of the solar cell, the surface increases to the surface directed to the surface electrode of the compound thin film solar cell, and the composition of the entire light absorption layer is 0.98 ≦ (S + Se) / (Cu + Zn + Sn). Fulfill.

The manufacturing method of the compound thin film solar cell which solves the said subject contains the element contained in the said light absorption layer the precursor layer of the light absorption layer containing five elements, Cu, Zn, Sn, S, and Se A precursor layer forming step for forming on the back electrode by applying an ink containing fine particles of a compound, and a heat treatment step for forming the light absorption layer by heating the precursor layer, the heat treatment step The change rate of each atomic concentration of S concentration and Se concentration in the film thickness direction of the light absorption layer formed by the above, the maximum value of the atomic concentration in the film thickness direction is Rmax, and the atomic concentration minimum in the film thickness direction When the value is represented by Rmin and the change rate of the atomic concentration is represented by Rr, the following formula (1):
Rr = (Rmax ÷ Rmin−1) × 100 (1)
The change rate of the S concentration is 10% or less, the change rate of the Se concentration is 10% or more and 30% or less, and the Se concentration increases along a direction away from the back electrode, The composition of the entire light absorption layer satisfies 0.98 ≦ (S + Se) / (Cu + Zn + Sn).

  According to the above configuration, the band gap of the light absorption layer gradually decreases in the direction from the back electrode to the front electrode along the film thickness direction. The collection efficiency of the electrons excited in the absorption layer is increased. Moreover, since the ratio of the chalcogen element in the light absorption layer is sufficiently secured, the light absorption layer functions well as a p-type semiconductor. Therefore, the photoelectric conversion efficiency in the compound thin film solar cell is increased.

  In the light absorption layer, the composition of the entire light absorption layer preferably satisfies (S + Se) / (Cu + Zn + Sn) ≦ 1.15.

  According to the above configuration, since segregation of S and Se on the surface of the light absorption layer that joins the buffer layer is suppressed, a good pn junction is formed by the light absorption layer and the buffer layer. Therefore, the photoelectric conversion efficiency is further increased in the compound thin film solar cell including this light absorption layer.

  In the manufacturing method of the light absorption layer, in the heat treatment step, heat treatment is performed in an atmosphere containing sulfur and selenium, and sulfur and sulfur are contained in the atmosphere so as to satisfy 0.33 ≦ Se / (S + Se) ≦ 0.67. It is preferable to introduce selenium.

According to the above method, it is possible to suitably form a light absorption layer that satisfies the above conditions for the S concentration and the Se concentration.

  ADVANTAGE OF THE INVENTION According to this invention, the photoelectric conversion efficiency in a compound thin film solar cell can be improved.

Sectional drawing which shows the whole structure of a compound thin film solar cell about one Embodiment of a compound thin film solar cell. The band structure figure which shows an example of the energy band of a buffer layer and a light absorption layer about the compound thin film solar cell of one Embodiment, and the conventional compound thin film solar cell. The band structure figure which shows an example of the energy band of a buffer layer and a light absorption layer about the compound thin film solar cell of one Embodiment and the compound thin film solar cell of a reference example. The figure which shows the SEM image of the cross section in the compound thin film solar cell of Example 1 with a composition analysis area | region. The figure which shows the SEM image of the cross section in the compound thin film solar cell of Example 2 with a composition analysis area | region. The figure which shows the SEM image of the cross section in the compound thin film solar cell of Example 3 with a composition analysis area | region. The figure which shows the SEM image of the cross section in the compound thin film solar cell of the comparative example 1 with a composition analysis area | region. The figure which shows the SEM image of the cross section in the compound thin film solar cell of the comparative example 2 with a composition analysis area | region. The figure which shows the SEM image of the cross section in the compound thin film solar cell of the comparative example 3 with a composition analysis area | region.

  With reference to FIG. 1, one embodiment of a light absorption layer, a compound thin film solar cell, a method for manufacturing a light absorption layer, and a method for manufacturing a compound thin film solar cell will be described.

[Configuration of Compound Thin Film Solar Cell]
As shown in FIG. 1, the compound thin-film solar cell 10 includes a substrate 11, a back electrode 12, a light absorption layer 13, a buffer layer 14, a semi-insulating layer 15, a window layer 16, and a surface electrode 17. The back electrode 12, the light absorption layer 13, the buffer layer 14, the semi-insulating layer 15, the window layer 16, and the surface electrode 17 are located on the substrate 11 in order from the position close to the substrate 11.

  As the substrate 11, for example, a glass substrate, a metal plate, a plastic film, or the like is used.

  The back electrode 12 is made of, for example, Mo (molybdenum), Ni (nickel), Cu (copper), Ti (titanium), Fe (iron), Al (aluminum), titania, stainless steel, or the like. Alternatively, the constituent material of the back electrode 12 may be a material composed of carbon such as carbon or graphene, or may be a transparent metal oxide having conductivity such as ITO (Indium Tin Oxide) or ZnO. May be.

The light absorption layer 13 is composed of a CZTS-based compound semiconductor that is one of p-type compound semiconductors, that is, a CZTS semiconductor. The CZTS semiconductor is a IB _2- (IIB-IVB) -VIB group ₄ compound semiconductor, and the light absorption layer 13 is composed of group IB (group 11) Cu, group IIB (group 12) Zn (zinc), IVB. It includes five elements of Sn (tin) of Group (Group 14), S (sulfur) of Group VIB (Group 16), and Se (selenium) of Group VIB (Group 16).

  The rate of change RSr of the S concentration (atomic%) in the film thickness direction of the light absorption layer 13 is 10% or less. Moreover, the change rate RSer of Se concentration (atomic%) in the film thickness direction of the light absorption layer 13 is 10% or more and 30% or less.

As for the change rate of each atomic concentration of S concentration and Se concentration, the maximum value of atomic concentration in the film thickness direction of the light absorption layer 13 is Rmax, the minimum value of atomic concentration in the film thickness direction of the light absorption layer 13 is Rmin, and the change rate. Is represented by the following formula (1).
Rr = (Rmax ÷ Rmin−1) × 100 (1)

That is, the change rate RSr of the S concentration in the film thickness direction of the light absorption layer 13 is RSmx, the maximum value of the S concentration in the film thickness direction of the light absorption layer 13, and the minimum value of the S concentration in the film thickness direction of the light absorption layer 13 Is represented by the following formula (2). The S concentration is a value indicating the ratio of the total number of S atoms to the total number of atoms of the elements included in the target region of the composition analysis in the light absorption layer 13 as a percentage.
RSr = (RSmx ÷ RSmn−1) × 100 (2)

  The direction of change in the S concentration in the film thickness direction of the light absorption layer 13 is not particularly limited, and the S concentration may increase or decrease along the direction from the back electrode 12 to the front electrode 17. However, it may be maximum or minimum at the central portion in the film thickness direction.

The Se concentration change rate RSer in the film thickness direction of the light absorption layer 13 is RSemx, which is the maximum value of Se concentration in the film thickness direction of the light absorption layer 13, and the minimum value of Se concentration in the film thickness direction of the light absorption layer 13. Is represented by the following formula (3). The Se concentration is a value indicating the percentage of the total number of Se atoms with respect to the total number of atoms of elements included in the composition analysis target region in the light absorption layer 13 as a percentage.
RSer = (RSemx ÷ RSem−1) × 100 (3)

  In the light absorption layer 13, the Se concentration increases toward the buffer layer 14 along the direction from the back electrode 12 to the front electrode 17, that is, in the direction away from the back electrode 12. In other words, the Se concentration increases from the surface of the light absorption layer 13 toward the surface electrode 17 from the surface directed toward the back electrode 12.

  The maximum value RSmx and the minimum value RSmn of the S concentration and the maximum value RSemx and the minimum value RSemn of the Se concentration are obtained by performing composition analysis on a plurality of regions having different positions in the film thickness direction in the light absorption layer 13. Desired. As a result of the composition analysis, the maximum value of the obtained S concentrations in each region is the maximum value RSmx, and the minimum value is the minimum value RSmn. In addition, as a result of the composition analysis, the maximum value of the Se concentrations obtained in each region is the maximum value RSemx, and the minimum value is the minimum value RSemn.

  The composition analysis is performed on the polished cross section of the light absorption layer 13 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 13 in the film thickness direction. For example, the cross section of the light absorption layer 13 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 13. The Se concentration increases along the direction from the back electrode 12 to the front electrode 17. The Se concentration in each composition analysis region is along the direction from the back electrode 12 to the front electrode 17 in the order in which the composition analysis regions are arranged. It means becoming bigger.

  As described above, in the film thickness direction of the light absorption layer 13, the change rate RSr of S concentration is suppressed to 10% or less, while the Se concentration is a range in which the change rate RSer is 10% or more and 30% or less. And increases from the back electrode 12 toward the buffer layer 14. According to such a configuration, the following effects occur.

  That is, as shown by a broken line in the band structure diagram of FIG. 2, in the conventional light absorption layer, that is, in the light absorption layer having a small S concentration change rate RSr and a Se concentration change rate RSer, the band gap of the light absorption layer. Changes along the film thickness direction from the back electrode to the buffer layer with a certain size.

  On the other hand, in the light absorption layer 13 of the present embodiment, as indicated by the solid line in the band structure diagram of FIG. 2, the energy level Ec at the lower end of the conduction band extends from the back electrode 12 toward the buffer layer 14 and the upper end of the valence band. It becomes low at the rate of change larger than the energy level Ev. Therefore, the band gap of the light absorption layer 13 gradually decreases from the back electrode 12 toward the buffer layer 14 along the film thickness direction. As a result, in the light absorption layer 13 of the present embodiment, electrons, which are minority carriers photoexcited in the light absorption layer 13, are compared with the conventional light absorption layer in the pn junction interface between the light absorption layer 13 and the buffer layer 14. Since the recombination of carriers at the interface between the light absorption layer 13 and the back electrode 12 is suppressed, the collection efficiency of excited electrons is increased.

  The composition of the entire light absorption layer 13 satisfies 0.98 ≦ (S + Se) / (Cu + Zn + Sn). Furthermore, the composition of the entire light absorption layer 13 preferably satisfies (S + Se) / (Cu + Zn + Sn) ≦ 1.15.

  The (S + Se) / (Cu + Zn + Sn) ratio is an atomic ratio, and the ratio of the sum of atomic percent of S and atomic percent of Se to the sum of atomic percent of Cu, atomic percent of Zn and atomic percent of Sn in the light absorption layer 13 Indicates. The (S + Se) / (Cu + Zn + Sn) ratio in the entire light absorption layer 13 is calculated from data obtained by performing mapping by EDS on the entire cross section of the polished light absorption layer 13.

  If the (S + Se) / (Cu + Zn + Sn) ratio is 0.98 or more in the entire light absorption layer 13, the ratio of the chalcogen element in the light absorption layer 13 does not become excessively small. Is suppressed, and a decrease in function as a p-type semiconductor is suppressed.

  Further, if there is a large amount of segregation of S or Se on the surface facing the surface electrode 17 in the light absorption layer 13, that is, the surface bonded to the buffer layer 14, the pn junction due to the heterojunction between the light absorption layer 13 and the buffer layer 14 Formation is inhibited. On the other hand, the change rate RSer of Se concentration is suppressed to 30% or less, and the (S + Se) / (Cu + Zn + Sn) ratio is suppressed to 1.15 or less. Since segregation of Se on the surface to be bonded to 14 is suppressed, a good pn junction is formed.

  Further, since the change rate RSr of the S concentration is suppressed to 10% or less, and the (S + Se) / (Cu + Zn + Sn) ratio is suppressed to 1.15 or less, the buffer layer 14 in the light absorption layer 13 and Segregation of S on the surfaces to be joined is suppressed. Furthermore, since the change rate RSer of Se concentration increases from the back electrode 12 toward the buffer layer 14, the S concentration is relatively lowered in the vicinity of the surface of the light absorption layer 13 that joins the buffer layer 14. Formation of a heterogeneous phase of ZnS on the surface bonded to the layer 14 is suppressed.

  Further, when the S concentration is high in the vicinity of the surface of the light absorption layer 13 where the buffer layer 14 is joined, as shown by the broken line in the band structure diagram of FIG. 3, the pn junction portion between the light absorption layer 13 and the buffer layer 14. The energy level Ec at the lower end of the conduction band of the light absorption layer 13 is higher than the energy level Ec at the lower end of the conduction band of the buffer layer 14, so that a Cliff type band structure is easily formed. In this case, recombination of holes and electrons is likely to occur at the interface portion between the light absorption layer 13 and the buffer layer 14, and the leakage current at the pn junction portion increases, so that the photoelectric conversion efficiency decreases. Therefore, as indicated by the solid line in the band structure diagram of FIG. 3, at the pn junction portion between the light absorption layer 13 and the buffer layer 14, the energy level Ec at the lower end of the conduction band of the buffer layer 14 is It is preferable to form a spike type band structure that is higher than the energy level Ec at the lower end of the conduction band of the light absorption layer 13 to the extent that it does not become a barrier. The S concentration change rate RSr is suppressed to 10% or less, and the (S + Se) / (Cu + Zn + Sn) ratio is suppressed to 1.15 or less, thereby joining the buffer layer 14 in the light absorption layer 13. Since the S concentration does not become too high in the vicinity of the surface, such a band structure is easily realized.

The buffer layer 14 is made of an n-type compound semiconductor. The buffer layer 14 is made of, for example, CdS, Zn (S, O, OH), ZnS, ZnSe, In ₂ S ₃ or the like. The semi-insulating layer 15 is made of an i-type compound semiconductor. The semi-insulating layer 15 is made of a metal oxide such as ZnO, for example. The window layer 16 is composed of an n-type compound semiconductor. The window layer 16 is made of, for example, ZnO to which Al, Ga (gallium), B (boron), or the like is added, or ITO.

  The surface electrode 17 is located on a part of the upper surface of the window layer 16. The surface electrode 17 is comprised from metals, such as Al and Ag (silver), for example. Alternatively, the constituent material of the surface electrode 17 may be a material composed of carbon such as carbon or graphene, or may be a transparent metal oxide having conductivity such as ITO or ZnO.

In addition, the compound thin film solar cell 10 may have layers other than each layer mentioned above. For example, the compound thin-film solar cell 10 may include an antireflection layer positioned on the window layer 16. The antireflection layer is made of, for example, MgF _2, and the thickness of the antireflection layer is preferably about 100 nm. Since the antireflection layer suppresses reflection of light incident on the compound thin film solar cell 10, the light absorption layer 13 can absorb more light when the compound thin film solar cell 10 includes the antireflection layer. . In addition, the compound thin film solar cell 10 may include an intermediate layer that is a layer having a composition for increasing the photoelectric conversion efficiency of the compound thin film solar cell 10 between the back electrode 12 and the light absorption layer 13.

[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 12, a light absorption layer 13, a buffer layer 14, a semi-insulating layer 15, a window layer 16, and a surface electrode 17 on the substrate 11 in this order.

  The back electrode 12 is formed on the upper surface of the substrate 11 using, for example, sputtering, vapor deposition, CVD (Chemical Vapor Deposition), or the like.

  The step of forming the light absorbing layer 13 includes a precursor layer forming step of forming a precursor layer of the light absorbing layer 13 on one surface of the back electrode 12, and the precursor layer is heated to form the light absorbing layer 13. Heat treatment step.

The precursor layer is formed using a light absorbing layer forming ink containing fine particles composed of the constituent elements of the light absorbing layer 13. 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 light absorbing layer forming ink includes, as fine particles, Cu _2-x S _y Se _2-y (0 ≦ x ≦ 1, 0 ≦ y ≦ 2), Zn _2-x 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) It may contain particles of at least one compound selected from the group consisting of compounds. 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 particles made of the above compound, particles of three types of compounds included in the above 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 13.

  The light absorbing layer forming ink is prepared by dispersing the fine particles in a solvent. 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, aromatic hydrocarbons, and the like. 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, thiol organic substances, selenol organic substances, alcohols having 10 or more carbon atoms, and the like can be used. Moreover, Se particle | grains, S particle | grains, Se compound, S compound, etc. may be used as a 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 13 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 13. Of carbon remains.

  In the precursor layer forming step, the light absorbing layer forming ink is applied to the upper surface of the back electrode 12 to form a film containing fine particles. Then, by drying the film containing fine particles, the solvent contained in the coated film is removed. Thereby, the coating film as a precursor layer is formed. 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 spraying method, a gravure printing method, a screen printing method, a reverse offset printing method, a relief printing method, and the like. And printing methods such as law.

  Next, a heat treatment step is performed, and by heating the precursor layer in the heat treatment step, crystallization of a large number of fine particles contained in the precursor layer proceeds, and the fine particles are sintered. Thereby, the light absorption layer 13 is formed.

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 maximum temperature of the heat treatment is preferably 250 ° C. or higher. When a glass substrate is used as the substrate 11, the maximum temperature of the heat treatment is preferably a temperature that the glass can withstand, specifically, 650 ° C. or less, more preferably 600 ° C. or less. Moreover, it is preferable that the time heated at the maximum temperature in a heat treatment process is 5 minutes or more and 180 minutes or less.

  The S concentration and Se concentration of the light absorption layer 13 can be adjusted by adjusting the concentration of S and Se contained in the atmosphere of the heat treatment, the maximum temperature of the heat treatment, and the heating time. Specifically, in order to obtain the light absorption layer 13 described above, it is preferable that any one of the following conditions 1 to 3 is satisfied.

  Condition 1: Heating of the precursor layer is started in a state where the precursor layer is placed in an environment where solid sulfur and solid selenium exist, and solid sulfur and solid at the start of heating The ratio with the selenium-like selenium satisfies 0.33 ≦ Se / (S + Se) ≦ 0.67 in terms of molar ratio.

Condition 2: The atmosphere of the heat treatment contains H ₂ S and H ₂ Se at the same time, and the ratio of S and Se introduced into the atmosphere is expressed as a molar ratio of 0.33 ≦ Se / (S + Se) ≦. It satisfies 0.67.
Condition 3: 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.

  That is, in conditions 1 and 2, heat treatment is performed in an atmosphere containing sulfur and selenium, and sulfur and selenium are contained in the atmosphere so as to satisfy 0.33 ≦ Se / (S + Se) ≦ 0.67. be introduced.

  By satisfying these conditions, the change rate RSr of S concentration in the film thickness direction is 10% or less, the change rate RSer of Se concentration is 10% or more and 30% or less, and the Se concentration is in the buffer layer 14. It becomes easy to obtain the light absorption layer 13 which becomes large toward it.

  In addition, although the thickness of the light absorption layer 13 after a heating should just be about 0.3 micrometer-10 micrometers, it is preferable that they are 1 micrometer or more and 3 micrometers or less, for example. The formed light absorption layer 13 may be subjected to a surface cleaning treatment with a known inorganic acid, organic acid, alkaline solution, pure water, or the like.

  The buffer layer 14 is formed on the upper surface of the light absorption layer 13 by using, for example, a CBD (Chemical Bath Deposition) method, a MOCVD (Metal Organic Chemical Vapor Deposition) method, an ALD (Atomic Layer Deposition) method, or the like.

The semi-insulating layer 15 is formed on the upper surface of the buffer layer 14 by using, for example, MOCVD or sputtering. The window layer 16 is formed on the upper surface of the semi-insulating layer 15 by using, for example, MOCVD or sputtering.
The surface electrode 17 is formed on a part of the upper surface of the window layer 16 by using, for example, sputtering, vapor deposition, CVD, or the like.

  In addition, the light absorption layer 13 can also be formed by performing heat processing with respect to the precursor layer which is a film | membrane formed using sputtering. However, in the case where the light absorption layer is formed from a film formed by sputtering and the case where the light absorption layer is formed from a coating film containing fine particles as described above, the crystallization process of the film during heat treatment is Different. That is, when sputtering is used, first, a layer for each metal constituting the light absorption layer or one layer containing these metals is formed by sputtering as a precursor layer. 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 satisfying the above-described conditions for the S concentration and the Se concentration brings particularly high photoelectric conversion efficiency when the light absorption layer is formed from a coating film containing fine particles.

  However, regardless of the manufacturing method, in the light absorption layer 13, the change rate RSr of the S concentration in the film thickness direction is 10% or less, the change rate RSer of the Se concentration is 10% or more and 30% or less, and the Se concentration Increases along the direction from the back electrode 12 to the front electrode 17 and the composition of the entire light absorption layer 13 satisfies 0.98 ≦ (S + Se) / (Cu + Zn + Sn), the photoelectric conversion efficiency can be improved.

[Example]
The above-described light absorption layer, compound thin film solar cell, method for producing the light absorption layer, and method for producing the compound thin film solar cell 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 mixed liquid after the reaction was filtered to take out Cu—Zn—Sn—Se fine particles, and the Cu—Zn—Sn—Se fine particles were washed with methanol. The Cu—Zn—Sn—Se fine particles after washing and thiourea are mixed so that the mass ratio of the fine particles to thiourea is 3: 2, and pyridine and methanol are further added to the mixture to obtain Cu— A light absorbing layer forming ink containing Zn—Sn—Se fine particles was prepared. The solid content in the light absorbing layer forming ink is 6.5% by mass.
<Formation of back electrode>
A soda lime glass substrate was used as the substrate, and a back electrode composed of Mo was formed by sputtering.

<Formation of light absorption layer>
As the precursor layer 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 layer formed on the upper surface of the back electrode, is placed in a case containing 32 mg of solid sulfur and 40 mg of solid selenium, and in an atmosphere into which nitrogen gas has been introduced. A heat treatment was performed at 600 ° C. for 20 minutes. Thereby, the light absorption layer of Example 1 comprised from the CZTS thin film with a film thickness of about 2 micrometers was obtained. In Example 1, the ratio of S and Se introduced into the heat treatment atmosphere is expressed as a molar ratio, which is Se / (S + Se) = 0.337.

<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 light absorbing layer and a compound thin-film solar cell of Example 2 were obtained by the same process as in Example 1 except that the conditions in the heat treatment step for forming the light absorbing layer were changed as follows. In the heat treatment step in Example 2, a coating film, which is a precursor layer formed on the upper surface of the back electrode, is placed in a case containing 16 mg of sulfur and 40 mg of selenium, and 600 atmospheres in an atmosphere in which nitrogen gas is introduced. Heat treatment was carried out at 20 ° C. for 20 minutes. In Example 2, the ratio of S and Se introduced into the heat treatment atmosphere is expressed as a molar ratio, which is Se / (S + Se) = 0.504.

(Example 3)
A light absorption layer and a compound thin-film solar cell of Example 3 were obtained by the same process as in Example 1 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 3, a coating film, which is a precursor layer formed on the upper surface of the back electrode, is put in a case containing 8 mg of sulfur and 40 mg of selenium, and 600 atmospheres in an atmosphere introduced with nitrogen gas. Heat treatment was carried out at 20 ° C. for 20 minutes. In Example 3, the ratio of S and Se introduced into the heat treatment atmosphere is expressed as a molar ratio, which is Se / (S + Se) = 0.670.

(Comparative Example 1)
A light-absorbing layer and a compound thin-film solar cell of Comparative Example 1 were obtained by the same steps as in Example 1 except that the conditions in the heat treatment step when forming the light-absorbing layer were changed as follows. In the heat treatment process in Comparative Example 1, a coating film, which is a precursor layer formed on the upper surface of the back electrode, is placed in a case containing 4 mg of sulfur and 40 mg of selenium, and 600 atmospheres in an atmosphere in which nitrogen gas is introduced. Heat treatment was carried out at 20 ° C. for 20 minutes. In Comparative Example 1, the ratio of S and Se introduced into the heat treatment atmosphere is expressed as a molar ratio, which is Se / (S + Se) = 0.802.

(Comparative Example 2)
A light-absorbing layer and a compound thin-film solar cell of Comparative Example 2 were obtained by the same steps as in Example 1 except that the conditions in the heat treatment step for forming the light-absorbing layer were changed as follows. In the heat treatment process in Comparative Example 2, a coating film, which is a precursor layer formed on the upper surface of the back electrode, is placed in a case containing 40 mg of sulfur and 10 mg of selenium, and in an atmosphere in which nitrogen gas is introduced, 580 Heat treatment was carried out at 20 ° C. for 20 minutes. In Comparative Example 2, the ratio of S and Se introduced into the heat treatment atmosphere is expressed as a molar ratio, which is Se / (S + Se) = 0.092.

(Comparative Example 3)
A light-absorbing layer and a compound thin-film solar cell of Comparative Example 3 were obtained by the same steps as in Example 1 except that the conditions in the heat treatment step for forming the light-absorbing layer were changed as follows. In the heat treatment step in Comparative Example 3, the coating film, which is a precursor layer formed on the upper surface of the back electrode, is placed in a case containing 32 mg of sulfur and 10 mg of selenium, and 600 atmosphere under an atmosphere in which nitrogen gas is introduced. After performing a heat treatment at 20 ° C. for 20 minutes, the heat treatment was performed at 500 ° C. for 20 minutes in an atmosphere into which nitrogen gas was introduced in a case where sulfur and selenium were not added. In Comparative Example 1, the ratio of S and Se introduced into the heat treatment atmosphere is expressed as a molar ratio, which is Se / (S + Se) = 0.113.

[Evaluation method]
(Composition ratio)
About the compound thin film solar cell of each Example and each comparative example, after polishing the cross section of a light absorption layer flatly with Ar ion, the light absorption layer of SEM-EDS (energy dispersive X-ray analysis for a scanning electron microscope) was carried out. The atomic concentrations of S and Se atoms in the upper, middle, and lower three regions were determined. 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 S concentration change rate RSr and the Se concentration change rate RSer were calculated based on the S concentration and Se concentration of the upper, middle, and lower portions, respectively.

  Further, the (S + Se) / (Cu + Zn + Sn) ratio of the entire light absorption layer was calculated based on mapping by SEM-EDS on the entire surface of the light absorption layer. The acceleration voltage in SEM-EDS at the time of composition analysis was 15 kV.

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

[result]
FIG. 4 shows an image of a cross section of the compound thin film solar cell of Example 1 by a scanning electron microscope (SEM). The region indicated by A1 in FIG. 4 indicates the upper composition analysis region, the region indicated by B1 indicates the middle composition analysis region, and the region indicated by C1 indicates the lower composition analysis region. In addition, a region indicated by D1 in FIG. 4 indicates a region used for calculating the (S + Se) / (Cu + Zn + Sn) ratio of the entire light absorption layer.

  Similarly, FIG. 5 shows an SEM image of a cross section of Example 2, FIG. 6 shows Example 3, FIG. 7 shows Comparative Example 1, FIG. 8 shows Comparative Example 2, and FIG. Each of the area indicated by A2 in FIG. 5, the area indicated by A3 in FIG. 6, the area indicated by A4 in FIG. 7, the area indicated by A5 in FIG. 8, and the area indicated by A6 in FIG. The composition analysis area | region of the upper part in each Example or each comparative example is shown. Each of the area indicated by B2 in FIG. 5, the area indicated by B3 in FIG. 6, the area indicated by B4 in FIG. 7, the area indicated by B5 in FIG. 8, and the area indicated by B6 in FIG. The composition analysis area | region of the center part in each Example or each comparative example is shown. Each of the area indicated by C2 in FIG. 5, the area indicated by C3 in FIG. 6, the area indicated by C4 in FIG. 7, the area indicated by C5 in FIG. 8, and the area indicated by C6 in FIG. The composition analysis area | region of the lower part in each Example or each comparative example is shown. Each of the area indicated by D2 in FIG. 5, the area indicated by D3 in FIG. 6, the area indicated by D4 in FIG. 7, the area indicated by D5 in FIG. 8, and the area indicated by D6 in FIG. The area | region used for calculation of (S + Se) / (Cu + Zn + Sn) ratio of the whole light absorption layer in each Example or each comparative example is shown.

  Table 1 shows the S concentration and Se concentration in each of the upper, middle, and lower composition analysis regions for each example and each comparative example, and the change rate of the S concentration calculated from these atomic concentrations, RSr and Se concentration. The rate RSer, the (S + Se) / (Cu + Zn + Sn) ratio of the entire light absorption layer, and the photoelectric conversion efficiency are shown. In Table 1, the change rates of the S concentration and the Se concentration show values rounded off to the second decimal place.

  As shown in Table 1, (a) the change rate RSr of S concentration in the light absorption layer is 10% or less, and (b) the change rate RSer of Se concentration in the light absorption layer is 10% or more and 30% or less. The Se concentration increases along the direction from the back electrode 12 to the front electrode 17, and (c) the composition of the entire light absorption layer satisfies 0.98 ≦ (S + Se) / (Cu + Zn + Sn). In Examples 1 to 3 in which all three conditions of (c) are satisfied, higher photoelectric conversion efficiency can be obtained as compared with Comparative Examples 1 to 3 in which at least one of these conditions is not satisfied. confirmed.

  In Comparative Example 1, the S concentration change rate RSr is larger than the range (a), and the Se concentration change rate RSer is larger than the range (b). In addition, the S concentration and the Se concentration increase toward the surface electrode 17, and the (S + Se) / (Cu + Zn + Sn) ratio is larger than 1.15. Therefore, a large amount of S and Se is segregated on the surface of the light absorption layer that is bonded to the buffer layer, suggesting that formation of a good pn junction between the light absorption layer and the buffer layer is hindered.

  In Comparative Example 2, the Se concentration change rate RSer is smaller than the range (b), and the Se concentration is substantially constant in each composition analysis region. Therefore, the band gap of the light absorption layer is substantially constant from the back electrode to the buffer layer, and movement of electrons excited in the light absorption layer in the direction of the surface electrode is hindered compared to the example. Is suggested. Further, in Comparative Example 2, since the S concentration is extremely higher than the Se concentration, it is also suggested that a cliff-type band structure is formed at the pn junction portion between the light absorption layer and the buffer layer.

  In Comparative Example 3, since the (S + Se) / (Cu + Zn + Sn) ratio is smaller than the range of (c), the ratio of the chalcogen element in the light absorption layer is excessively small. This increases, suggesting that the function of the light absorption layer as a p-type semiconductor is reduced.

  Thus, the present inventors analyzed the relationship between the S concentration and Se concentration in the light absorption layer and the photoelectric conversion efficiency, and in particular, the tendency of the change in S concentration in the film thickness direction of the light absorption layer and the Se concentration. It has been found that the photoelectric conversion efficiency can be improved by controlling the concentration of these atoms so that the tendency of change differs.

As described above, according to the embodiment, the effects listed below can be obtained as described with reference to the examples.
(1) In the film thickness direction of the light absorption layer 13, the change rate RSr of S concentration is 10% or less, and the change rate RSer of Se concentration is 10% or more and 30% or less. The composition increases in the direction from the electrode 12 toward the surface electrode 17 and the composition of the entire light absorption layer 13 satisfies 0.98 ≦ (S + Se) / (Cu + Zn + Sn). According to such a configuration, the band gap of the light absorption layer 13 gradually decreases from the back electrode 12 toward the buffer layer 14, so that the collection efficiency of excited electrons increases. Moreover, since the ratio of the chalcogen element in the light absorption layer 13 is sufficiently ensured, the light absorption layer 13 functions well as a p-type semiconductor. Therefore, the photoelectric conversion efficiency in the compound thin film solar cell 10 is increased.

  (2) The composition of the entire light absorption layer 13 satisfies (S + Se) / (Cu + Zn + Sn) ≦ 1.15. According to such a configuration, since segregation of S and Se on the surface of the light absorption layer 13 where the buffer layer 14 is bonded is suppressed, a good pn junction is formed by the light absorption layer 13 and the buffer layer 14.

  (3) In the heat treatment step, the light absorption layer 13 that satisfies the above conditions for the S concentration and the Se concentration can be easily obtained by performing the heat treatment under the condition according to any one of the above conditions 1 to 3. In particular, heat treatment is performed in an atmosphere containing sulfur and selenium, and sulfur and selenium are introduced into the atmosphere so as to satisfy 0.33 ≦ Se / (S + Se) ≦ 0.67. It is possible to suitably form the light absorption layer 13 that satisfies the above conditions for the Se concentration.

  DESCRIPTION OF SYMBOLS 10 ... Compound thin film solar cell, 11 ... Substrate, 12 ... Back electrode, 13 ... Light absorption layer, 14 ... Buffer layer, 15 ... Semi-insulating layer, 16 ... Window layer, 17 ... Surface electrode

Claims (6)

A light-absorbing layer containing five elements of Cu, Zn, Sn, S, and Se, and used for a compound thin film solar cell,
The rate of change of each atomic concentration of the S concentration and the Se concentration in the film thickness direction of the light absorption layer, the maximum value of the atomic concentration in the film thickness direction as Rmax, the minimum value of the atomic concentration in the film thickness direction as Rmin, When represented by the following formula (1) where the change rate of the atomic concentration is Rr,
Rr = (Rmax ÷ Rmin−1) × 100 (1)
The rate of change of the S concentration is 10% or less,
The change rate of the Se concentration is 10% or more and 30% or less, and the Se concentration is reduced from the surface of the light absorption layer toward the back electrode of the compound thin film solar cell. It grows toward the surface facing the surface electrode of the solar cell,
The composition of the whole light absorption layer is a light absorption layer satisfying 0.98 ≦ (S + Se) / (Cu + Zn + Sn). The light absorption layer according to claim 1, wherein the composition of the entire light absorption layer satisfies (S + Se) / (Cu + Zn + Sn) ≦ 1.15. A back electrode;
A surface electrode;
A light absorption layer located between the back electrode and the front electrode, the light absorption layer containing five elements of Cu, Zn, Sn, S, and Se, and
The rate of change of each atomic concentration of the S concentration and the Se concentration in the film thickness direction of the light absorption layer, the maximum value of the atomic concentration in the film thickness direction as Rmax, the minimum value of the atomic concentration in the film thickness direction as Rmin, When represented by the following formula (1) where the change rate of the atomic concentration is Rr,
Rr = (Rmax ÷ Rmin−1) × 100 (1)
The rate of change of the S concentration is 10% or less,
The change rate of the Se concentration is 10% or more and 30% or less, and the Se concentration increases along a direction from the back electrode to the front electrode,
The composition of the whole said light absorption layer is 0.98 <= (S + Se) / (Cu + Zn + Sn) The compound thin film solar cell. A light-absorbing layer containing five elements of Cu, Zn, Sn, S, and Se, and a precursor layer of the light-absorbing layer used in a compound thin film solar cell, and an element contained in the light-absorbing layer A precursor layer forming step formed by applying an ink containing fine particles comprising a compound containing;
A heat treatment step of forming the light absorption layer by heating the precursor layer,
The rate of change of each atomic concentration of the S concentration and Se concentration in the film thickness direction of the light absorption layer formed in the heat treatment step, the maximum value of the atomic concentration in the film thickness direction as Rmax, and the atoms in the film thickness direction When the minimum value of concentration is represented by Rmin and the change rate of the atomic concentration is represented by Rr,
Rr = (Rmax ÷ Rmin−1) × 100 (1)
The rate of change of the S concentration is 10% or less,
The change rate of the Se concentration is 10% or more and 30% or less, and the Se concentration is reduced from the surface of the light absorption layer toward the back electrode of the compound thin film solar cell. It grows toward the surface facing the surface electrode of the solar cell,
The composition of the whole light absorption layer is a manufacturing method of the light absorption layer which satisfy | fills 0.98 <= (S + Se) / (Cu + Zn + Sn). In the heat treatment step, heat treatment is performed in an atmosphere containing sulfur and selenium, and sulfur and selenium are introduced into the atmosphere so as to satisfy 0.33 ≦ Se / (S + Se) ≦ 0.67. The manufacturing method of the light absorption layer of description. A precursor layer of a light absorption layer containing five elements of Cu, Zn, Sn, S, and Se is applied on the back electrode by applying ink containing fine particles made of a compound containing an element contained in the light absorption layer. A precursor layer forming step to be formed into,
A heat treatment step of forming the light absorption layer by heating the precursor layer,
The rate of change of each atomic concentration of the S concentration and Se concentration in the film thickness direction of the light absorption layer formed in the heat treatment step, the maximum value of the atomic concentration in the film thickness direction as Rmax, and the atoms in the film thickness direction When the minimum value of concentration is represented by Rmin and the change rate of the atomic concentration is represented by Rr,
Rr = (Rmax ÷ Rmin−1) × 100 (1)
The rate of change of the S concentration is 10% or less,
The change rate of the Se concentration is 10% or more and 30% or less, and the Se concentration increases along a direction away from the back electrode,
The composition of the whole light absorption layer is a manufacturing method of the compound thin film solar cell which satisfies 0.98 <= (S + Se) / (Cu + Zn + Sn).