JP2014086527A - Compound semiconductor thin film, manufacturing method of the same and solar cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a compound semiconductor thin film having reduces surface roughness.SOLUTION: A compound semiconductor thin film comprises a laminate including a lower layer containing particles of a group I-II-IV-VI compound and an upper layer for planarizing a surface of the lower layer.

Description

  The present invention relates to a compound semiconductor thin film, a manufacturing method thereof, and a solar cell including the compound semiconductor thin film.

A solar cell is a photoelectric conversion element that converts light energy into electric energy by using the photovoltaic effect, and has recently been attracting attention from the viewpoints of prevention of global warming and replacement of exhausted resources. The solar cell is a semiconductor is used, for example, single crystal Si, polycrystalline Si, amorphous _{Si, CdTe, CuIn 1-x} Ga x Se 2 (CIGS _{compound), Cu 2 ZnSn (S,} Se) 4 (CZTS Compound), GaAs, InP, and the like are known. The CIGS solar cell has a large light absorption coefficient and a maximum energy conversion efficiency exceeding 20% as compared with the crystalline Si solar cell which is mainstream in the current market. In addition, CIGS solar cells have relatively few manufacturing processes and are excellent in cost performance. Therefore, commercial mass production has already begun as a next-generation solar cell. In CZTS solar cells, rare metals are not used as the main constituent elements, and there is also an advantage that the environmental load is small.

  In a solar cell using a CIGS compound or a CZTS compound, the light absorption layer is the most important component, and various methods for producing the light absorption layer have been proposed. For example, Patent Document 1 and Non-Patent Document 1 propose a method for producing a light absorption layer made of a CZTS thin film by sputtering and vapor deposition, which are vacuum processes. Non-Patent Documents 2 and 3 propose a method for producing a light absorption layer made of a CIGS thin film by sputtering and vapor deposition which are vacuum processes.

  However, in the vacuum process, expensive vacuum equipment is required, the manufacturing process is complicated, and the power generation cost is high. In addition, there is also a drawback that it is difficult to maintain the uniformity of the distribution of each element in the plane when forming a film with a large area.

  Patent Document 2 proposes to reduce the film formation cost by applying a semiconductor fine particle to form a light absorption layer. According to the method of applying fine particles, an expensive vacuum device is not required and the process is simplified, so that the power generation cost is expected to be greatly reduced. It is also expected that the distribution of each element in the plane becomes uniform and the conversion efficiency is improved. However, since fine particles are used, the flatness of the outermost surface of the obtained compound semiconductor thin film is insufficient. As the surface roughness of the light absorption layer increases, leakage current tends to occur, leading to a decrease in conversion efficiency.

JP 2009-26891 A JP 2009-076842 A

Progress In Photovoltaics: Research and Applications 19 (2011) 93-96 Journal of Applied Physics 102 (2007) 074922. Progress In Photovoltaics: Research and Applications 16 (2008) 235.

  This invention is made | formed in view of the said situation, Comprising: It aims at providing the solar cell using the compound semiconductor thin film with which surface roughness was reduced, its manufacturing method, and this compound semiconductor thin film.

  In order to solve the above problems, a first aspect of the present invention includes a laminate including a lower layer containing fine particles of a I-II-IV-VI group compound and an upper layer for flattening the surface of the lower layer. A compound semiconductor thin film is provided.

It is preferable that the forbidden band width Eg _(lower) of the _lower layer and the forbidden band width Eg _(upper) of the _upper layer satisfy the relationship represented by the following formula (1).

0.00eV ≦ (Eg _(upper) −Eg _(lower) ) ≦ 0.30 eV (1)
It is preferable that the thickness T _{(upper) of the lower} layer and the thickness T _{(lower) of the} upper layer satisfy the relationship represented by the following formula (2).

0.01 ≦ T _(upper) / T _(lower) ≦ 1.00 (2)
The compound semiconductor thin film preferably has an average roughness Ra on the outermost surface of 200 nm or less.

  The average particle diameter of the fine particles is preferably 1 nm or more and 2000 nm or less.

The fine particles may have a composition represented by Cu _2-x Zn _{1 + y} SnS _z Se _4-z (0 ≦ x ≦ 1, 0 ≦ y ≦ 1, 0 ≦ z ≦ 4).

The fine particles include first fine particles having a composition represented by Cu _{2-x Sy} Se _2-y (0 ≦ x ≦ 1, 0 ≦ y ≦ 2), Zn _{2-x Sy} Se _2-y ( Second fine particles having a composition represented by 0 ≦ x ≦ 1, 0 ≦ y ≦ 2), and Sn _2−x S _y Se _2−y (0 ≦ x ≦ 1, 0 ≦ y ≦ 2) And third fine particles having a composition.

In a second aspect of the present invention, a step of applying a lower layer forming ink containing fine particles of a I-II-IV-VI group compound semiconductor on a substrate and heat-treating to form a lower layer;
A step of applying an upper layer forming ink on the lower layer and heat-treating to form an upper layer to obtain a laminate including a lower layer containing fine particles and an upper layer covering the surface. .

According to a third aspect of the present invention, a step of applying a lower layer forming ink containing fine particles of an I-II-IV-VI group compound semiconductor on a substrate to form a lower layer precursor;
A step of applying an upper layer forming ink on the lower layer precursor to form an upper layer precursor;
And heat-treating the upper layer precursor together with the lower layer precursor to obtain a laminate including a lower layer containing fine particles and an upper layer covering the surface.

  According to a fourth aspect of the present invention, there is provided a solar cell comprising a light absorption layer including the above-described compound semiconductor thin film.

  ADVANTAGE OF THE INVENTION According to this invention, the compound semiconductor thin film with which surface roughness was reduced, its manufacturing method, and the solar cell using this compound semiconductor thin film are provided.

It is a vertical side view which shows typically the structure of the solar cell which concerns on one Embodiment.

  Embodiments of the present invention will be described below with reference to the drawings.

  The compound semiconductor thin film concerning one Embodiment is used suitably as a light absorption layer of a solar cell. FIG. 1 is a longitudinal side view showing a configuration of a solar cell according to an embodiment.

  The illustrated solar cell 100 includes a substrate 101, a back electrode 102, a light absorption layer 105, a buffer layer 106, an i layer 107, an n layer 108, and a surface electrode 109 that are sequentially provided thereon.

  As the substrate 101, for example, soda lime glass, a metal plate, or a plastic film can be used. As the back electrode 102, a metal such as molybdenum (Mo), nickel (Ni), or copper (Cu) can be used. Further, a carbon-based electrode such as carbon and graphene, or a transparent conductive film such as ITO and ZnO may be used as the back electrode 101.

  The light absorption layer 105 is composed of the compound semiconductor thin film of the present embodiment, and includes a lower layer 103 containing fine particles and an upper layer 104 that flattens the surface of the lower layer 103. Each of the lower layer 103 and the upper layer 104 has a composition of a group I-II-IV-VI compound, and the lower layer and the upper layer will be described in detail below.

A buffer layer 106, an i layer 107, and an n layer 108 are sequentially formed on the light absorption layer 105. As the buffer layer 106, for example, CdS, Zn (S, O, OH), or In ₂ S ₃ can be used. As the i layer 107, for example, a metal oxide such as ZnO can be used. As the n layer 108, for example, ZnO to which Al, Ga, B, or the like is added can be used.

  As the surface electrode 109 on the n-layer 108, for example, a metal such as Al or Ag can be used. Alternatively, carbon-based electrodes such as carbon and graphene, or transparent conductive films such as ITO and ZnO may be used.

Although not shown, an antireflection film can be formed on the n layer 108. The antireflection film has a role of suppressing reflection of light and absorbing more light by the light absorption layer. The antireflection film can be formed with a thickness of about 100 nm using, for example, magnesium fluoride (MgF ₂ ).

In this embodiment, the I-II-IV-VI group compound has a basic composition of Cu ₂ ZnSn (S, Se) ₄ , for example, Cu _2−x Zn _{1 + y} SnS _z Se _4−z ( 0 ≦ x ≦ 1, 0 ≦ y ≦ 1, 0 ≦ z ≦ 4). Such compounds containing Cu (Group IB), Zn (Group IIB), Sn (Group IVB), S (Group VIB) and / or Se (Group VIB) are generally referred to as CZTS compounds.

The lower layer 103 containing fine particles has a composition of a I-II-IV-VI group compound. The lower layer 103 contains, for example, fine particles having a composition represented by Cu _2-x Zn _{1 + y} SnS _z Se _4-z (0 ≦ x ≦ 1, 0 ≦ y ≦ 1, 0 ≦ z ≦ 4). The By including such one kind of fine particles, the lower layer 103 having the composition of the I-II-IV-VI group compound can be obtained.

Or it is good also as a composition of the I-II-IV-VI group compound as the whole lower layer 103 by using combining the several fine particle from which a composition differs. For example, the first fine particles having a composition represented by Cu _2-x S _y Se _2-y (0 ≦ x ≦ 1, 0 ≦ y ≦ 2) and Zn _2-x S _y Se _2-y (0 ≦ a second fine particle having a composition represented by x ≦ 1, 0 ≦ y ≦ 2) and a third fine particle represented by Sn _2−x S _y Se _2−y (0 ≦ x ≦ 1, 0 ≦ y ≦ 2). Fine particles can be used. By including the fine particles having these three kinds of compositions, the entire lower layer 103 has a composition of a group I-II-IV-VI compound semiconductor.

  In both cases of using one kind of fine particles and a plurality of kinds of fine particles, the lower layer 103 as a whole has a composition (number of moles) that satisfies 0.6 ≦ Cu / (Zn + Sn) ≦ 0.99, It is preferable that these elements are contained. If the number of moles of Cu, Zn and Zn contained in the lower layer 103 is within such a range, the carrier concentration becomes an appropriate value, and the resistance of the compound thin film becomes too low due to the formation of semimetals such as CuS and CuSe. Nor. For this reason, a decrease in photoelectric conversion efficiency is avoided. The value of Cu / (Zn + Sn) is more preferably 0.8 or more and 0.9 or less.

  The average particle size of the fine particles contained in the lower layer 103 is desirably 1 nm or more and 2000 nm or less. If the average particle diameter is within such a range, the surface roughness of the lower layer 103 will not be too large, and the aggregation of the fine particles will not easily occur. The lower limit of the average particle size of the fine particles is more preferably 10 nm or more, and further preferably 20 nm or more. On the other hand, the upper limit of the average particle size of the fine particles is more preferably 500 nm or less. The average particle diameter of the fine particles can be obtained by averaging the shortest diameters of the semiconductor fine particles observed using SEM (scanning electron microscope) or TEM (transmission electron microscope).

  The lower layer 103 can be formed using an ink for lower layer formation prepared by dispersing fine particles having a predetermined composition in an organic solvent. The organic solvent is not particularly limited, and can be selected from, for example, alcohols, ethers, esters, aliphatic hydrocarbons, alicyclic hydrocarbons, and aromatic hydrocarbons. Preferred organic solvents are alcohols having less than 10 carbon atoms such as methanol, ethanol and butanol, diethyl ether, pentane, hexane, cyclohexane and toluene, with methanol, pyridine, toluene and hexane being particularly preferred.

  The lower layer forming ink may contain a dispersant in order to enhance the dispersibility of the fine particles in the organic solvent. Examples of the dispersant include thiols, selenols, and alcohols having 10 or more carbon atoms.

  When the binder is contained in the lower layer forming ink, a denser semiconductor thin film can be formed. Examples of the binder include Se particles, S particles, Se compounds, and S compounds. Also, sodium sulfide, sodium selenide, potassium selenide, sodium selenate, thiosulfate, and the like can be used. The concentration of the S compound in the solid organic solvent such as thiourea is not particularly limited, but a desired effect can be obtained if it is about 1 to 20% by mass.

  In forming the lower layer 103 of the compound semiconductor stack, first, a lower layer forming ink is applied or printed on a substrate to form a coating film. Examples of the ink application method include a doctor method, a spin coating method, a slit coating method, and a spray method. Examples of the printing method include a gravure printing method, a screen printing method, a reverse offset printing method, and a relief printing method. Law.

  The coating film containing the fine particles is dried to remove the organic solvent, thereby forming a lower layer precursor. Then, the lower layer 103 is formed by crystallizing the compound semiconductor by heat treatment. The heat treatment may be performed by rapid thermal annealing (RTA) as well as annealing by a heating furnace.

  In order to crystallize the compound semiconductor, the heat treatment temperature here is preferably 350 ° C. or higher. In the case where a glass substrate is used as the substrate, it is necessary to perform heat treatment at a temperature that the glass substrate can withstand. Specifically, the heat treatment temperature in this case is preferably 650 ° C. or lower, and more preferably 550 ° C. or lower.

The heat treatment atmosphere can be selected from a nitrogen atmosphere, argon atmosphere, forming gas atmosphere, hydrogen atmosphere, Se gas atmosphere, S gas atmosphere, H ₂ Se gas atmosphere, H ₂ S gas atmosphere, and the like.

  The thickness of the lower layer 103 thus formed is determined depending on the particle size of the contained fine particles, but can be about 0.5 to 10 μm. The lower layer 103 is preferably about 2 μm thick, for example.

  The lower layer 103 can be produced without using an expensive vacuum apparatus, and the distribution of each element in the surface obtained is uniform. However, since the lower layer 103 contains the fine particles as described above, the surface of the lower layer 103 does not become flat but has irregularities. In the present embodiment, the upper layer 104 for flattening the surface is formed on the lower layer 103, and the light absorption layer 105 is configured by a laminate of the lower layer 103 and the upper layer 104. As a result of improving the flatness of the interface between the light absorption layer 105 and the buffer layer 106 provided thereon, the leakage current is reduced.

  The upper layer 104 can have a composition of, for example, a group I-II-IV-VI compound. In this case, it may be the same as or different from the composition of the I-II-IV-VI group compound in the upper layer 103 described above. Since the purpose is to improve the flatness of the surface of the lower layer 103, it is desired that the upper layer 104 does not contain fine particles, but within a range that does not substantially affect the surface roughness of the light absorption layer 105. For example, mixing of fine particles is allowed.

  The upper layer 104 can be formed, for example, by applying an upper layer forming ink containing a copper raw material, a zinc raw material, a tin raw material, and an organic solvent, and then performing a heat treatment in an S / Se atmosphere. The copper raw material can be selected from organic copper compounds such as copper acetate and copper acetate hydrate, for example, inorganic copper compounds such as copper chloride, copper iodide, copper sulfate, and copper nitrate. The zinc raw material can be selected from organic zinc compounds such as zinc acetate and zinc acetate hydrate, for example, inorganic zinc compounds such as zinc chloride, zinc iodide, zinc sulfate, and zinc nitrate. The tin raw material can be selected from organic tin compounds such as tin acetate and tin acetate hydrate, for example, inorganic tin compounds such as tin chloride, tin iodide, tin sulfate, and tin nitrate.

  The upper layer 104 of the compound semiconductor laminate is formed by directly dissolving copper sulfide / copper selenide, tin sulfide / tin selenide, zinc, and sulfur / selenium in a reducing agent such as hydrazine or hydrazine hydrate. Ink may be prepared.

The upper layer forming ink can be applied or printed by the same method as in the case of the lower layer forming ink. The organic solvent is removed from the coating film containing the upper layer forming ink to obtain the upper layer precursor. Furthermore, an upper layer precursor can be formed by a metal plating method using a solution containing CuSO ₄ , ZnSO ₄ , SnCl ₂ , and C ₆ H ₅ O ₇ Na ₃ . After forming the upper layer precursor by metal plating, the S / Se element can be added in a heat treatment atmosphere.

  As in the case of the lower layer, in the case of the upper layer, heat treatment is performed to crystallize the compound semiconductor. The heat treatment can be performed at the temperature and atmosphere as described above.

  In some cases, after forming the lower layer precursor, the upper layer precursor may be formed without heat treatment, and the lower and upper compound semiconductors may be crystallized by a single heat treatment.

The compound semiconductor thin film of the present embodiment is constituted by the lower layer containing fine particles and the upper layer provided thereon. In the present embodiment, the forbidden band width Fg _(lower) of the _lower layer 103 containing fine particles and the forbidden band width Eg _(upper) of the upper layer 104 that flattens the surface have a relationship represented by the following formula (1). It is preferable to satisfy.

0.00eV ≤ Eg _(upper) -Eg _(lower) ≤ 0.30 eV (1)
For example, it is known that the Eg of CuZnSnSe is about 1.0 eV, and the Eg of CuZnSnS is about 1.5 eV. For example, when the upper layer 104 is made of an I-II-IV-VI group compound and the composition thereof is the same as the composition of the I-II-IV-VI group compound in the lower layer 103, Eg _(upper) -Eg _(lower The value of ₎ is zero.

For example, the content of S in the upper layer 104 can be adjusted to control the value of (Eg _(upper) −Eg _(lower) ). By providing the Eg slope on the surface side, the open circuit voltage is improved, and the energy conversion efficiency is increased. If Eg _(upper) −Eg _(lower) is 0.30 eV or less, the Eg slope does not become too large. In addition, there is no possibility that the energy conversion efficiency is lowered due to the generation of collected carriers.

The thickness T _(upper) thick T _(lower) and the upper layer 104 of the lower layer 103, it is preferable to satisfy the relationship represented by the following formula (2). The thickness of the lower layer 103 can be controlled, for example, by adjusting the distance between the blade and the substrate in the doctor blade method or adjusting the rotation speed of the spin coating. The thickness of the upper layer 104 can be controlled, for example, by adjusting the distance between the blade and the substrate in the doctor blade method or adjusting the rotation speed of the spin coating.

  The thickness of each layer can be measured by, for example, a stylus type surface shape measuring device DEK-TAK. Even if the composition of the I-II-IV-VI group compound in the upper layer 104 is the same as the composition of the I-II-IV-VI group compound in the lower layer 103, a scanning electron microscope (SEM) It is possible to determine the thickness of each layer by confirming the boundary between the lower layer 103 and the upper layer 104 by observing using a transmission electron microscope (TEM).

0.01 ≦ T _(upper) / T _(lower) ≦ 1.00 (2)
If the value of (T _(upper) / T _(lower) ) is within such a range, it will be avoided that the Eg slope distance becomes a barrier to carrier collection that occurs, and the energy conversion efficiency may decrease. Absent. In addition, since the upper layer 104 can sufficiently cover the unevenness of the surface of the lower layer 103, the light absorption layer 105 having a flat surface can be obtained.

  The light absorption layer 105 preferably has an average roughness Ra of the outermost surface measured with a scanning range of 5 μm using an atomic force microscope AFM of 200 nm or less. When the average roughness of the outermost surface measured in this way is 200 nm or less, a shunt path is not easily generated, so that a leakage current hardly occurs. Therefore, the possibility that the energy conversion efficiency is reduced is reduced. The average roughness of the outermost surface of the light absorption layer 105 is more preferably 100 nm or less, and most preferably 50 nm or less.

  In the solar cell of this embodiment, since the light absorption layer is constituted by the laminate including the lower layer containing fine particles and the upper layer for flattening the surface, the surface roughness of the light absorption layer is reduced. In such a solar cell of this embodiment, the leakage current is reduced and the energy conversion efficiency can be increased.

The above-described method using the lower layer forming ink and the upper layer forming ink can also be applied to the production of a compound semiconductor thin film having a general composition. Each ink may be prepared in an appropriate composition according to the composition of the target compound semiconductor. For _{example, Cu x In y Ga 1-} y Se z S 2-z (0 ≦ x ≦ 1,0 ≦ y ≦ 1,0 ≦ z ≦ 2) CIGS compound having a composition represented by the semiconductor, and Ag _x an In _{y Ga 1-y Se z S 2} -z (0 ≦ x ≦ 1,0 ≦ y ≦ 1,0 ≦ z ≦ 2) CuInTe such AIGS compound semiconductor having a composition represented by _2. For example, in the case of a CIGS solar cell, a compound including a lower layer containing CuInSe ₂ (Eg = 1.04 eV) fine particles and an upper layer of CuIn _0.7 Ga _0.3 Se ₂ (Eg = 1.23 eV) for flattening the surface roughness of the lower layer. A semiconductor thin film can be comprised.

  EXAMPLES Hereinafter, although this invention is demonstrated in detail based on an Example, this invention is not limited to this Example.

Example 1
(Production of Cu-Zn-Sn-Se-S fine particles)
A first solution is prepared by dissolving CuI, ZnI ₂ , and SnI ₂ in pyridine so that the molar ratio of Cu: Zn: Sn is 1.5: 1.5: 1.25. In addition, a second solution is prepared by dissolving Na ₂ Se and Na ₂ S in methanol so that the molar ratio of Se: S is 2: 2.

  The first solution and the second solution are mixed to obtain a mixed solution having a Cu: Zn: Sn: Se: S molar ratio of 2.5: 1.5: 1.25: 2: 2. This mixed solution is reacted at 0 ° C. in an inert gas atmosphere to produce Cu—Zn—Sn—Se—S fine particles. It is observed with a scanning microscope that the average particle diameter of the Cu—Zn—Sn—Se—S fine particles thus obtained is 80 nm. The reaction solution is filtered and washed with methanol, and then the obtained Cu—Zn—Sn—Se—S fine particles are dispersed in a mixed solution of pyridine and methanol to obtain a Cu—Zn—Sn—Se—S fine particle dispersion. Get.

(Preparation of lower layer forming ink)
Thiourea as a binder is added to the Cu-Zn-Sn-Se-S fine particle dispersion so that the mass ratio of Cu-Zn-Sn-Se-S fine particles and thiourea is 3: 2, and the mixture is added. obtain. Methanol is further added to this mixture to prepare a lower layer forming ink having a solid content of 5% by mass.

(Preparation of upper layer forming ink)
An ink for forming an upper layer is prepared by dissolving copper acetate hydrate, zinc acetate dihydrate, and tin chloride dihydrate in a mixed solution of 2-methoxyethanol and monoethanolamine having a volume ratio of 10: 1. To prepare. In the upper layer forming ink, the concentration of copper acetate hydrate is 0.875M, the concentration of zinc acetate dihydrate is 0.4375M, and the concentration of tin chloride dihydrate is 0.4375M. is there. The S / Se element is added from the heat treatment atmosphere.

  A solar cell having the structure shown in FIG. 1 is manufactured using the lower layer forming ink and the upper layer forming ink prepared as described above.

(Formation of back electrode 102)
Soda lime glass is prepared as the substrate 101, and the back electrode 102 made of the Mo layer is formed by sputtering.

(Formation of the light absorption layer 105)
A lower layer forming ink is applied onto the back electrode 102 by a doctor method, and the solvent is evaporated in an oven at 250 ° C. to form a lower layer precursor. Furthermore, by heating at 500 ° C. for 30 minutes in a selenium atmosphere, the lower layer 103 of the compound semiconductor thin film having a thickness of 2 μm is obtained. The surface of the lower layer 103 is not flat, and there are irregularities due to Cu—Zn—Sn—Se—S fine particles.

  An upper layer forming ink is applied onto the lower layer 103 by a doctor method, and the solvent is evaporated in an oven at 250 ° C. to obtain a lower layer precursor. By heating for 30 minutes in an atmosphere of 500 ° C. containing selenium and sulfur, the upper layer 104 of the compound semiconductor thin film having a thickness of 0.2 μm is formed.

  A light absorption layer 105 is configured by a laminate of the lower layer 103 and the upper layer 104 provided thereon. Since the upper layer 104 exists, the surface roughness of the light absorption layer 105 is smaller than that of the lower layer 103.

(Formation of buffer layer 106)
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) is heated to 70 ° C. A buffer layer 106 made of CdS having a film thickness of 50 nm is formed on the light absorption layer 105 by immersing the structure in which the light absorption layer 105 is formed in this liquid mixture.

(Formation of i layer 107)
An i layer 107 made of ZnO having a thickness of 50 nm is formed on the buffer layer 106 by MOCVD using diethyl zinc and water as raw materials.

(Formation of n layer 108)
An n layer 108 made of ZnO: B having a thickness of 1 μm is formed on the i layer 107 by MOCVD using diethyl zinc, water, and diborane as raw materials.

(Formation of surface electrode 109)
A surface electrode 109 made of Al having a thickness of 0.3 μm is formed on the n layer 108 by vapor deposition.

  As described above, the CZTS solar cell of Example 1 is obtained. Since the solar cell of Example 1 has the compound semiconductor thin film whose surface roughness is reduced as the light absorption layer 105, a decrease in performance due to the surface roughness is avoided.

(Example 2)
Except that the upper layer forming ink is applied on the lower layer precursor from which the solvent has been removed to form the lower layer precursor, and the lower layer precursor and the upper layer precursor are collectively heat-treated, as in Example 1. The light absorption layer 105 is formed. In this heat treatment, heating is performed at 500 ° C. in a selenium atmosphere for 30 minutes and then in a sulfur atmosphere for 10 minutes.

  Furthermore, the buffer layer, i layer, n layer, and surface electrode are sequentially formed in the same manner as in Example 1 to obtain the CZTS solar cell of Example 2. Since the solar cell of Example 2 has the compound semiconductor thin film with reduced surface roughness as the light absorption layer 105, a decrease in performance due to the surface roughness is avoided.

(Comparative example)
Except not forming the upper layer 104, it carries out similarly to Example 1, and obtains the CZTS solar cell of a comparative example. In the solar cell of the comparative example, since the light absorption layer is formed by the layer (lower layer) containing fine particles, the surface roughness of the light absorption layer is large. The solar cell of the comparative example is accompanied by a decrease in performance due to this surface roughness.

DESCRIPTION OF SYMBOLS 100 ... Solar cell; 101 ... Glass substrate; 102 ... Back surface electrode 103 ... Compound semiconductor lower layer; 104 ... Compound semiconductor upper layer; 105 ... Light absorption layer 106 ... Buffer layer; 107 ... I layer; .

Claims (10)

A lower layer containing fine particles of an I-II-IV-VI group compound;
A compound semiconductor thin film comprising a laminate including an upper layer for planarizing a surface of the lower layer. 2. The compound semiconductor thin film according to claim 1, wherein the forbidden band width Eg _(lower) of the _lower layer and the forbidden band width Eg _(upper) of the _upper layer satisfy a relationship represented by the following formula (1).
0.00eV ≦ (Eg _(upper) −Eg _(lower) ) ≦ 0.30 eV (1) 3. The compound semiconductor thin film according to claim 1, wherein a thickness T _{(upper) of the lower} layer and a thickness T _{(lower) of the} upper layer satisfy a relationship represented by the following formula (2).
0.01 ≦ T _(upper) / T _(lower) ≦ 1.00 (2)   The compound semiconductor thin film according to claim 1, wherein the compound semiconductor thin film has an average roughness Ra of an outermost surface of 200 nm or less.   5. The compound semiconductor thin film according to claim 1, wherein an average particle diameter of the fine particles is 1 nm or more and 2000 nm or less. The fine particles have a composition represented by Cu _2-x Zn _{1 + y} SnS _z Se _4-z (0 ≦ x ≦ 1, 0 ≦ y ≦ 1, 0 ≦ z ≦ 4). The compound semiconductor thin film of any one of 1-5. The fine particles include first fine particles having a composition represented by Cu _{2-x Sy} Se _2-y (0 ≦ x ≦ 1, 0 ≦ y ≦ 2), Zn _{2-x Sy} Se _2-y ( Second fine particles having a composition represented by 0 ≦ x ≦ 1, 0 ≦ y ≦ 2), and Sn _2−x S _y Se _2−y (0 ≦ x ≦ 1, 0 ≦ y ≦ 2) The compound semiconductor thin film according to claim 1, further comprising third fine particles having a composition. It is a manufacturing method of the compound semiconductor thin film according to any one of claims 1 to 6,
Applying a lower layer forming ink containing fine particles of an I-II-IV-VI group compound semiconductor on a substrate and heat-treating to form a lower layer;
A step of applying an upper layer forming ink on the lower layer and heat-treating to form an upper layer to obtain a laminate including a lower layer containing fine particles and an upper layer covering the surface. Method. It is a manufacturing method of the compound semiconductor thin film according to any one of claims 1 to 6,
Applying a lower layer forming ink containing fine particles of an I-II-IV-VI group compound semiconductor on a substrate to form a lower layer precursor;
A step of applying an upper layer forming ink on the lower layer precursor to form an upper layer precursor;
And a step of heat-treating the upper layer precursor together with the lower layer precursor to obtain a laminate including a lower layer containing fine particles and an upper layer covering the surface.   A solar cell comprising a light absorption layer including the compound semiconductor thin film according to claim 1.

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