JP2012124284A - Photoelectric element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain higher power generation efficiency without requiring a material containing Cd, in a photoelectric element in which a sulfide-based compound semiconductor such as CZTS is used in a light absorbing layer.SOLUTION: A photoelectric element 10 comprises a light absorbing layer 16 being a p-type semiconductor layer, a buffer layer 18, and a window layer 20. The light absorbing layer 16 is a film of a sulfide-based compound semiconductor containing Cu, Zn, Sn, and S. The buffer layer 18 contains InSand In(OH).

Description

  The present invention relates to a photoelectric element.

  A photoelectric element converts photon energy into an electrical signal (photoelectric conversion) through some physical phenomenon. Photoelectric elements are used for solar cells, photoconductive cells, photodiodes, phototransistors, and the like.

A solar cell is a kind of photoelectric element, and can efficiently convert light energy of sunlight into electric energy. A general solar cell has a structure in which a p-type semiconductor layer and an n-type semiconductor layer are joined. The p-type semiconductor used in solar cells, monocrystalline Si, polycrystalline Si, amorphous _{Si, GaAs, InP, CdTe,} CuIn 1-x Ga x Se 2 (CIGS), Cu 2 ZnSnS 4 (CZTS) Hitoshigachi It has been. Among these, chalcogenite-based compound semiconductors typified by CIGS and CZTS have a large light absorption coefficient, so that a thin film advantageous for cost reduction can be obtained. In particular, thin film solar cells using CIGS have high conversion efficiency, and conversion efficiency exceeding that of polycrystalline Si is also obtained. However, CIGS has a problem that it contains elements with high environmental impact and rare elements. On the other hand, CZTS has a band gap energy (1.4 to 1.5 eV) suitable for solar cells, does not contain elements with high environmental impact and rare elements, and is rich in material resources and can be manufactured at low cost. Recently, attention has been paid (for example, Patent Document 1).

JP 2009-26891 A

  However, photoelectric elements using sulfide compound semiconductors such as CZTS in the light absorption layer have lower power generation efficiency than conventional photoelectric elements using CIGS, and further improvement in this respect is necessary for practical use. It has been demanded. A certain amount of power generation efficiency can be achieved by combining a CdS film with a CZTS using a CdS film as a buffer layer, but industrially, it is desirable to use a material that does not contain Cd and has a high environmental load.

  Accordingly, an object of the present invention is to obtain higher power generation efficiency without requiring a material containing Cd in a photoelectric element using a sulfide compound semiconductor such as CZTS for a light absorption layer.

The present invention relates to a photoelectric device including a light absorption layer that is a p-type semiconductor layer, a buffer layer, and a window layer, and the light absorption layer, the buffer layer, and the window layer are provided in this order. In the photoelectric device according to the present invention, the light absorption layer is a film of a sulfide compound semiconductor containing Cu, Zn, Sn, and S, and the buffer layer contains In ₂ S ₃ and In (OH) ₃ .

As a result of intensive studies by the present inventors, in a photoelectric device using a sulfide compound semiconductor such as CZTS as a light absorption layer, it is further increased by providing a buffer layer containing In ₂ S ₃ and In (OH) _3. It became clear that power generation efficiency was obtained.

Since the conduction band bottom (CBM) is different between CIGS and CZTS, the combination of the light absorption layer of CZTS and the conventional buffer layer is a conduction band offset between the light absorption layer, the buffer layer, and the window layer. Whereas (CBO) was not adequately optimized, by providing a buffer layer containing In ₂ S ₃ and In (OH) ₃ , the conduction band offset was optimized, resulting in power generation The efficiency is thought to have improved.

Buffer layer, the first layer is _{an In} 2 _{S 3} film formed from _{an In} 2 _{S 3,} and a second layer comprising an In _{(OH) 3,} it is preferable to have a. In this case, it is more preferable that the second layer is formed closer to the window layer than the first layer. For example, the buffer layer may be a layer formed by depositing In ₂ S ₃ on the light absorption layer and then depositing In (OH) ₃ by the CBD method. By such two-stage film formation, a buffer layer having a region containing In (OH) ₃ at a relatively higher concentration on the window layer side than on the light absorption layer side can be formed.

Thus, the presence of a portion containing In (OH) ₃ at a high concentration in the buffer layer suppresses the leakage current, which is considered to contribute to the improvement of power generation efficiency.

In another aspect, the present invention relates to a method for manufacturing a photoelectric element. In the manufacturing method according to the present invention, an In ₂ S ₃ film is formed on a light absorption layer that is a film of a sulfide-based compound semiconductor containing Cu, Zn, Sn, and S, and then In (OH) ₃ is converted into a CBD method. Forming a buffer layer containing In ₂ S ₃ and In (OH) ₃ and forming a window layer on the opposite side of the buffer layer from the light absorption layer.

  According to the production method of the present invention, a photoelectric device having higher power generation efficiency can be obtained while using a sulfide-based compound semiconductor such as CZTS for the light absorption layer.

  According to the present invention, in a photoelectric device using a sulfide compound semiconductor such as CZTS for a light absorption layer, higher power generation efficiency can be obtained without requiring a material containing Cd.

It is a perspective view which shows one Embodiment of a photoelectric element.

  Hereinafter, preferred embodiments of the present invention will be described in detail. However, the present invention is not limited to the following embodiments.

  FIG. 1 is a perspective view showing an embodiment of a photoelectric element. 1 includes a substrate 12, a lower electrode 14 provided on the substrate 12, a light absorption layer 16 provided on the lower electrode 14, and a buffer layer provided on the light absorption layer 16. 18, a window layer 20 provided on the buffer layer 18, and an upper electrode 22 provided on the window layer 20. The light absorption layer 16, the buffer layer 18, and the window layer 20 are laminated in this order.

  The substrate 12 supports each member formed thereon. The substrate 12 may be a conductor or an insulator. The substrate 12 is preferably selected from quartz glass, non-alkali glass, and low alkali glass (LAG), and metals, semiconductors, oxides, sulfides, nitrides, carbides, silicides, carbon, or combinations thereof. It is a plate-shaped body of material.

The lower electrode 14 is formed of a material having high electrical conductivity and good adhesion to the substrate 12. The lower electrode 14 is formed of, for example, a material selected from Mo, In—Sn—O, In—Zn—O, ZnO: B, SnO ₂ : F, SnO ₂ : Sb, and TiO ₂ : Nb. The notation “ZnO: B” means ZnO doped with B. Others are the same. When a glass substrate is used as the substrate 12, Mo is preferable from the viewpoints of adhesion, electrical conductivity, incident light reflectance, and difficulty in sulfidation.

The light absorption layer 16 is a p-type semiconductor layer containing a sulfide compound semiconductor containing Cu, Zn, Sn, and S. As the sulfide compound semiconductor, Cu ₂ ZnSnS ₄ (CZTS) is preferable. Since CZTS has a large light absorption coefficient, if CZTS is used, the thickness of the light absorption layer 16 can be extremely reduced. The light absorption layer 16 is generally a thin film, but may be an aggregate of particles. The thickness of the light absorption layer 16 is preferably 0.3 μm to 2 μm.

  The ratio of Cu in CZTS is preferably slightly smaller than the stoichiometric composition because high conversion efficiency can be obtained. Specifically, Cu / (Zn + Sn) is preferably 0.69 to 0.99, more preferably 0.8 to 0.9 in terms of atomic ratio.

The buffer layer 18 includes In ₂ S ₃ and In (OH) ₃ . In order to achieve higher power generation efficiency, the molar ratio of In (OH) ₃ to In ₂ S ₃ in the buffer layer 18 is preferably 0.001 to 0.3.

More specifically, the buffer layer 18 includes a first layer 1 mainly composed of In ₂ S ₃ and a second layer including In (OH) ₃ provided on the window layer 20 side of the first layer 1. 2. The first layer 1 is substantially free of In _{(OH) 3,} which is _{an In} 2 _{S 3} film formed from _{an In} 2 _{S 3.} The second layer 2 only needs to have a region containing In (OH) ₃ at a higher concentration than the first layer 1 and is usually formed by intrusion of In (OH) ₃ into the In ₂ S ₃ film. Including That is, the second layer 2 often includes a region containing In ₂ S ₃ . Therefore, when the buffer layer is observed with a transmission electron microscope or the like, the boundary between the first layer 1 and the second layer 2 is not always clear.

Buffer layer 18 having a first layer 1 and the second layer 2 is, for example, after a film of _{an In} 2 _{S 3} on the light-absorbing layer 16 to form a _{an In} 2 _{S 3} film, _{an In} 2 _{S 3} film on the In (OH) ₃ can be formed by a two-stage film formation method in which a film is formed by a CBD method (Chemical Bath Deposition method).

The In ₂ S ₃ film may be formed by, for example, a method of immersing the light absorption layer 16 in an acidic reaction solution for CBD containing thioacetamide, indium salt (such as indium chloride) and water in which these are dissolved. it can. In this case, the formed In ₂ S ₃ film (first layer 1) is preferably dried by heating before the second layer 2 is formed.

The thickness of the In ₂ S ₃ film is preferably 10 nm or more, more preferably 30 nm or more. Further, the thickness of the In ₂ S ₃ film is preferably 120 nm or less, more preferably 90 nm or less. By controlling the thickness of the In ₂ S ₃ film within these numerical ranges, particularly high power generation efficiency can be obtained. The thickness of the In ₂ S ₃ film can be adjusted by the immersion time for film formation. The immersion time for forming an In ₂ S ₃ film having a suitable thickness is usually about 15 to 40 minutes.

The second layer 2 is formed by, for example, immersing an In ₂ S ₃ film in a reaction solution for CBD containing thiourea, an indium salt (such as indium chloride), and water in which these are dissolved, and thereby converting In (OH) ₃ into In It can be formed by the CBD method of depositing on the ₂ S ₃ film. The reaction solution is heated to about 50 to 85 ° C. if necessary. According to this method, at least a part of the deposited In (OH) ₃ often intrudes into the In ₂ S ₃ film.

  The pH of the reaction solution for forming the second layer 2 is preferably 2.5 to 4.5. In order to stably form a film having a certain thickness, the reaction solution for CBD preferably further contains zinc chloride. In the reaction solution for CBD, the concentration of thiourea is preferably 0.03 to 0.3 mol / L, the concentration of indium is preferably 0.001 to 0.1 mol / L, and the concentration of zinc chloride is preferably 0.00. 001 to 0.1 mol / L.

  The thickness of the second layer 2 is preferably 0.1 to 10 nm. For example, when the solution temperature is 60 ° C., the immersion time for forming the second layer 2 having a suitable thickness is about 10 to 100 minutes. After the immersion, the second layer 2 is preferably dried by heating.

  The window layer 20 has a low resistance and transmits most of light in the visible to near infrared region.

The window layer 20 includes, for example, Zn _x Mg _1-x O (ZMO): Ga (x is a numerical value greater than 0 and less than 1), ZnO: B, ZnO: Al (AZO), In—Sn—O, In It is a film composed of a semiconductor selected from —Zn—O, SnO ₂ : Sb, TiO ₂ : Nb, and ZnO: Ga (GZO).

  The upper electrode 22 is provided to efficiently extract the current collected by the window layer 20 to the outside. The upper electrode 22 is usually formed in a comb shape. The upper electrode 22 is not necessarily required depending on the form of the photoelectric element. Examples of the material of the upper electrode 22 include Al, Cu, Ag, and Au, and Al is preferable. The upper electrode 22 may be an alloy containing one or more metals selected from Al, Cu, Ag, and Au. Specific examples of such alloys include Al-Ti alloys, Al-Mg alloys, Al-Ni alloys, Cu-Ti alloys, Cu-Sn alloys, Cu-Zn alloys, Cu-Au alloys, and Ag-Ti. There are an alloy, an Ag—Sn alloy, an Ag—Zn alloy, an Ag—Au alloy, and the like.

  The photoelectric element according to this embodiment can be manufactured by, for example, a method of sequentially forming a lower electrode, a light absorption layer, a buffer layer, a window layer, and an upper electrode on a substrate. Each film can be formed by a method usually employed as a method for forming a thin film, such as a sputtering method, a vacuum deposition method, or a PLD method.

The CZTS film as the light absorption layer 16 is a precursor film in which Cu, Sn, and ZnS are laminated in a predetermined order on a substrate using a sputtering method, a vacuum evaporation method, a pulse laser deposition (PLD) method, or the like. And this precursor can be formed by sulfurization under an atmosphere containing H ₂ S (for example, in a mixed gas atmosphere of 5 to 20% by volume of H ₂ S and N ₂ atmosphere). The temperature during sulfidation is about 500 to 600 ° C. When producing the precursor, various CZTS films having different compositions can be formed by changing the thickness of Cu, Sn, or ZnS.

The ZMO: Ga film as the window layer 20 can be formed, for example, by sputtering using ZnO, MgO, and Ga ₂ O ₃ simultaneously as targets.

  As a method for forming various thin films other than sputtering, vacuum deposition, and PLD, for example, (1) a solution in which an organic metal or the like is dissolved is coated on the substrate 12 and dried in the air. There is a sol-gel + sulfurization method in which a degenerative reaction is caused to form a metal oxide thin film, and the light absorption layer 16 is formed by heat-treating the metal oxide thin film in a hydrogen sulfide atmosphere.

  The present invention is not limited to the embodiment described above, and various modifications can be made without departing from the gist of the present invention. For example, additional layers other than the substrate, the lower electrode, the light absorption layer, the buffer layer, the window layer, and the upper electrode may be provided between the layers. As additional layers, for example, an adhesive layer, a light scattering layer, and an antireflection layer may be provided. A solar cell module including a plurality of photoelectric elements connected in series or in parallel can also be configured.

  Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited to these examples.

1. Production of Solar Cell An evaluation solar cell comprising a photoelectric element having the same configuration as the photoelectric element 10 of FIG. 1 was produced according to the following procedure.

(Examples 1-5)
(1) A Mo film as the lower electrode 14 was formed on a low alkali glass (LAG) substrate by sputtering.
(2) A CZTS precursor film was formed on the Mo film by sputtering.
(3) CZTS film (light absorption layer 16) is formed from the precursor film by sulfiding treatment at 550 to 580 ° C. for 3 hours in a mixed gas atmosphere of 20 vol% H ₂ S and N _{2 at} atmospheric pressure. I let you.
(4) The CZTS film is formed from a CBD containing thioacetamide (0.3 mol / L), hydrochloric acid (0.009 mol / L), indium chloride (25 mmol / L), and acetic acid (0.3 mol / L). The In ₂ S ₃ film was formed on the light absorption layer 16 by dipping in the reaction solution (70 ° C., pH 1.8). The CBD bath was prepared by dissolving thioacetamide in 70 ° C. water, adding hydrochloric acid thereto, and further adding an acetic acid solution of indium chloride. After adding the acetic acid solution of indium chloride, the reaction solution became cloudy after about 7 minutes and gradually turned yellow. Immediately after the reaction solution turned yellow, the CZTS film was immersed in the reaction solution. As the In ₂ S ₃ film formation progressed, the reaction solution changed from yellow to orange. The formed In ₂ S ₃ film was dried by heating at 200 ° C. for 20 minutes. The film thickness of the In ₂ S ₃ film after drying was adjusted to 20 nm (Example 1), 27 nm (Example 2), 30 nm (Example 3), 43 nm (Example 4) or 81 nm (implemented) by adjusting the immersion time. Example 5). The film thickness was measured by cross-sectional observation with FE-SEM (HITACHI S-4800). It should be noted that the film thickness of less than 20 nm could not be measured by this FE-SEM.
Reaction solution for CBD containing thiourea (0.15 mol / L), indium chloride (0.01 mol / L), acetic acid (0.0016 mol / L) and zinc chloride (0.01 mol / L) (25 (° C.) was prepared in a beaker, a laminate including an In ₂ S ₃ film was immersed therein, and film formation of In (OH) ₃ was advanced while the beaker was heated in a water bath at 64 ° C. Thereafter, the laminate was dried by heating at 200 ° C. for 20 minutes.
A buffer layer 18 mainly composed of a first layer containing In ₂ S ₃ and a second layer containing In (OH) ₃ was formed by the above-described two-stage film formation by the CBD method.
(5) A ZnO: Al film (window layer 20) was formed on the buffer layer by sputtering.
(6) Twelve comb-shaped electrodes (Al films) arranged in a matrix were formed on the window layer 20 by sputtering.
(7) The window layer 20, the buffer layer 18 and the light absorption layer 16 other than the columnar part located under the 4 mm × 5 mm square surrounding one comb electrode are removed to expose the lower electrode 14. It was. On the lower electrode 14, twelve columnar laminates composed of a comb-shaped electrode, a window layer 20, a buffer layer 18, and a light absorption layer 16 were arranged in a matrix of 4 rows × 3 columns.
(8) A silver paste was applied to the exposed end surface of the lower electrode 14.

(Comparative Example 1)
A solar cell for evaluation was produced in the same procedure as in Example 4 except that In (OH) ₃ was not formed on the In ₂ S ₃ film when the buffer layer was formed.

(Comparative Example 2)
A solar cell was fabricated in the same procedure as in the example except that a CdS film (thickness 80 nm) was formed as a buffer layer.

2. Evaluation of voltage-current characteristics (IV measurement)
Each solar cell was subjected to IV measurement using air mass (AM) 1.5 and 1 SUN simulated sunlight. The open-circuit voltage V _{OC, the} short-circuit current density J _SC , and the fill factor FF were determined, and the power generation efficiency (η) was calculated from η = V _oc · J _sc · FF / (energy per unit area of simulated sunlight).
The obtained results are shown in Table 1.

The power generation efficiency of the solar cell of the example was significantly improved as compared with Comparative Example 1 in which In (OH) ₃ was not formed. In particular, Examples 3, 4, and 5 achieved higher power generation efficiency than Comparative Example 2 having a CdS film as a buffer layer.

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

  DESCRIPTION OF SYMBOLS 1 ... 1st layer, 2 ... 2nd layer, 10 ... Photoelectric element, 12 ... Board | substrate, 14 ... Lower electrode, 16 ... Light absorption layer, 18 ... Buffer layer, 20 ... Window layer, 22 ... Upper electrode.

Claims (5)

In a photoelectric device comprising a light absorption layer that is a p-type semiconductor layer, a buffer layer, and a window layer, wherein the light absorption layer, the buffer layer, and the window layer are provided in this order.
The light absorption layer is a film of a sulfide compound semiconductor containing Cu, Zn, Sn and S;
It said buffer layer comprises _{an In} 2 _{S 3} and In _{(OH) 3,} the photoelectric element. The buffer layer has a first layer is _{an In} 2 _{S 3} film formed from _{an In} 2 _{S 3,} and a second layer comprising an In _{(OH) 3,} the photoelectric element according to claim 1.   The photoelectric device according to claim 2, wherein the second layer is formed closer to the window layer than the first layer. 2. The buffer layer according to claim 1, wherein the buffer layer is a layer that can be formed by depositing In ₂ S ₃ on the light absorption layer and then depositing In (OH) ₃ by a CBD method. Photoelectric element. In ₂ S ₃ is formed on a light absorption layer that is a film of a sulfide compound semiconductor containing Cu, Zn, Sn, and S, and then In (OH) _{3 is formed} by the CBD method. Forming a buffer layer comprising ₂ S ₃ and In (OH) ₃ ;
Forming a window layer on the opposite side of the buffer layer from the light absorbing layer;
A method for manufacturing a photoelectric element.

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