JP2012124286A - Photoelectric element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To achieve both an improvement of quantum efficiency in a short wavelength region and a reduction in the amount of Cd while maintaining high power generation efficiency, 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 CdS and 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). In a photoelectric device using a sulfide-based compound semiconductor such as CZTS as a light absorption layer, a certain level of power generation efficiency can be achieved by providing a CdS film as a buffer layer.

JP 2009-26891 A

  However, a photoelectric device combining a light absorption layer containing a sulfide compound semiconductor such as CZTS and a buffer layer containing CdS has low quantum efficiency in a short wavelength region of 500 nm or less, and further improvement is desired in this respect. It is.

  Therefore, the present invention aims to further improve the quantum efficiency in the short wavelength region and to reduce the amount of Cd in a photoelectric device in which a light absorption layer containing a sulfide compound semiconductor such as CZTS and a buffer layer containing CdS are combined. The purpose is to reduce.

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 sulfide compound semiconductor film containing Cu, Zn, Sn, and S, and the buffer layer contains CdS and In (OH) ₃ .

As a result of intensive studies by the present inventors, in a photoelectric element in which a light absorption layer containing a sulfide compound semiconductor such as CZTS and a buffer layer containing CdS are combined, the buffer layer further contains In (OH) _3. As a result, the quantum efficiency in the short wavelength region was improved. When the CdS film as the buffer layer is thinned, the light transmittance is increased, so that it is expected that the quantum efficiency in the short wavelength region is improved. However, when the CdS film is made thinner, the open circuit voltage (V _oc ) and the _fill factor (FF) are decreased, and as a result, the power generation efficiency tends to decrease. On the other hand, according to the present invention, it is possible to improve the quantum efficiency in the short wavelength region and reduce the amount of Cd while maintaining high power generation efficiency.

The buffer layer preferably has a first layer that is a CdS film formed of CdS and a second layer containing In (OH) ₃ . In this case, it is more preferable that the second layer is formed on the window layer side. Alternatively, the buffer layer may be a layer that can be formed by depositing CdS on the light absorption layer and then depositing In (OH) ₃ by the CBD method. In this way, a buffer layer having a region containing In (OH) ₃ at a higher concentration on the window layer side than on the light absorption layer side can be formed by the two-stage film formation.

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, CdS is formed on a light absorption layer which is a film of a sulfide compound semiconductor containing Cu, Zn, Sn and S, and then In (OH) ₃ is formed by CBD. Thus, a step of forming a buffer layer containing CdS and In (OH) ₃ and a step of forming a window layer on the opposite side of the buffer layer from the light absorption layer are provided.

  According to the manufacturing method of the present invention, while using a sulfide compound semiconductor such as CZTS for the light absorption layer, it is possible to obtain a photoelectric element that exhibits high quantum efficiency in a short wavelength region and to reduce the amount of Cd. it can.

  According to the present invention, it is possible to further improve the quantum efficiency in a short wavelength region in a photoelectric device in which a light absorption layer including a sulfide compound semiconductor such as CZTS and a buffer layer including CdS are combined. The amount of Cd can be reduced.

It is a perspective view which shows one Embodiment of a photoelectric element. It is a graph which shows the relationship between quantum efficiency and a wavelength.

  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 CdS and In (OH) ₃ . In order to achieve higher power generation efficiency, the molar ratio of In to Cd is preferably 0.05 to 1 in the buffer layer 18. This molar ratio can be quantitatively measured by ICP.

More specifically, the buffer layer 18 includes a first layer 1 mainly composed of CdS, and a second layer 2 including In (OH) ₃ provided on the window layer 20 side of the first layer 1. Have. The first layer 1 is a CdS film substantially free of In (OH) ₃ and formed from CdS. The second layer 2 only needs to have a region containing In (OH) ₃ at a higher concentration than that of the first layer 1, and usually a region formed by intrusion of In (OH) ₃ into the CdS film. Including. That is, the second layer 2 often includes a region containing CdS. 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.

The buffer layer 18 having the first layer 1 and the second layer 2 is formed, for example, by forming CdS on the light absorption layer 16 to form a CdS film, and then using In (OH) ₃ on the CdS film by the CBD method ( It can be formed by a two-stage film formation method in which a film is formed by a chemical bath deposition method.

  The CdS film can be formed by, for example, a method of immersing the light absorption layer 16 in an alkaline reaction solution for CBD containing cadmium salt (such as cadmium acetate), thiourea, and water in which these are dissolved. In this case, the formed CdS film (first layer 1) is preferably dried by heating before the second layer 2 is formed.

  The thickness of the CdS film is preferably 5 nm or more, more preferably 15 nm or more. Further, the thickness of the CdS film is preferably 40 nm or less, more preferably 30 nm or less. By controlling the thickness of the CdS film within these numerical ranges, higher power generation efficiency can be obtained. The thickness of the CdS film can be adjusted by the immersion time for film formation. The immersion time for forming a CdS film having a suitable thickness is usually about 3 to 20 minutes.

The second layer 2 is formed by immersing a CdS film in a reaction solution for CBD containing, for example, thiourea, an indium salt (indium chloride, etc.) and water in which these are dissolved, so that In (OH) ₃ is deposited on the CdS film. It can be formed by a CBD method of depositing. The reaction solution for CBD is heated to about 40 to 85 ° C. if necessary. According to this method, at least a part of the deposited In (OH) ₃ often penetrates into the CdS 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 preferably further contains zinc chloride. In the reaction solution, the thiourea concentration is preferably 0.03 to 0.3 mol / L, the indium concentration is preferably 0.001 to 0.1 mol / L, and the zinc chloride concentration is preferably 0.001 to 0. .1 mol / L.

  The thickness of the second layer 2 is preferably 0.1 to 10 nm. The immersion time for forming the second layer 2 having a suitable thickness is usually 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 not necessarily required depending on the form of the optoelectronic device. 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 ZnMgO film, 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.

(Example)
(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) A CZTS membrane was prepared by reacting a CBD reaction solution (80 ° C., pH 12) containing cadmium acetate (3.60 mmol / L), ammonia (2.96 mol / L), and thiourea (0.1 mol / L). (At 25 ° C.)) for 9 minutes, and a CdS film was formed on the light absorption layer 16 while stirring with a stirrer. The reaction solution was initially colorless and transparent, but gradually turned yellow. The formed CdS film was washed with ethanol and then dried by heating at 200 ° C. for 10 minutes.
Reaction solution (pH 3) 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) 4 and 25 ° C.) in a beaker, and while heating the beaker with a water bath at 64 ° C., the laminate containing the CdS film is immersed in the beaker for 50 minutes, and the film formation of In (OH) ₃ proceeds. I let you. Thereafter, the laminate was dried by heating at 200 ° C. for 20 minutes.
The buffer layer 18 mainly composed of the first layer containing CdS and the 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 CdS film (thickness 80 nm) was formed as a buffer layer, and In (OH) ₃ was not formed. Except for this, a solar cell was fabricated in the same procedure as in the example.

(Comparative Example 2)
A CdS film (thickness 40 nm) was formed as a buffer layer, and In (OH) ₃ was not formed. Except for this, a solar cell was fabricated in the same procedure as in the example.

2. Composition analysis of buffer layer Using the sample solution obtained by eluting the buffer layer formed in the example, dielectric bond plasma issuance analysis was performed to measure the amount (mg) of each element contained in the buffer layer. The measurement results are shown in Table 1. From this measurement result, the molar ratio of In to Cd in the buffer layer is calculated to be 0.4. The sample solution contained Cu, Zn, Sn and S derived from the CZTS film and Mo derived from the Mo film.

3. Evaluation of Quantum Efficiency Regarding the solar cells of Examples and Comparative Example 1, the quantum efficiency in the wavelength region of 300 to 1000 nm was measured. The measurement was performed using CEP-2000 manufactured by Spectrometer Co., Ltd. FIG. 2 is a graph showing the relationship between quantum efficiency and wavelength. As shown in FIG. 2, the solar cell of the example formed by forming In (OH) ₃ on the CdS film is more than the comparative example in which In (OH) ₃ was not formed. The quantum efficiency in a short wavelength region of 500 nm or less was significantly improved.

4). 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. From the measurement results, the short-circuit current density (J _sc ), the open-circuit voltage (V _oc ), and the fill factor (FF) are obtained, and the power generation efficiency (η) is further calculated as η = V _oc · J _sc · FF / (pseudo sunlight Calculated from energy per unit area). The obtained results are shown in Table 2.

The solar cell of Comparative Example 2 that does not contain In (OH) ₃ and has the same 40 nm-thickness buffer layer as the Example has an open end compared to the solar cell of Comparative Example 1 that has a 80 nm-thick buffer layer. The voltage (V _oc ) and the fill factor (FF) were reduced, and as a result, the power generation efficiency was greatly reduced. On the other hand, the solar cell of the example maintained the same or higher power generation efficiency than the solar cell of Comparative Example 1. From this experimental result, it was confirmed that according to the present invention, it is possible to improve the quantum efficiency in the short wavelength region while maintaining high power generation efficiency and to reduce the amount of Cd.

  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;
The photoelectric element in which the buffer layer contains CdS and In (OH) ₃ . The photoelectric element according to claim 1, wherein the buffer layer has a first layer that is a CdS film formed of CdS and a second layer containing In (OH) ₃ .   The photoelectric device according to claim 2, wherein the second layer is formed closer to the window layer than the first layer. The photoelectric element according to claim 1, wherein the buffer layer is a layer that can be formed by depositing CdS on the light absorption layer and then depositing In (OH) ₃ by a CBD method. CdS is deposited on a light-absorbing layer that is a sulfide-based compound semiconductor film containing Cu, Zn, Sn, and S, and then In (OH) ₃ is deposited by the CBD method, so that CdS and In ( Forming a buffer layer comprising 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.

Images