JP2019003996A - Solar battery manufacture method and solar battery - Google Patents

Abstract

To provide a technology to improve solar battery efficiency.SOLUTION: A solar battery according to an embodiment of the present invention includes a silicon solar cell structure made of a silicon material, a transparent conductive layer laminated on the silicon solar cell structure and made of a transparent conductive material, and a compound solar cell structure laminated on the transparent conductive layer and composed of a compound material not including a silicon material.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a solar cell manufacturing method and a solar cell.

  In recent years, various types of multi-junction solar cells have been proposed in order to achieve high efficiency and low cost. For example, Non-Patent Document 1 proposes a hybrid multi-junction solar cell configured by bonding a silicon solar cell structure and a compound solar cell structure, as illustrated in FIG.

  FIG. 1 schematically illustrates the structure of a conventional solar cell 100 proposed in Non-Patent Document 1. The solar cell 100 includes a back electrode 111, a silicon solar cell structure 120, a compound solar cell structure 130, and a surface electrode 112 in order from the back surface 101 side to the front surface 102 side.

The silicon solar cell structure 120 is laminated on the back electrode 111 and configured using a silicon material. Specifically, the silicon solar cell structure 120 includes, in order from the back surface 101 side, a p ⁺ -type silicon layer 1211 containing a p-type impurity at a high concentration, a silicon layer 121, and an n ⁺ containing an n-type impurity at a high concentration. Type silicon layer 122. The silicon solar cell structure 120 constitutes one solar cell (subcell). A solar cell structure including an n ⁺ type semiconductor layer on the front side and a p ⁺ type semiconductor layer on the back side, such as the silicon solar cell structure 120, is hereinafter referred to as “n-on-p type”. . Since the silicon solar cell structure 120 is disposed on the back surface 101 side, it is also referred to as a “bottom cell”.

The compound solar cell structure 130 is configured by crystal-growing a predetermined structure made of a compound material not containing a silicon material on a compound semiconductor substrate, bonding it to the silicon solar cell structure 120, and removing the compound semiconductor substrate. The Specifically, the compound solar cell structure 130 includes a top cell 134, a tunnel junction 133, a middle cell 132, and a p ⁺ -type GaAs (gallium arsenide) layer containing a p-type impurity at a high concentration on a compound semiconductor substrate. 131, the p ⁺ -type GaAs (gallium arsenide) layer 131 is bonded onto the silicon solar cell structure 120, and the compound semiconductor substrate is removed. A p ⁺ -type GaAs (gallium arsenide) layer 131 containing a p-type impurity, a middle cell 132, a tunnel junction 133, and a top cell 134 are provided. Each of the middle cell 132 and the top cell 134 constitutes one n-on-p type compound semiconductor cell (subcell), similarly to the bottom cell. The middle cell 132 is made of, for example, GaAs, and the top cell 134 is made of, for example, InGaP (indium gallium phosphorus).

  Thereby, since the solar cell 100 is comprised by three subcells in which an energy gap becomes small in steps from the surface 102 side to the back surface 101 side, it can photoelectrically convert sunlight. Therefore, the solar cell 100 can convert sunlight into electric power with high efficiency. A germanium substrate may be used for the bottom cell. Compared to this case, since the solar cell 100 uses the silicon solar cell structure 120, it can be manufactured at low cost. Note that an antireflection film 113 is appropriately provided on the surface side 102 in order to increase the transmittance of sunlight.

Naoteru Shigekawa, Jianbo Liang, Ryusuke Onitsuka, Takaaki Agui, Hiroyuki Juso, and Tatsuya Takamoto, "Current-voltage and spectral-response characteristics of surface-activated-bonding-based InGaP / GaAs / Si hybrid triple-junction cells", Japanese Journal of Applied Physics 54, 08KE03 (2015).

The present inventors have found that the conventional solar cell illustrated in FIG. 1 has the following problems. That is, in the solar cell 100, the n ⁺ -type silicon layer of the silicon solar cell structure 120 and the p ⁺ -type GaAs layer of the compound solar cell structure 130 are bonded by a surface activated bonding method. At this time, the thickness of the n ⁺ -type silicon layer is several nm, but since this n ⁺ -type silicon layer also serves as the emitter of the silicon solar cell structure 120, it is difficult to increase the thickness. As a result, the electrical resistance of the junction interface 103 between the silicon solar cell structure 120 and the compound solar cell structure 130 becomes high, and there is a problem that it is difficult to further increase the efficiency. The inventors have found.

  In one aspect, the present invention has been made in view of such a situation, and an object thereof is to provide a technique for improving the efficiency of a solar cell.

  The present invention employs the following configuration in order to solve the above-described problems.

  That is, a solar cell manufacturing method according to one aspect of the present invention includes a step of preparing a silicon solar cell structure composed of a silicon material, and a compound material that does not include a silicon material and has a wider band gap than silicon. Providing a structured compound solar cell structure, forming a transparent conductive layer composed of a transparent conductive material on the surface of the silicon solar cell structure by a plasma deposition method, and the compound solar cell Bonding a structure to the transparent conductive layer.

  In the said structure, a silicon solar cell structure and a compound solar cell structure are joined via a transparent conductive layer. Thereby, compared with the case where a silicon solar cell structure and a compound solar cell structure are joined directly, the electrical resistance of a joining interface can be made low. Therefore, according to the said structure, efficiency improvement of a solar cell can be achieved.

  Note that the silicon solar cell structure is not particularly limited as long as it is made of a silicon material. The silicon solar cell structure may be, for example, an n-on-p type solar cell structure formed by impurity diffusion into a high-resistance Si single crystal substrate, or ion implantation and activation annealing.

  Moreover, the compound solar cell structure may not be particularly limited as long as it does not include a silicon material and is composed of a compound material having a wider band gap than silicon. The compound solar cell structure is formed, for example, by removing the compound semiconductor substrate after bonding to an n-on-p type compound semiconductor cell or silicon solar cell structure that is crystal-grown on the compound semiconductor substrate. It may be a -on-p type solar cell structure or the like.

Further, the plasma vapor deposition method is a method of irradiating a transparent conductive material with plasma such as Ar to ionize the transparent conductive material to form a film. Since the transparent conductive material is supplied to the surface of the silicon solar cell structure with low energy as compared with the sputtering method, a high-quality transparent conductive layer is formed.
・ References Hisashi Kitami, Masaru Miyashita, Toshiyuki Sakemi, Yasushi Aoki, and Takanori Kato, "Quantitative analysis of ionization rates of depositing particles in reactive plasma deposition using mass-energy analyzer and Langmuir probe", Japanese Journal of Applied Physics 54, 01AB05 (2015)

  In the solar cell manufacturing method according to the above aspect, the transparent conductive material may be indium tin oxide (hereinafter also referred to as “ITO”), fluorine-doped tin oxide (hereinafter also referred to as “FTO”), or oxidation. It can be zinc (ZnO).

  In the solar cell manufacturing method according to the above aspect, the content of the transparent conductive material in the transparent conductive layer can be 99.9% by mass or more. When using ITO as a transparent conductive material, an ink containing ITO is applied to the bonding interface, and the silicon solar cell structure and the compound solar cell structure are baked to form a transparent conductive layer. And a silicon solar cell structure and a compound solar cell structure can be joined. However, in this case, since the ink contains an organic compound other than ITO, the ITO content of the formed transparent conductive layer (ITO layer) becomes low, and the electrical resistance of the transparent conductive layer (ITO layer) is It will be high. On the other hand, in the said structure, since a transparent conductive layer is formed by a plasma vapor deposition method, content of the conductive material of the said transparent conductive layer is prescribed | regulated by the purity of the transparent conductive material used by a plasma vapor deposition method. Therefore, by using a high-purity transparent conductive material in the plasma deposition method, the content of the conductive material in the transparent conductive layer is 99.9% by mass or more, preferably 99.99% by mass or more. Thus, the electrical resistance of the transparent conductive layer can be prevented from becoming high (for example, 1.2E-4 Ω (cm or less (the above-mentioned reference)). Can be achieved.

  In the solar cell manufacturing method according to the above aspect, the arithmetic average roughness of the surface of the transparent conductive layer for bonding the compound solar cell structure may be 1 nm or less. According to the said structure, a silicon solar cell structure and a compound solar cell structure can be joined with sufficient intensity | strength. Arithmetic average roughness is represented by the average value of the uneven shape of the extracted section of a part of the surface (roughness curved surface) to be measured.

In the solar cell manufacturing method according to the above aspect, the compound solar cell structure is bonded to the transparent conductive layer, and is connected to the p-type semiconductor layer and a p-type semiconductor layer doped with a p-type impurity. -On-p type compound semiconductor cell. According to the said structure, the multijunction solar cell which forms a tunnel junction with the p-type semiconductor layer and transparent conductive layer of a compound solar cell structure can be provided. As described above, the n-on-p type compound semiconductor cell is a compound semiconductor cell that includes an n ⁺ type compound semiconductor layer on the front surface side and a p ⁺ type compound semiconductor layer on the back surface side.

  In the solar cell manufacturing method according to the above aspect, the compound solar cell structure is bonded to the transparent conductive layer, connected to the n-type semiconductor layer doped with an n-type impurity, the n-type semiconductor layer, and p A p-type semiconductor layer doped with a p-type impurity; and an n-on-p-type compound semiconductor cell connected to the p-type semiconductor layer. The n-type semiconductor layer, the p-type semiconductor layer, May constitute a tunnel junction. According to the said structure, the multijunction solar cell by which a tunnel junction is formed in the part adjacent to the transparent conductive layer of a compound solar cell structure can be provided.

  In the solar cell manufacturing method according to the one aspect described above, the compound solar cell structure includes a plurality of layers connected via tunnel junctions in the order in which each band gap narrows from the front surface to the back surface (silicon bottom cell). An n-on-p type compound semiconductor cell may be provided. According to this configuration, it is possible to provide a multijunction solar cell in which three or more n-on-p type semiconductor cells (subcells) are stacked.

  A solar cell according to one aspect of the present invention includes a type silicon solar cell structure made of a silicon material, a transparent conductive layer laminated on the silicon solar cell structure, and made of a transparent conductive material, And a compound solar cell structure that is connected to the transparent conductive layer, does not contain a silicon material, and is made of a compound material having a wider band gap than silicon.

  In the solar cell according to the one aspect, the content of the transparent conductive material in the transparent conductive layer can be 99.9% by mass or more.

  In the solar cell according to the above aspect, the compound solar cell structure is connected to the transparent conductive layer, and is connected to the p-type semiconductor layer and a p-type semiconductor layer doped with a p-type impurity, and n-on. -P-type compound semiconductor cell.

  In the solar cell according to the one aspect, the compound solar cell structure is stacked on the transparent conductive layer, connected to the n-type semiconductor layer doped with an n-type impurity, and the p-type semiconductor layer. An impurity-doped p-type semiconductor layer and an n-on-p-type compound semiconductor cell connected to the p-type semiconductor layer may be provided, and the n-type semiconductor layer and the p-type semiconductor layer are tunneled. You may make it comprise joining.

  ADVANTAGE OF THE INVENTION According to this invention, the technique which aims at the highly efficient solar cell can be provided.

FIG. 1 schematically illustrates the structure of a conventional solar cell. FIG. 2 schematically illustrates the structure of the solar cell according to the embodiment. FIG. 3A illustrates the manufacturing process of the solar cell according to the embodiment. FIG. 3B illustrates the manufacturing process of the solar cell according to the embodiment. FIG. 3C illustrates the manufacturing process of the solar cell according to the embodiment. FIG. 3D illustrates the manufacturing process of the solar cell according to the embodiment. FIG. 4 shows the optical properties of the ITO layer. FIG. 5 shows the surface state of the ITO layer. FIG. 6 shows the relationship between the resistance value of the ITO layer and the heat treatment. FIG. 7 schematically illustrates the configuration of a solar cell according to a modification. FIG. 8A shows the characteristics of the solar cell according to the example. FIG. 8B shows the characteristics of the solar cell according to the example. FIG. 9A shows the characteristics of the solar cell according to the example. FIG. 9B shows the characteristics of the solar cell according to the example. FIG. 10A shows the characteristics of the solar cell according to the comparative example. FIG. 10B shows the characteristics of the solar cell according to the comparative example. FIG. 11 shows the relationship between the resistance value between the ITO layers of each example and the heat treatment.

  Hereinafter, an embodiment according to an aspect of the present invention (hereinafter, also referred to as “this embodiment”) will be described with reference to the drawings. However, this embodiment described below is only an illustration of the present invention in all respects. It goes without saying that various improvements and modifications can be made without departing from the scope of the present invention. That is, in implementing the present invention, a specific configuration according to the embodiment may be adopted as appropriate. In the following description, for convenience of description, the description will be made with reference to the direction in the drawing.

§1 Configuration Example First, the configuration of the solar cell 1 according to the present embodiment will be described with reference to FIG. FIG. 2 schematically illustrates the configuration of the solar cell 1 according to this embodiment. As illustrated in FIG. 2, the solar cell 1 according to this embodiment is a multi-junction solar cell, and in order from the back surface 11 side to the front surface 12 side, the back electrode 14, the silicon solar cell structure 2, An ITO layer 4, a compound solar cell structure 3, and a surface electrode 15 are provided. The front surface 12 is a surface on which sunlight is incident, and the back surface 11 is a surface on the opposite side.

The silicon solar cell structure 2 is laminated on the back electrode 14 and configured using a silicon material. In this embodiment, the silicon solar cell structure 2 includes, in order from the back surface 11 side, a p ⁺ -type silicon layer 211 containing a p-type impurity at a high concentration, a silicon layer 21, and an n ⁺ containing an n-type impurity at a high concentration. Type silicon layer 122. Thereby, the silicon solar cell structure 2 is an n-on-p type. The p-type impurity doped into the p ⁺ -type silicon layer 211 is, for example, boron, and the p-type impurity concentration can be, for example, about 3E19 cm ⁻³ . The n-type impurity doped in the n ⁺ -type silicon layer 122 is, for example, phosphorus, and the n-type impurity concentration can be, for example, about 3E19 cm ⁻³ . Since the silicon solar cell structure 2 constitutes one solar cell (subcell) and is disposed on the back surface 11 side, it may be referred to as “(Si) bottom cell”.

  The ITO layer 4 is laminated on the surface on the surface 12 side of the silicon solar cell structure 2, and is configured using ITO which is a transparent conductive material. The ITO layer 4 corresponds to the “transparent conductive layer” of the present invention. In the present embodiment, the ITO layer 4 is laminated on the surface of the silicon solar cell structure 2 by a plasma deposition method, so that the content of ITO is 99.9% by mass, preferably 99.99% by mass. It is comprised so that it may become.

The compound solar cell structure 3 is laminated on the ITO layer 4 and does not include a silicon material, and is configured using a compound material having a wider band gap than silicon. The compound material used may be, for example, a gallium-based compound or a CIG (S) -based compound. In this embodiment, a gallium-based compound is used, and the compound solar cell structure 3 according to this embodiment includes, in order from the back surface 11 side, a p ⁺ -type GaAs layer 34 containing p-type impurities at a high concentration, and a middle cell. 33, a tunnel junction 32, and a top cell 31.

The p ⁺ -type GaAs layer 34 is bonded on the ITO layer 4, in other words, adjacent to the ITO layer 4. The p-type impurity doped in the p ⁺ -type GaAs layer 34 is, for example, carbon, and the p-type impurity concentration can be, for example, about 1E19 cm ⁻³ . This p ⁺ -type GaAs layer 34 corresponds to a “p-type semiconductor layer” of the compound solar cell structure of the present invention. The ITO layer 4 functions as an n-type semiconductor layer. Therefore, in this embodiment, the p ⁺ type GaAs layer 34 and the ITO layer 4 form a tunnel junction between the middle cell 33 and the silicon solar cell structure 2 (bottom cell).

The middle cell 33 is connected to the p ⁺ -type GaAs layer 34, the tunnel junction 32 is connected to the middle cell 33, and the top cell 31 is connected to the tunnel junction 32. Each of the middle cell 33 and the top cell 31 constitutes one solar cell (subcell), similarly to the bottom cell. The middle cell 33 and the top cell 31 are connected in order from the back surface 11 side to the front surface 12 side, and correspond to the “n-on-p type compound semiconductor cell” of the present invention. The tunnel junction 32 forms a tunnel junction between the top cell 31 and the middle cell 33. If the top cell 31 is configured to have an energy gap larger than that of the middle cell 33, the configurations of the top cell 31 and the middle cell 33 may be appropriately determined according to the embodiment.

  As a specific example, the top cell 31 may be configured by an n-type InGaP layer and a p-type InGaP layer that are sequentially stacked from the surface 12 side. Further, an n-type AlInP (aluminum / indium / phosphorus) layer may be inserted as a window layer on the front surface 12 side of the n-type InGaP layer, and a p-type AlInP layer on the back surface 11 side of the p-type InGaP layer. You may insert as a back surface electric field layer. The tunnel junction 32 may be formed of a p-type AlGaAs (aluminum / gallium / arsenic) layer and an n-type InGaP layer that are sequentially stacked from the surface 12 side. The middle cell 33 may be composed of an n-type GaAs layer and a p-type GaAs layer that are sequentially stacked from the surface 12 side. Further, an n-type AlInP layer may be inserted as a window layer on the surface 12 side of the n-type GaAs layer, and a p-type InGaP layer is inserted as a back surface field layer on the back surface 11 side of the p-type GaAs layer. Also good.

  The back electrode 14 and the front electrode 15 are appropriately configured so that electricity generated by the cells (31, 33) of the compound solar cell structure 3 and the silicon solar cell structure 2 can be output. In the present embodiment, the back electrode 14 is provided so as to cover the entire back surface 11. On the other hand, the surface electrode 15 is provided on a part of the surface 12 so that sunlight can enter the top cell 31. The material of the back electrode 14 and the front electrode 15 may be appropriately selected according to the embodiment. For example, the back electrode 14 is made of Al (aluminum), and the front electrode 15 is made of AuGe (gold / germanium) / Ni ( (Nickel) / Au (gold).

In the present embodiment, the antireflection film 16 is provided on the surface 12 side in order to increase the transmittance of sunlight. The material of the antireflection film 16 may be appropriately selected according to the embodiment, and can be made, for example, with a multilayer structure of SiO ₂ (silicon dioxide) and SiN (silicon nitride). In addition, the thickness and planar dimension of each layer of the solar cell 1 according to the present embodiment may be appropriately determined according to the embodiment.

§2 Manufacturing Method Next, the manufacturing process of the solar cell 1 according to the present embodiment will be described with reference to FIGS. 3A to 3D. 3A to 3D schematically illustrate a process of manufacturing the solar cell 1 according to this embodiment. The manufacturing process of the solar cell 1 described below corresponds to the “solar cell manufacturing method” of the present invention. However, the manufacturing process described below is only an example, and each process may be changed as much as possible. In addition, in the manufacturing process described below, the process can be omitted, replaced, and added as appropriate according to the embodiment.

(First step)
First, as a first step, as illustrated in FIG. 3A, a silicon solar cell structure 2 is prepared. The method for preparing the silicon solar cell structure 2 can be appropriately selected according to the embodiment. For example, a silicon substrate is prepared, an n-type impurity is doped from one surface of the prepared silicon substrate, and a p-type impurity is doped from the other surface. Thereby, the silicon solar cell structure 2 including the p ⁺ silicon layer 211, the silicon layer 21, and the n ⁺ type silicon layer 22 that are sequentially stacked can be manufactured.

(Second step)
Next, as a second step, a compound solar cell structure 3 is prepared as illustrated in FIG. 3B. The method for preparing the compound solar cell structure 3 can be appropriately selected according to the embodiment. For example, a GaAs substrate 30 is prepared, and a top cell 31, a tunnel junction 32, a middle cell 33, and a p ⁺ -type GaAs layer are sequentially formed on the prepared GaAs substrate 30 by crystal growth. Note that the order of the first step and the second step may be interchanged, or may be performed in parallel.

(Third step)
Next, as illustrated in FIG. 3C, as a third step, the ITO layer 4 is deposited on the surface 12 side of the silicon solar cell structure 2, that is, the surface 221 of the n ⁺ -type silicon layer 22 by plasma deposition. Form. As described above, the plasma deposition method is a method in which a transparent conductive material is irradiated with plasma such as Ar, and the transparent conductive material is ionized to form a film. By using a high-purity transparent conductive material for the plasma deposition method, the ITO content in the ITO layer 4 can be 99.9% by mass or more, preferably 99.99% by mass or more. The thickness of the ITO layer 4 may be appropriately set according to the embodiment, and can be set to 30 nm or more, for example.

(4th step)
Next, as a fourth step, as illustrated in FIG. 3D, the compound solar cell structure 3 is bonded (bonded) to the surface 41 on the surface 12 side of the ITO layer 4. As a method for bonding the compound solar cell structure 3 to the ITO layer 4, for example, a surface activated bonding method may be used. In the surface activated bonding method, the surfaces of two objects to be bonded are irradiated with a beam (for example, an argon beam) or plasma to clean and activate each surface, and then pressure is applied between the surfaces. It is the joining method which joins. A series of steps of this surface activated bonding method is performed in a vacuum.

  FIG. 4 shows the light before and after irradiating the 90 nm thick ITO layer formed on the glass substrate under the same conditions as the film formation on the silicon solar cell structure with the argon beam by the surface activated bonding method. The result of having measured the transmittance | permeability with the spectrophotometer is shown. As shown in FIG. 4, there is almost no change in the optical characteristics of the ITO layer before and after irradiation with the argon beam. Therefore, even after the compound solar cell structure 3 is bonded to the ITO layer 4 by the surface activation bonding method, the ITO layer 4 functions appropriately as a transparent conductive layer.

  When the compound solar cell structure 3 is bonded to the ITO layer 4 by the surface activation bonding method, the arithmetic average roughness of the surface 41 to which the compound solar cell structure 3 of the ITO layer 4 formed in the third step is bonded. (Ra) is preferably 1 nm or less. When the arithmetic average roughness of the surface 41 of the ITO layer 4 is 1 nm or less, the compound solar cell structure 3 and the ITO layer 4 can be bonded with sufficient strength by the surface activated bonding method.

  FIG. 5 shows the result of observing the arithmetic average roughness of the surface of the ITO layer having a thickness of 90 nm formed on the Si substrate under the same conditions as those on the silicon solar cell structure by AFM. Arithmetic average roughness is represented by the average value of the uneven shape of the extracted section of a part of the surface (roughness curved surface) to be measured. The arithmetic average roughness of the ITO layer shown in FIG. 5 was 0.3 nm. The ITO layer having this arithmetic mean roughness could be bonded to the compound solar cell structure with sufficient strength by the surface activated bonding method. Therefore, by forming the ITO layer 4 under the above conditions, the arithmetic average roughness of the surface 41 of the ITO layer 4 is 1 nm or less, and the compound solar cell structure 3 and the ITO layer 4 are sufficiently strong by the surface activated bonding method. It was found that it can be joined with. An ITO layer having the same thickness was formed on the Si substrate by sputtering. The arithmetic average roughness of the surface was 1.9 nm. A bonding experiment of the ITO layer was conducted, but no bonding was formed.

  After performing the fourth step, the GaAs substrate 30 is removed by selective etching. Thereby, the compound solar cell structure 3 can be produced. The antireflection film 16, the back electrode 14, and the front electrode 15 are appropriately formed. For example, after the antireflection film 16 is formed on the surface of the compound solar cell structure 3 on the top cell 31 side, the back electrode 14 and the front electrode 15 are deposited. In order to obtain good ohmic characteristics, a heat treatment at 400 ° C. is performed when the surface electrode 15 is formed. Thereby, this manufacturing process is complete | finished and the solar cell 1 (FIG. 2) which concerns on this embodiment can be produced.

[Feature]
As described above, in the solar cell 1 according to this embodiment, the silicon solar cell structure 2 and the compound solar cell structure 3 are joined via the ITO layer 4 which is a transparent conductive layer. Thereby, as shown in the Example mentioned later, compared with the case where the silicon solar cell structure 2 and the compound solar cell structure 3 are joined directly, the electrical resistance of the junction interface 13 can be made low. Therefore, according to this embodiment, high efficiency of the solar cell can be achieved. In this embodiment, since the silicon solar cell structure 2 includes one subcell and the compound solar cell structure 3 includes two subcells, the solar cell 1 is configured as a three-junction solar cell.

  In addition, as a method for bonding two structures through an ITO layer, there is a method using an ink containing ITO in addition to the method using the plasma deposition method and the surface activation bonding method of the above embodiment. In this method, ink containing ITO is applied to the bonding interface, and the silicon solar cell structure and the compound solar cell structure are baked in a state of being bonded together at the bonding interface. Thereby, a silicon solar cell structure and a compound solar cell structure can be joined via an ITO layer.

However, in the method using the ink containing ITO, since the ink contains an organic compound other than IT, the content of ITO in the formed ITO layer becomes low. In addition, the electrical resistance of the ITO layer becomes high due to heating during bonding. According to the document “Masumi Nakami (Osaka City Industrial Research Institute),“ Development and characteristics of ITO transparent conductive film forming ink ”and surface technology, Vol. 60 (2009), No. 10, p. 631” When a transparent conductive film (thickness: 309 nm) composed of ink containing ITO nanoparticles printed by a screen printing method is heated in the atmosphere at 480 degrees for 30 minutes, the resistivity of the transparent conductive film is 3.4 × 10 ⁻² (Ω · cm). Moreover, when the transparent conductive film (film thickness 345 nm) comprised with the ink containing the ITO nanoparticle printed by the screen printing method was heated for 30 minutes at 480 degree | times in nitrogen atmosphere, the resistivity of the said transparent conductive film is 6.5 × 10 ⁻³ (Ω · cm).

  On the other hand, in the method using the plasma vapor deposition method and the surface activated bonding method of the above embodiment, the ITO content in the ITO layer is a high ratio that is difficult to achieve by the method using the ink (for example, 99.99). 9% by mass). Therefore, an increase in the electrical resistance of the ITO layer can be suppressed even if heating is performed in the process of forming the surface electrode 15.

FIG. 6 shows the results of measuring the resistivity (Ω · cm) by the TLM method before heating the ITO layer (thickness 90 nm) formed on the glass plate and after heating at each temperature for 1 minute. As shown in FIG. 6, the resistivity of the ITO layer before heating is 1.3 × 10 ⁻⁵ (Ω · cm), and the resistivity of the ITO layer after heating at 400 ° C. is 6.7 × 10 ^−5. (Ω · cm). For this reason, it was found that the increase in the electrical resistance of the ITO layer 4 can be suppressed even when heated at 400 degrees in the process of forming the surface electrode. Specifically, it was found that the electrical resistance of the ITO layer 4 can be reduced to about 1/100 compared with the case where ink is used. Therefore, according to this embodiment, since the electrical resistance of the ITO layer 4 which is a transparent conductive layer can be prevented from increasing, the efficiency of the solar cell can be increased.

§3 Modifications Although one embodiment of the present invention has been described above, the above description is merely an illustration of the present invention in all respects. It goes without saying that various improvements and modifications can be made without departing from the scope of the present invention. For example, the following changes are possible. In the following, the same reference numerals are used for the same components as in the above embodiment, and the description of the same points as in the above embodiment is omitted as appropriate. The following modifications can be combined as appropriate.

<3.1>
The solar cell 1 according to the embodiment is configured to form a tunnel junction between the p ⁺ -type GaAs layer 34 and the ITO layer 4. However, the configuration of the solar cell 1 may not be limited to such an example, and may be appropriately determined according to the embodiment.

FIG. 7 schematically illustrates the configuration of the solar cell 1A according to this modification. In this solar cell 1A, the compound solar cell structure 3A includes an n ⁺ -type GaAs layer 35 containing n-type impurities at a high concentration on the back surface 11 side in addition to the configuration of the compound solar cell structure 3. That is, the n ⁺ type GaAs layer 35 is laminated on the ITO layer 4, and the p ⁺ type GaAs layer 34 is laminated on the n ⁺ type GaAs layer 35.

The n ⁺ -type GaAs layer 35 corresponds to the “n-type semiconductor layer” of the present invention. The n-type impurity doped in the n ⁺ -type GaAs layer 35 is, for example, Si, and the n-type impurity concentration can be, for example, about 1E19 cm ⁻³ . Except for the point where the n ⁺ -type GaAs layer 35 is laminated, the solar cell 1 A is configured in the same manner as the solar cell 1. Moreover, this solar cell 1 </ b> A can be manufactured in the same manufacturing process as the solar cell 1. That is, in the process of crystal growth in the second step, the n ⁺ -type GaAs layer 35 is further formed, and other steps are performed in the same manner as in the above embodiment, so that the solar cell 1A according to the present modification can be obtained. Can be produced.

In solar cell 1A according to this modification, p ⁺ type GaAs layer 34 and n ⁺ type GaAs layer 35 form a tunnel junction. As a result, the electrical resistance of the bonding interface 13 can be further reduced as shown in the examples described later. Therefore, according to this modification, the efficiency of the solar cell can be increased.

<3.2>
In the said embodiment, ITO was utilized as a material of a transparent conductive layer. However, the transparent conductive layer only needs to be made of a transparent conductive material, and the transparent conductive material may not be limited to ITO. In addition to ITO, for example, FTO or ZnO can be used as the transparent conductive material.

  Moreover, in the said embodiment, after forming the ITO layer 4 in the surface at the surface side of the silicon solar cell structure 2, the ITO layer 4 and the compound solar cell structure 3 are joined. However, the manufacturing process of the solar cell 1 may not be limited to such an example. For example, an ITO layer is formed on the surface of the silicon solar cell structure 2 and an ITO layer is further formed on the back surface of the compound solar cell structure 3, and then formed on both structures (2, 3). The ITO layers may be joined by a surface activated joining method.

<3.3>
The configuration of the silicon solar cell structure 2 and the compound solar cell structure 3 may not be limited to the example of the above embodiment, and may be appropriately determined according to the embodiment.

  For example, a material in which silicon and germanium are mixed may be used as the material of the silicon solar cell structure 2. The silicon solar cell structure 2 may be produced by crystal growth from a silicon substrate.

  In addition, for example, the compound solar battery structure 3 includes two compound semiconductor cells (the top cell 31 and the middle cell 33), that is, has a structure of a two-junction solar battery. However, the structure of the compound solar cell structure 3 may not be limited to such an example, and the number of compound semiconductor cells in the compound solar cell structure 3 may be one or three. It may be the above. The configuration of each compound semiconductor cell may be appropriately determined so that the energy gap decreases in order from the front surface side to the back surface side.

§4 Examples Hereinafter, examples of the present invention will be described. However, the present invention is not limited to this embodiment.

(First embodiment)
A solar cell having the same configuration as that of the above-described embodiment (FIG. 2) was manufactured as a first example by performing the manufacturing process of the above-described manufacturing method. The detailed manufacturing conditions of the first embodiment are as follows.

A p ⁺ -type silicon layer containing p-type impurities at a high concentration is formed on the back surface of the high-resistance p-Si substrate by boron ion implantation and activation annealing, and a high concentration is formed on the surface by phosphorus ion implantation and activation annealing. An n ⁺ -type silicon layer containing n-type impurities was formed to form a silicon solar cell structure. An ITO layer was formed on the surface of the silicon solar cell structure (the surface of the n ⁺ -type silicon layer) using a plasma deposition method.

A tunnel formed by sequentially growing an n-on-p type top cell made of InGaP, a layer containing p-type impurities at a high concentration, and a layer containing n-type impurities at a high concentration on a GaAs substrate by crystal growth. A junction, an n-on-p middle cell made of GaAs, and a p ⁺ -type GaAs layer were formed in this order. The p ⁺ -type GaAs layer surface is bonded to the silicon solar cell structure surface, that is, the ITO layer surface, the GaAs substrate is polished from the back surface to be thinned, and removed by wet etching with a mixed solution of H2SO4, H2O2, and H2O. Thus, the top cell surface was exposed.

  After a comb-shaped surface electrode was formed by vapor deposition, lift-off, and heat treatment at 400 ° C. for 1 minute, a 1 mm × 1 mm mesa reaching the bottom cell was formed by dry etching, and an antireflection film was formed by sputtering. A back electrode was formed on the bottom cell back surface. Thus, the first example was produced.

(Second embodiment)
In the process of crystal growth, an n ⁺ -type GaAs layer is further formed on the back surface of the p ⁺ -type GaAs layer of the first embodiment, so that a solar cell having the same configuration as that of the above-described modified example (FIG. 7) can be obtained. It produced as 2nd Example.

(Comparative example)
By directly joining the compound solar cell structure and the silicon solar cell structure by the surface activated bonding method, a solar cell having the same configuration as the conventional example (FIG. 1) was produced as a comparative example. The solar cell according to the comparative example has the same configuration as that of the first example, except that the ITO layer is not provided.

  Light is irradiated from the surface side of each example and comparative example under irradiation conditions of AM (air mass) 1.5G (Global) and 1 sun, and the characteristics of each example and comparative example (planar dimensions: 1 mm × 1 mm) are sourced. Measurement was performed with a major unit (B2902A, manufactured by Agilent). 8A and 8B show the measurement results of the first example. 9A and 9B show the measurement results of the second example. 10A and 10B show the measurement results of the comparative example.

  As shown in the measurement results of the comparative example, when the silicon solar cell structure and the compound solar cell structure are directly joined, the differential resistance value at the open circuit voltage is 1800 (Ω), and the electrical resistance at the junction interface is compared. It became high. Compared to this, in the first example and the second example, the electrical resistance at the bonding interface could be lowered as compared with the comparative example. In particular, in the second example, the differential resistance value at the open circuit voltage was 1130 (Ω), and the electrical resistance at the junction interface could be made very low. As a result, the conversion efficiency of the comparative example was measured to be 24.0%, whereas the conversion efficiency of the first example was measured to be 24.6%, and the conversion efficiency of the second example was 25.1%. And measured. Therefore, according to the present invention, it has been found that high efficiency of the solar cell can be achieved.

In order to evaluate the electrical resistance of the bonded interface after the heat treatment in the first and second examples, an ITO layer was formed on an n ⁺ type silicon substrate and an ohmic electrode was formed on the back surface. A first test piece (corresponding to the first example) was produced by attaching a p ⁺ -type GaAs substrate having an ohmic electrode formed in advance on the back surface through the ITO layer. In addition, an ITO layer is formed on an n ⁺ type silicon substrate, and an n ⁺ type GaAs substrate having an ohmic electrode formed in advance on the back surface is bonded to the second test piece (through the ITO layer). Corresponding to the second example). Each test piece was diced to a size of 2 mm × 2 mm, and the bonding area was defined. Then, the resistance (Ω · cm ² ) between the silicon substrate and the GaAs substrate after heat treatment of each test piece and after heat treatment at each temperature for 1 minute was measured with a source measure unit (B2902A, manufactured by Agilent). FIG. 11 shows the measurement results.

As shown in FIG. 11, in comparison with the first experiment piece was adhered to the ITO layer and the p ⁺ -type GaAs substrate, toward the second experiment piece was adhered to the ITO layer and the n ⁺ -type GaAs substrate prior to heating and Both were low resistance after heating. Thus, it was found that the second example (modified example, FIG. 7) can achieve higher efficiency than the first example (embodiment, FIG. 2).

1 ... solar cell,
11 ... Back side, 12 ... Front side, 13 ... Bonding interface,
14 ... Back electrode, 15 ... Front electrode, 16 ... Antireflection film,
2 ... silicon solar cell structure,
21 ... silicon layer, 22 ... n ⁺ type silicon layer,
3 ... Compound solar cell structure,
31 ... Top cell (n-on-p type compound semiconductor cell),
32 ... Tunnel junction,
33 ... Middle cell (n-on-p type compound semiconductor cell),
34... P ⁺ type GaAs layer (p type semiconductor layer),
35 ... n ⁺ -type GaAs layer (n-type semiconductor layer),
4 ... ITO layer (transparent conductive layer)

Claims (7)

Providing a silicon solar cell structure composed of a silicon material;
Providing a compound solar cell structure that is composed of a compound material that does not include a silicon material and has a wider band gap than silicon; and
Forming a transparent conductive layer made of a transparent conductive material on the surface of the silicon solar cell structure by a plasma deposition method;
Bonding the compound solar cell structure to the transparent conductive layer;
Comprising
Solar cell manufacturing method. The content of the transparent conductive material in the transparent conductive layer is 99.9% by mass or more.
The method for manufacturing a solar cell according to claim 1. The arithmetic average roughness of the surface of the transparent conductive layer for bonding the compound solar cell structure is 1 nm or less,
The solar cell production method according to claim 1 or 2. The transparent conductive material is indium tin oxide, fluorine-doped tin oxide, or zinc oxide.
The solar cell manufacturing method of any one of Claim 1 to 3. A silicon solar cell structure composed of a silicon material;
A transparent conductive layer connected to the silicon solar cell structure and made of a transparent conductive material;
A compound solar cell structure that is laminated on the transparent conductive layer, does not contain a silicon material, and is composed of a compound material having a wide band gap compared to silicon;
Comprising
Solar cell. The content of the transparent conductive material in the transparent conductive layer is 99.9% by mass or more.
The solar cell according to claim 5. The transparent conductive material is indium tin oxide, fluorine-doped tin oxide, or zinc oxide.
The solar cell according to claim 5 or 6.