WO2017159281A1

WO2017159281A1 - Solar cell

Info

Publication number: WO2017159281A1
Application number: PCT/JP2017/006995
Authority: WO
Inventors: 繁山田; 幸美市川; 政和平井; 修平吉葉; 保聡屋敷
Original assignee: 国立研究開発法人科学技術振興機構
Priority date: 2016-03-16
Filing date: 2017-02-24
Publication date: 2017-09-21
Also published as: JP2019083220A

Abstract

The present invention provides a solar cell in which a photogenerated carrier, when being dispersed in a superlattice structure comprising a crystalline Si-layer/SiO₂ layer, does not need to traverse a high energy barrier of the SiO₂ layer, thereby relaxing constraints regarding the thickness of the SiO₂ layer and significantly suppressing loss of the carrier during dispersal, and making it possible to obtain a solar cell having increased internal quantum efficiency and high photoelectric conversion efficiency. Because junctions (pi-junction and ni-junction) are formed in a lateral direction, the need to use a substrate containing an impurity dopant is eliminated, and it becomes possible to avoid the problem of unintended irreversible dispersion of a dopant from the substrate as a result of annealing at relatively high temperature for Si-layer crystallization, making it possible to perform contact formation substantially in accordance with the design thereof.

Description

Solar cell

The present invention relates to a solar cell technology, and more particularly to a technology of a silicon solar cell with high photoelectric conversion efficiency using a quantum size effect.

The importance of developing clean energy alternatives to fossil fuels is increasing as social interest in environmental conservation increases in recent years. Under such a social background, solar cells have become widespread as power devices using natural energy. However, in order for solar cells to be used as a major energy supply source, further cost reduction is required. Improvement in photoelectric conversion efficiency is required.

Various types of solar cell structures have been proposed so far. For example, in Patent Document 1 (Japanese Patent Laid-Open No. 2014-27119), a light absorption layer includes, for example, a semiconductor layer made of silicon and the semiconductor. An embodiment having a superlattice structure in which barrier layers made of a material having a larger band gap than the layer (for example, silicon oxide) are alternately stacked is disclosed.

JP 2014-27119 A

FIG. 1 is a diagram for conceptually explaining the flow of photogenerated carriers (electrons and holes) in a solar cell having the superlattice structure described above. In the example shown in this figure, a superlattice structure 210 in which a plurality of crystalline Si layers 201 and SiO ₂ layers 202 are alternately stacked is provided on the main surface of an n-type Si substrate 200. A p-type Si layer 203, a transparent conductive film layer 204, and a surface electrode 205 are sequentially formed on the upper surface. When light enters this solar cell, electrons 206 and holes 207 are generated in the Si layer 201 constituting the superlattice structure 210, the electrons 206 are directed to the n-type Si substrate 200 side, and the holes 207 are p-type Si. A current flows by diffusing in the vertical direction across the SiO ₂ layer 202 toward the layer 203 side.

However, since such a current is generated when the carrier crosses (permeates) the high energy barrier of the SiO ₂ layer 202 by the tunnel effect, the thickness of the SiO ₂ layer 202 is optimized to increase the thickness. In addition to being required to be controlled with accuracy, it is not desirable from the viewpoint of obtaining a solar cell with high photoelectric conversion efficiency by incurring loss of carriers during diffusion and increasing internal quantum efficiency. In addition, with annealing at a relatively high temperature to crystallize the Si layer 201, dopant (in this case, donor P, etc. in this case) diffuses unavoidably from the n-type Si substrate. There is also concern that contact formation cannot be performed as designed.

The present invention has been made in view of such problems, and the object of the present invention is to efficiently convert photogenerated carriers into currents to increase internal quantum efficiency, thereby increasing the photoelectric conversion efficiency. It is an object of the present invention to provide a solar cell, and to provide a solar cell capable of avoiding the problem of dopant diffusion inevitably caused by annealing at a high temperature and capable of performing contact formation as designed.

In order to solve the above-described problems, a solar cell according to the present invention includes a structure portion having a superlattice structure in which a plurality of crystalline Si layers and SiO ₂ layers are alternately stacked on a substrate having an insulating surface. The crystalline Si layer is substantially an intrinsic semiconductor layer, and the first side surface of the superlattice structure is a p-type superlattice in which a plurality of p-type crystalline Si layers and SiO ₂ layers are alternately stacked. And the second side surface opposite to the first side surface has an n-type superlattice structure in which a plurality of n-type crystalline Si layers and SiO ₂ layers are alternately stacked, and the p-type The crystalline Si layer and the intrinsic crystalline Si layer are pi-junctioned, and the n-type crystalline Si layer and the intrinsic crystalline Si layer are ni-junctioned.

In one aspect, a plurality of the superlattice structures are formed on the substrate so as to be separated from each other.

In one embodiment, adjacent superlattice structures of the plurality of superlattice structures are arranged such that the p-type superlattice structures face each other and the n-type superlattice structures face each other.

In one embodiment, adjacent superlattice structures of the plurality of superlattice structures are arranged such that the p-type superlattice structure and the n-type superlattice structure face each other.

Furthermore, in one aspect, a p-type electrode or an n-type electrode is formed in a region where the superlattice structures are separated from each other.

Further, in a preferred embodiment, the superlattice structure has a mesa shape having an inclination on a side surface on which the p-type superlattice structure and the n-type superlattice structure are formed.

For example, the acceptor in the p-type crystalline Si layer and the donor in the n-type crystalline Si layer are ion-implanted impurities.

Preferably, the crystalline Si layer has a thickness of 5 nm or less.

Also preferably, the substrate is a translucent substrate.

In one embodiment, the thickness of the SiO ₂ layer constituting the superlattice structure is thicker toward the light receiving side.

In one embodiment, the distance between the first side surface and the second side surface of the superlattice structure is 10 μm or less.

Furthermore, in one aspect, an opening is provided in a central region of the superlattice structure.

In one aspect, the superlattice structure includes an electrode pattern in which a p-type electrode and an n-type electrode are formed in regions spaced from each other in series.

In another aspect, the superlattice structure is provided with an electrode pattern in which a p-type electrode and an n-type electrode formed in regions separated from each other are connected in parallel.

The solar cell of the present invention is a tandem solar cell in which a top cell provided on the light incident side and a bottom cell provided below the top cell are laminated, and the top cell includes the above-described structure. The bottom cell may be made of crystalline Si, and a light receiving surface electrode may be provided on the surface of the top cell, and a back electrode may be provided on the back surface of the bottom cell.

Since this in the solar cell according to the invention, light generated carriers do not have to cross the high energy barrier with the SiO ₂ layer in spreading the superlattice structure in made of crystalline Si layer / SiO ₂ layer, SiO ₂ While restrictions on the thickness of the layer are relaxed, loss of carriers during diffusion is remarkably suppressed, and a solar cell with high photoelectric conversion efficiency can be obtained by increasing the internal quantum efficiency.

Also, since the junction (pi junction and ni junction) is formed in the lateral direction, the problem of irreversible diffusion of dopants from the unintended substrate associated with annealing at a relatively high temperature to crystallize the Si layer is avoided. It is possible to perform contact formation almost as designed.

It is a figure for demonstrating notionally the flow of the photo-generated carrier (electron and hole) in the solar cell provided with the conventional superlattice structure. It is sectional drawing for demonstrating the outline of the basic structure of the solar cell which concerns on this invention. It is a figure for demonstrating notionally the structure of the solar cell of the aspect by which the superlattice structure employ | adopted by this invention is spaced apart and formed in multiple numbers on the board | substrate with an insulating surface. FIG. 2 is a diagram for conceptually explaining the structure of a solar cell in which a plurality of adjacent superlattice structures are arranged such that p-type superlattice structures face each other and n-type superlattice structures face each other. is there. It is a figure for demonstrating the aspect in which the superlattice structure has the mesa shape which has the inclination in the side surface in which the p-type superlattice structure and the n-type superlattice structure are formed. It is a figure for demonstrating the structural example by which the opening part is provided in the center area | region of the superlattice structure. It is a figure for demonstrating the structural example provided with the electrode pattern which connects the p-type electrode and n-type electrode which were formed in the area | region where a superlattice structure mutually spaces apart. It is a perspective view of a solar cell provided with the electrode pattern shown to FIG. 7A. It is a figure for demonstrating the structural example provided with the electrode pattern which connects the p-type electrode and n-type electrode which were formed in the area | region where a superlattice structure mutually spaces apart. It is a perspective view of a solar cell provided with the electrode pattern shown to FIG. 8A. It is a figure for demonstrating the example which applied the structure which concerns on this invention to the tandem type solar cell.

Hereinafter, the structure of the solar cell according to the present invention will be described with reference to the drawings. In the following description, the crystalline Si layer may be a single crystal Si layer or a polycrystalline Si layer.

[Outline of basic structure]
FIG. 2 is a cross-sectional view for explaining the outline of the basic structure of the solar cell according to the present invention. In the example shown in this figure, a superlattice structure is formed by alternately laminating a plurality of crystalline Si layers 101 and SiO ₂ layers 102 on the main surface of a substrate 100 having an insulating surface. The crystalline Si layer 101 is substantially an intrinsic semiconductor. The first side surface (left side surface in this figure) of the superlattice structure is a p-type superlattice structure 110p in which a plurality of p-type crystalline Si layers and SiO ₂ layers are alternately stacked. The second side surface (right side surface in this figure) opposite to the first side surface is an n-type superlattice structure 110n in which a plurality of n-type crystalline Si layers and SiO ₂ layers are alternately stacked. Then, the p-type crystalline Si layer constituting the p-type superlattice structure 110p and the intrinsic crystalline Si layer constituting the superlattice structure are pi-joined to form an n-type crystalline Si layer constituting the n-type superlattice structure 110n. The intrinsic crystalline Si layer constituting the superlattice structure is in ni junction. Note that a p-type electrode 105p and an n-type electrode 105n are formed on each of the p-type superlattice structure 110p and the n-type superlattice structure 110n, and what is indicated by reference numeral 104 in the figure is a protective film.

That is, the solar cell according to the present invention includes a structural portion having a superlattice structure in which a plurality of crystalline Si layers and SiO ₂ layers are alternately stacked on a substrate having an insulating surface, and the crystalline Si layer Is substantially an intrinsic semiconductor layer, and the first side surface of the superlattice structure has a p-type superlattice structure in which a plurality of p-type crystalline Si layers and SiO ₂ layers are alternately stacked, and The second side surface opposite to the first side surface has an n-type superlattice structure in which a plurality of n-type crystalline Si layers and SiO ₂ layers are alternately stacked, and the p-type crystalline Si layer and the The intrinsic crystalline Si layer has a pi junction, and the n-type crystalline Si layer and the intrinsic crystalline Si layer have a ni junction.

When light enters this solar cell, electrons 106 and holes 107 are generated in the intrinsic crystalline Si layer 101 constituting the superlattice structure, and the electrons 106 are directed to the n-type electrode 105n side, and the holes 107 are p-type. A current flows by diffusing to the electrode 105p side. That is, in the solar cell having this configuration, it is not necessary to cross the high energy barrier of the SiO ₂ layer when the photogenerated carriers diffuse in the superlattice structure composed of the crystalline Si layer / SiO ₂ layer. In addition, in the plane of each of the intrinsic crystalline Si layers 101 constituting the superlattice structure, a state in which carriers are spread throughout the entire plane without being localized regardless of the application of an electric field is maintained. For this reason, restrictions on the thickness of the SiO ₂ layer are eased, loss of carriers during diffusion is remarkably suppressed, and a solar cell with high photoelectric conversion efficiency can be obtained by increasing internal quantum efficiency.

Moreover, the pi junction between the p-type crystalline Si layer constituting the p-type superlattice structure 110p and the intrinsic crystalline Si layer constituting the superlattice structure, and the n-type crystalline Si constituting the n-type superlattice structure 110n Each of the ni junctions of the intrinsic crystalline Si layer constituting the layer and the superlattice structure is formed on the side surface of the superlattice structure. In the conventional structure, it is necessary to use a substrate containing a dopant impurity in order to form a pi junction or an ni junction between the superlattice and the substrate. However, in the present invention, it is not necessary to use such a substrate. As a result, the problem of irreversible diffusion of dopants from the unintended substrate associated with annealing at a relatively high temperature (for example, 1000 to 1100 ° C.) to crystallize the Si layer formed by a technique such as sputtering is avoided. can do. For this reason, in the case of the present invention, contact formation can be performed almost as designed.

The substrate 100 only needs to have an insulating surface, and may be, for example, a substrate in which an insulating film such as SiO ₂ is provided on an n-type or p-type Si substrate, as well as an insulating substrate such as a quartz substrate. In the case of a solar cell having a structure in which the substrate side is a light incident surface, the substrate needs to be a translucent substrate. In the case of a solar cell having a light incident surface on the substrate side, a texture layer made of a material such as ZnO is provided on the surface of the superlattice structure to increase the reflectance on the back surface (surface opposite to the substrate). You may do it.

As described above, in the solar cell according to the present invention, the p-type crystalline Si layer constituting the p-type superlattice structure 110p and the intrinsic crystalline Si layer constituting the superlattice structure are pi-junctioned to form an n-type superlattice structure. The n-type crystalline Si layer constituting 110n and the intrinsic crystalline Si layer constituting the superlattice structure are in ni junction. That is, this solar cell has a p-type superlattice / i-type superlattice / n-type superlattice structure, and the pi junction and the ni junction are composed of only a superlattice in which a plurality of Si layers / SiO ₂ layers are stacked. ing. This is for providing a wide band gap of about 1.7 eV in the pi junction region and the ni junction region (that is, the band gap of the superlattice structure) to prevent a reduction in open-circuit voltage due to the formation of the junction. .

If p-type and n-type polysilicon electrodes having no superlattice structure are formed on opposite sides of a superlattice structure in which a plurality of crystalline Si layers and SiO ₂ layers are alternately stacked, The band gap of these junction regions is that of the polysilicon electrode (about 1.1 eV), and the open circuit voltage of the solar cell is regulated by this value, which may cause a decrease in the open circuit voltage in the junction region.

Such a pi junction and ni junction are formed by, for example, ion-implanting a p-type impurity (acceptor) or an n-type impurity (donor) on the side surface after the superlattice structure is formed.

In order to ensure the effect of increasing the open-circuit voltage by widening the band gap by the quantum size effect, it is preferable to design the thickness of the crystalline Si layer constituting the superlattice structure to 5 nm or less. Further, the refractive index of the superlattice structure can be changed by controlling the thickness of the SiO ₂ layer constituting the superlattice structure. By utilizing this principle, it is possible to increase the photoelectric conversion efficiency by designing the SiO ₂ layer constituting the superlattice structure to be thicker toward the light receiving side (light incident surface side).

[Outline of structure having a plurality of superlattice structures]
FIG. 2 shows only one superlattice structure as described above, but when configuring as a solar cell, it is of course possible to provide a plurality of such superlattice structures adjacent to each other. is there.

FIG. 3 is a diagram for conceptually explaining the structure of a solar cell in which a plurality of the above-described superlattice structures 110 are formed on a substrate 100 whose surface is insulatively spaced from each other.

In the embodiment shown in this figure, adjacent superlattice structures are regions in which the p-type superlattice structure 110p and the n-type superlattice structure 110n are arranged to face each other and the superlattice structures are separated from each other. In addition, a p-type electrode or an n-type electrode is formed. As shown in FIG. 4, a plurality of adjacent superlattice structures 110 are formed so that the p-type superlattice structures 110p face each other and the n-type superlattice It is good also as an aspect by which 110 n of structures are arrange | positioned facing each other.

In addition, as shown in FIG. 5, the superlattice structure 110 may have a mesa shape having an inclined side surface on which the p-type superlattice structure 110p and the n-type superlattice structure 110n are formed. Good. In the case of such a mesa-shaped superlattice structure 110, it is easy to control the distribution of implanted ions when the p-type superlattice structure 110p and the n-type superlattice structure 110n are formed by ion implantation. There are advantages.

Furthermore, as shown in FIG. 6, a structure in which an opening 108 is provided in the central region of the superlattice structure may be employed. When such an opening 108 is provided, when a relatively large number of defects are included in the crystalline Si layer 101 constituting the superlattice structure, the defects are not removed by hydrogenation. There is an advantage that it becomes easy to activate. The distance between the first side surface (the surface on which the p-type superlattice structure 110p is formed) and the second side surface (the surface on which the n-type superlattice structure 110n is formed) is preferably 10 μm or less. The diameter of 108 is, for example, about 2 μm.

[Example of electrode pattern design]
In a solar cell having a structure in which a plurality of superlattice structures are provided adjacent to each other, a special method for interconnecting the p-type electrode and the n-type electrode provided in each structural part having the superlattice structure There is no limit.

For example, as in the example illustrated in FIG. 7A, the superlattice structure may be configured to include an electrode pattern 109 that connects the p-type electrode and the n-type electrode formed in a region separated from each other in parallel. A perspective view of the solar cell 10 in this case is shown in FIG. 8A. Note that reference numeral 103 in the figure denotes a wall forming portion.

Further, for example, as in the example illustrated in FIG. 8A, the superlattice structure may be provided with an electrode pattern 109 in which a p-type electrode and an n-type electrode are connected in series, which are formed in regions separated from each other. A perspective view of the solar cell 10 in this case is shown in FIG. 8B.

[Application to tandem solar cells]
The solar cell having the above-described structure can be a tandem solar cell.

FIG. 9 is a diagram for explaining an example in which the structure according to the present invention is applied to a tandem solar cell. Reference numeral 10 indicates a top cell, and reference numeral 20 indicates crystalline Si. Bottom cell. In the example shown in this figure, the structure shown in FIG. 5 is used as the top cell. In addition, it cannot be overemphasized that the board | substrate in this case needs to have translucency.

That is, this solar cell is a tandem solar cell in which a top cell provided on the light incident side and a bottom cell provided below the top cell are stacked, and the top cell includes the above-described structure, The bottom cell is made of crystalline Si, and a light receiving surface electrode is provided on the surface of the top cell 10, and a back electrode is provided on the back surface of the bottom cell 20.

Thus, in the solar cell according to the present invention, it is not necessary to cross the high energy barrier of the SiO ₂ layer when the photogenerated carriers diffuse in the superlattice structure composed of the crystalline Si layer / SiO ₂ layer. Therefore, restrictions on the thickness of the SiO ₂ layer are eased, loss of carriers during diffusion is remarkably suppressed, and a solar cell with high photoelectric conversion efficiency can be obtained by increasing the internal quantum efficiency.

DESCRIPTION OF SYMBOLS 100 Substrate 101 Crystalline Si layer 102 SiO ₂ layer 103 Wall portion 104 Protective film 105p p-type electrode 105n n-type electrode 106 electron 107 hole 108 opening 109 electrode pattern 110p p-type superlattice structure 110n n-type superlattice structure

Claims

A structure having a superlattice structure in which a plurality of crystalline Si layers and SiO 2 layers are alternately stacked on a substrate having an insulating surface;
The crystalline Si layer is substantially an intrinsic semiconductor layer;
The first side surface of the superlattice structure has a p-type superlattice structure in which a plurality of p-type crystalline Si layers and SiO 2 layers are alternately stacked, and the second side surface that opposes the first side surface. The side surface has an n-type superlattice structure in which a plurality of n-type crystalline Si layers and SiO 2 layers are alternately stacked,
The p-type crystalline Si layer and the intrinsic crystalline Si layer are pi-bonded, and the n-type crystalline Si layer and the intrinsic crystalline Si layer are ni-junction.
A solar cell characterized by that.
The solar cell according to claim 1, wherein a plurality of the superlattice structures are formed on the substrate so as to be separated from each other.
The superlattice structure adjacent to the plurality of superlattice structures is arranged such that the p-type superlattice structures face each other and the n-type superlattice structures face each other. Solar cell.
The solar cell according to claim 2, wherein the superlattice structure adjacent to the plurality of superlattice structures is arranged such that the p-type superlattice structure and the n-type superlattice structure face each other.
The solar cell according to claim 2, wherein a p-type electrode or an n-type electrode is formed in a region where the superlattice structures are separated from each other.
The superlattice structure according to any one of claims 1 to 5, wherein the superlattice structure has a mesa shape having an inclination on a side surface on which the p-type superlattice structure and the n-type superlattice structure are formed. Solar cells.
6. The solar cell according to claim 1, wherein the acceptor in the p-type crystalline Si layer and the donor in the n-type crystalline Si layer are ion-implanted impurities.
The solar cell according to any one of claims 1 to 5, wherein the thickness of the crystalline Si layer is 5 nm or less.
The solar cell according to any one of claims 1 to 5, wherein the substrate is a translucent substrate.
The solar cell according to any one of claims 1 to 5, wherein a thickness of the SiO 2 layer constituting the superlattice structure is thicker toward a light receiving side.
6. The solar cell according to claim 1, wherein a distance between the first side surface and the second side surface of the superlattice structure is 10 μm or less.
The solar cell according to any one of claims 1 to 5, wherein an opening is provided in a central region of the superlattice structure.
The solar cell according to claim 5, comprising an electrode pattern in which a p-type electrode and an n-type electrode formed in a region where the superlattice structure is separated from each other are connected in series.
The solar cell according to claim 5, further comprising an electrode pattern in which a p-type electrode and an n-type electrode formed in regions where the superlattice structure is separated from each other are connected in parallel.
A tandem solar cell in which a top cell provided on a light incident side and a bottom cell provided below the top cell are stacked,
The top cell comprises the structure according to any one of claims 1 to 5,
The bottom cell is made of crystalline Si,
A light receiving surface electrode is provided on the surface of the top cell, and a back electrode is provided on the back surface of the bottom cell.
A solar cell characterized by that.