JP2005197597A - Multijunction solar cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a multijunction solar cell in which balance is struck in photocurrent between cells, electric connectivity between cells is improved, deterioration in solar cell characteristics during accumulation, in other words, deterioration in leak current is inhibited, and current-voltage charateristics are improved. <P>SOLUTION: The multijunction solar cell comprises a plurality of laminated pin junction cells, and an intermediate member in dot- or stripe-form that is provided between at least two pin junction cells of the plurality of pin junction cells. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a multi-junction solar cell configured by laminating a plurality of pin junction cells, and by providing a dot-shaped or striped intermediate member between the pin junction cells, the multi-junction having improved current-voltage characteristics Type solar cell.

  Solar cells are attracting attention as an alternative energy source for fossil fuels such as petroleum, where there is concern about future supply and demand and carbon dioxide emissions that cause global warming, and various devices are being developed and put to practical use. ing.

  In the development of elements, increasing the photoelectric conversion efficiency is one problem, and a multi-junction solar cell in which a plurality of elements (cells) are stacked has been proposed as a solution. FIG. 4 is a schematic cross-sectional view showing an example of a multi-junction solar cell having a two-terminal structure including a glass substrate 1, a transparent electrode 2, an amorphous silicon cell 3, a microcrystalline silicon cell 4, and a back electrode 5. In such a multi-junction solar cell having a two-terminal structure, in order to obtain higher photoelectric conversion efficiency, the photocurrent balance between cells becomes very important.

For example, when the photocurrent of a cell on the light incident side is smaller than the photocurrent of other cells, an intermediate layer made of a material having a low refractive index such as silicon oxide is inserted between the cells and reflected. A method has been proposed in which a part of light transmitted through the light incident side cell is returned to the light incident side cell and absorbed to secure a photocurrent (Japanese Patent Laid-Open No. 61-183976). Publication: Japanese Patent Application Laid-Open No. 4-127580 and Japanese Patent Application Laid-Open No. 4-127580). FIG. 5 is a schematic cross-sectional view showing an example of a multijunction solar cell having such a light reflecting intermediate layer. In the figure, reference numeral 6 denotes an intermediate layer made of a material having a low refractive index.
However, this intermediate layer only reflects part of the light transmitted through the cells on the light incident side, and has little effect of scattering the light reflected by the intermediate layer or the light transmitted through the intermediate layer. It was not sufficient to realize photocurrent balance matching.

Further, in the development of a multi-junction solar cell having a two-terminal structure as shown in FIG. 4, one problem is to improve the electrical connectivity between cells. A method of inserting a low resistance or narrow band gap intermediate layer such as zinc has been proposed (see Japanese Patent Application Laid-Open No. 6-151016: Patent Document 3). FIG. 6 is a schematic cross-sectional view showing an example of a multijunction solar cell having such an intermediate layer for improving connectivity. In the figure, 7 indicates an intermediate layer having a low resistance or a narrow band gap.
However, the multijunction solar cell having the connectivity improving intermediate layer has a current leak path (arrow in the figure) through the low resistance intermediate layer when the multijunction solar cells are integrated as shown in FIG. There has been a problem that the characteristics of the solar cell after integration can be significantly reduced.

JP 61-183976 A JP-A-4-127580 JP-A-6-151016

  The present invention realizes matching of photocurrent balance between cells, improves electrical connectivity between cells, suppresses degradation of solar cell characteristics during integration, that is, degradation of leakage current, and has current-voltage characteristics. It is an object to provide an improved multijunction solar cell.

  As a result of intensive studies to solve the above problems, the present inventors have provided an intermediate member having an optical light scattering effect between cells instead of the conventional flat intermediate layer. The present inventors have found that the above problems can be solved and have completed the present invention.

Thus, according to the present invention, a plurality of pin junction cells are stacked, and a dot-shaped or stripe-shaped intermediate member is provided between at least one pin junction cell among the plurality of pin junction cells. A multi-junction solar cell is provided.
Moreover, according to this invention, the multijunction solar cell module which integrated said multijunction solar cell is provided.

  According to the present invention, the matching of the photocurrent balance between cells is realized, the electrical connectivity between cells is improved, the solar cell characteristics during integration, that is, the leakage current is suppressed, and the current-voltage A multi-junction solar cell with improved characteristics can be provided.

The basic structure of the multi-junction solar cell of the present invention is, for example, as shown in FIG. 1, a glass substrate 1, a transparent electrode 2, an amorphous silicon cell 3, a quadrangular pyramid-shaped dot-shaped intermediate member 8, It consists of a microcrystalline silicon cell 4 and a back electrode 5. Each of the amorphous silicon cell 3 and the microcrystalline silicon cell 4 may be a pin junction cell, and the dot-shaped intermediate member 8 may have a stripe shape. As described above, the multi-junction solar cell of the present invention is configured by stacking a plurality of pin junction cells, and among the plurality of pin junction cells, an intermediate member having a dot shape or a stripe shape between at least one pin junction cell. Is provided.
Except for the dot-like or stripe-like intermediate member, it can be formed by known materials and methods.

The material of the intermediate member is not particularly limited as long as it is a material that can exhibit the effects of the present invention, regardless of whether it is a semiconductor material or a metal material. In order to obtain a sufficient optical effect, it is desirable to use a material having a low refractive index as compared with the pin junction cell material, or a material having total reflection such as a metal. For example, a low-resistance light-transmitting material having a low refractive index such as zinc oxide doped with Al or Ga may be used, and a pin-junction cell material having a high reflectance such as gold or silver. A low-resistance metal material that can provide a good ohmic junction with the substrate may be used.
Further, in order to achieve photocurrent balance matching between pin junction cells and improve electrical connectivity between cells, a semiconductor material having a narrow band gap as compared with pin junction cells is preferable.
Therefore, the intermediate member is preferably made of a semiconductor material or metal material having a narrow band gap as compared with a low-resistance semiconductor or pin junction cell.

The shape of the intermediate member may be either a dot shape or a stripe shape.
The shape of the dot is not particularly limited as long as the shape is sufficient optical, that is, a light scattering effect can be obtained. Examples thereof include a pyramid shape such as a quadrangular pyramid shape, a truncated pyramid shape such as a quadrangular frustum shape, and a semicircular shape. In terms of light scattering effect, a pyramid shape that is not a flat plate shape and a semispherical shape are preferable. Here, the “pyramidal base” means a pyramid with a truncated head.
Therefore, the shape of the dot is preferably a quadrangular pyramid shape, a quadrangular frustum shape, or a hemispherical shape.

  The dot-like or stripe-like intermediate member of the present invention provides a higher light scattering effect than the conventional flat intermediate layer with respect to transmitted light and reflected light, and provides a photoelectric effect due to an increase in photocurrent. Improvement in conversion efficiency can be expected. In the thin film solar cell, generally, the light confinement effect is increased by light scattering, and the effective optical path length in the cell is increased, but the intermediate member of the present invention is compared with the conventional intermediate layer, Gives a higher light scattering effect.

  The dot size needs to be sufficient with respect to the wavelength of light in order to obtain sufficient optical characteristics. In the case of a quadrangular pyramid shape or a quadrangular frustum shape, the diagonal length of the bottom surface is 0. About 1 to 10 μm and a height of about 0.01 to 10 μm are preferable.

Also, the dot interval is greatly affected by the resistivity of the doped layer in the adjacent pin junction cell, and is generally preferably about the same as the dot size. The resistivity of each dot is preferably 10 ⁻¹ Ωcm ² or less in order to obtain sufficient electrical characteristics.
When the dot spacing is sufficiently small relative to the resistivity of the doped layer in adjacent pin junction cells, the current around the dot is concentrated through the doped layer into a low resistance dot, with respect to the junction between the pin junction cells. The effect of improving the bonding is almost the same as that of the conventional flat intermediate layer. In addition, since the dots are not directly connected to each other, even if the resistivity of the intermediate member is reduced for the purpose of improving the electrical connectivity between cells, the conventional multiplicity as shown in FIG. Current leakage during integration that has occurred in the intermediate layer of the junction solar cell is less likely to occur. As a result, the improvement effect due to the low resistance of the intermediate member and the suppression of current leakage during integration can be realized at the same time, and the photoelectric conversion characteristics of the multijunction solar cell are improved.
Furthermore, in the multijunction solar cell of the present invention, it is preferable that an insulating layer such as a silicon oxide film is provided between dots or stripes of the intermediate member.

  On the other hand, when the intermediate member is striped, the width is about 0.1 to 10 μm, and the interval is usually preferably about the same as the width of the dots.

  The formation method of an intermediate member is not specifically limited, According to the shape of an intermediate member, it can form by a well-known method. Specifically, a method of forming a film by a sputtering method using a mask, a CVD method, a coating method, or the like, or a method of partially removing a film formed by a sputtering method, a CVD method, a coating method, or the like by etching or the like. . For example, a zinc oxide film is formed on a pin junction cell by sputtering, and the resulting zinc oxide film is etched with dilute hydrochloric acid using the difference in etching rate caused by the anisotropy and minute spots of zinc oxide. Thus, a dot-shaped intermediate member made of zinc oxide can be obtained.

  In the multijunction solar cell of the present invention, a dot-like or stripe-like intermediate member may be provided between at least one pin junction cell among a plurality of stacked pin junction cells. Therefore, one multi-junction solar cell may have both a dot-shaped or striped intermediate member and a conventional flat intermediate layer. That is, in the case of a 3-junction solar cell, a dot-like or striped intermediate member may be provided between two pin junction cells, and a dot-like or striped intermediate member is provided between one pin junction cell. And a conventional flat intermediate layer may be provided between the other pin junction cells.

  In the multijunction solar cell of the present invention, at least one pin junction cell may be replaced with a pn junction cell or a Schottky junction cell. That is, the multijunction solar cell of the present invention may be a junction type of a pin junction cell and a pn junction cell and / or a Schottky junction cell.

  Furthermore, according to this invention, the multijunction solar cell module which integrated | stacked said multijunction solar cell by the well-known method is provided. Since this module has a reduced leakage current at the time of integration compared to a conventional module, the characteristics of the solar cell after integration will not be significantly reduced.

  Next, embodiments of the multi-junction solar cell of the present invention will be specifically described, but the present invention is not limited to these embodiments.

(Embodiment 1)
FIG. 1 is a schematic cross-sectional view showing an example of a multi-junction solar cell of the present invention having a square pyramid-shaped dot-shaped intermediate member. This multi-junction solar cell includes a glass substrate 1, a transparent electrode 2, an amorphous silicon cell 3, a quadrangular pyramid-shaped dot-shaped intermediate member 8, a microcrystalline silicon cell 4, and a back electrode 5.

This multi-junction solar cell is obtained as follows.
First, a transparent electrode 2 such as ITO having a thickness of about 0.1 to 10 μm is formed on a glass substrate 1 having a thickness of about 0.1 to 10 mm by a known method such as a sputtering method, and further a plasma CVD method or the like. The amorphous silicon cell (pin junction cell) 3 having a film thickness of about 0.1 to 0.5 μm is formed by the known method. Next, an Al or Ga-doped zinc oxide film is formed on the pin junction cell by a known method such as sputtering, and the difference in etching rate caused by anisotropy of zinc oxide and minute spots is used. Then, the obtained zinc oxide film is etched with dilute hydrochloric acid to obtain a dot-shaped intermediate member 8 made of zinc oxide having a diameter of about 1 μm. Next, a microcrystalline silicon cell (pin junction cell) 4 having a thickness of about 0.1 to 10 μm is formed by a known method such as a plasma CVD method. Next, a back electrode 5 made of silver or the like having a thickness of about 0.01 to 10 μm is formed by a known method such as sputtering, thereby completing a multi-junction solar cell.

  In this multi-junction solar cell, since the intermediate member is a quadrangular pyramid-shaped dot, the photocurrent balance between cells is matched, the electrical connectivity between cells is improved, and the solar cell characteristics during integration , That is, the leakage current is suppressed, and the current-voltage characteristics are improved.

(Embodiment 2)
FIG. 2 is a schematic cross-sectional view showing an example of the multijunction solar cell of the present invention having a hemispherical dot-shaped intermediate member. This multi-junction solar cell includes a glass substrate 1, a transparent electrode 2, an amorphous silicon cell 3, a semispherical dot-shaped intermediate member (metal dot) 9, a microcrystalline silicon cell 4, and a back electrode 5.
This multi-junction solar cell is basically the same as Embodiment 1 except that the intermediate member is a semispherical dot shape and the material thereof is metal (silver). The intermediate member can be formed by, for example, a vapor deposition method.
In the conventional flat intermediate layer, since the transmitted light is remarkably reduced, the metal material cannot be used. However, since the transmitted light can be secured in the gap between the dots by forming the dots, the metal material can be used for the intermediate member. . As a result, a good ohmic junction with the pin junction cell material is obtained.

(Embodiment 3)
FIG. 3 is a schematic cross-sectional view showing an example of the multi-junction solar cell of the present invention having a square pyramid-shaped dot-shaped intermediate member. This multi-junction solar cell includes a glass substrate 1, a transparent electrode 2, an amorphous silicon cell 3, a square pyramid-shaped dot-shaped intermediate member (metal dot) 10, an insulating film (silicon oxide film) 11, a microcrystalline silicon cell. 4 and the back electrode 5.
This multi-junction solar cell is basically the same as Embodiment 1 except that the intermediate member is in the form of a square frustum-shaped dot and an insulating layer made of a silicon oxide film is provided between the dots. . The intermediate member can be formed by, for example, sputtering and etching, and the silicon oxide film can be formed by, for example, PECVD.
Since the silicon oxide film provided between the dots also serves as a passivation film for the adjacent pin junction cell, surface recombination of the pin junction cell is suppressed and good solar cell characteristics can be obtained.

It is a schematic sectional drawing which shows an example of the multijunction solar cell of this invention (Embodiment 1). It is a schematic sectional drawing which shows an example of the multijunction solar cell of this invention (Embodiment 2). It is a schematic sectional drawing which shows an example of the multijunction solar cell of this invention (Embodiment 3). It is a schematic sectional drawing which shows an example of the conventional multijunction type solar cell which does not have an intermediate | middle layer. It is a schematic sectional drawing which shows an example of the conventional multijunction solar cell which has an intermediate | middle layer for light reflection. It is a schematic sectional drawing which shows an example of the conventional multijunction solar cell which has an intermediate | middle layer for electrical connectivity improvement. It is a schematic sectional drawing which shows the problem at the time of integrating the multijunction solar cell of FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Glass substrate 2 Transparent electrode 3 Amorphous silicon cell 4 Microcrystalline silicon cell 5 Back surface electrode 6 Intermediate layer made of a material having a small refractive index 7 Low resistance or narrow band gap intermediate layer 8 Square pyramid-shaped dot-shaped intermediate member 9 Half Circular spherical dot-shaped intermediate member (metal dot)
10 Square pyramid shaped dot-shaped intermediate member (metal dot)
11 Insulating film (silicon oxide film)

Claims (8)

  A multi-junction solar comprising a plurality of pin junction cells and a dot-like or stripe-shaped intermediate member provided between at least one pin junction cell among the plurality of pin junction cells battery.   The multi-junction solar cell according to claim 1, wherein the intermediate member is made of a semiconductor material or a metal material having a narrow band gap as compared with a low-resistance semiconductor material or a pin junction cell.   The multi-junction solar cell according to claim 1 or 2, wherein the dot has a quadrangular pyramid shape, a quadrangular frustum shape, or a semispherical shape.   The multijunction solar cell according to claim 3, wherein the quadrangular pyramid shape or the quadrangular frustum shape is a diagonal length of 0.1 to 10 μm and a height of 0.01 to 10 μm.   The multi-junction solar cell according to claim 3, wherein the semispherical shape has a diameter of 0.1 to 10 µm and a height of 0.01 to 10 µm.   The multijunction solar cell according to claim 1, wherein an insulating layer is provided between dots or stripes of the intermediate member.   The multi-junction solar cell according to claim 1, wherein at least one pin junction cell is replaced with a pn junction cell or a Schottky junction cell.   The multijunction solar cell module which integrated the multijunction solar cell as described in any one of Claims 1-7.

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