The claimed invention relates to a multi-junction compound solar cell, a multi-junction compound solar battery, and a method for manufacturing the same.
A multi-junction III-V group compound solar battery has been proposed as a solar battery suitable for a concentrating solar battery, which has the highest efficiency among solar batteries (see Patent Literature (hereinafter abbreviated as PTL) 1, for example). An example of a structure of such a multi-junction III-V group compound solar battery and a manufacturing method thereof will be described.
In order to obtain the multi-junction III-V group compound solar battery in the related art shown in
Top cell T including pn junction of InGaP is formed on sacrifice layer 4. It is necessary to initially form top cell T, instead of bottom cell B, in order to match with a grating constant of the GaAs substrate and to prevent misfit dislocation or defects such as pores from occurring. Top cell T is formed by epitaxial growth of InGaP or the like. The band gap of InGaP that constitutes top cell T is about 1.7 to 2.1 eV.
Next, middle cell M including pn junction of GaAs is formed on top cell T. Middle cell M is formed by epitaxial growth of GaAs or the like. The band gap of GaAs that constitutes middle cell M is about 1.3 to 1.6 eV.
Further, bottom cell B including pn junction of InGaAs is formed on middle cell M. Bottom cell B is formed by epitaxial growth of InGaAs or the like. The band gap of InGaAs that constitutes bottom cell B is 1.0 eV or lower.
In this way, a cell laminate is obtained in which three pn junctions of InGaP, GaAs and InGaAs are connected on GaAs substrate 1 in series. The obtained cell laminate is solar cell C of a three-junction III-V group compound solar battery.
In a case where solar cell C is used as a solar battery, solar light beams are incident from the side of top cell T and proceed toward bottom cell B (InGaAs). According to this configuration, light of a predetermined wavelength based on each band gaps of top cell T, middle cell M and bottom cell B is absorbed and converted into electric energy. Thus, it is possible to realize a solar battery with high efficiency.
However, in the cell laminate in the state shown in
In order to obtain the structure in which light can be incident from top cell T, rear surface electrode 9 is formed on an overall surface of bottom cell B by plating, in a first process. In a second process, solar cell C and GaAs substrate 1 are separated from each other. The separation is performed using weakness of sacrifice layer 4. Sacrifice layer 4 that remains on the separated solar cell C is removed by etching using hydrofluoric acid.
Next, front surface electrode 15 is formed to extract an electric potential from top cell T (see
Through these processes, a multi-junction compound solar battery of a double-sided electrode structure in the related art in which top cell T, middle cell M and bottom cell B are sequentially laminated and rear surface electrode 9 and front surface electrode 15 are provided, as shown in
In addition to the above-described technique, various techniques have been proposed as a technique relating to a multi-junction compound solar battery (for example, see PTLs 2 to 6).
For example, PTL 2 discloses an extraction electrode structure of a thin solar battery in which a first electrode and a second electrode are electrically connected to each other through a connecting groove provided inside a laminated band. According to this technique, it is possible to reduce the area of an extraction electrode section. However, this electrode structure is provided on the first electrode that extends from a connection terminal end portion of a plurality of solar cells that is connected in series, which does not increase the solar light receiving area of each solar cell.
For example, PTL 3 discloses a solar battery module including a plurality of solar cells in which a lower electrode (rear surface electrode) of each solar cell (tandem photoelectric conversion cell) and a transparent electrode (light receiving surface electrode) of an adjacent solar cell are electrically joined to each other through a grating electrode. According to this technique, it is possible to join the plurality of solar cells in series by the grating electrode. However, this technique does not increase the solar light receiving area of each solar cell.
PTL 1: Japanese Patent No. 4471584
PTL 2: Japanese Patent Application Laid-Open No. HEI 9-83001
PTL 3: Japanese Patent Application Laid-Open No. 2006-13403
PTL 4: Japanese Patent Application Laid-Open No. 2008-34592
PTL 5: US Patent Application Laid-Open No. 2001-0023962
PTL 6: US Patent Application Laid-Open No. 2010-0065115
As described above, the multi-junction compound solar battery in the related art includes front surface electrode 15 on the surface of top cell T. Since front surface electrode 15 is made of a metallic material such as Au, Ni or Ge that does not transmit solar light, the amount of solar light that is incident on top cell T decreases. Further, other techniques in the related art do not propose a method of increasing the solar light receiving area of a solar cell.
Due to the double-sided electrode structure of front surface electrode 15 and rear surface electrode 9, mounting of rear surface electrode 9 should be performed in a die bonding process, and mounting of front surface electrode 15 should be performed in a wire bonding process or a soldering process. That is, in order to achieve electric connection with the outside, two mounting processes of the mounting of rear surface electrode 9 and the mounting of front surface electrode 15 are necessary. As a result, a production lead time is prolonged.
Further, since the thicknesses of top cell T, middle cell M and bottom cell B that constitute solar cell C are only 5 μm to 20 μm, if stress is applied from the outside, solar cell C is easily damaged. Thus, solar cell C may be damaged due to stress generated by the process of separating solar cell C and GaAs substrate 1 using weakness of sacrifice layer 4, the die bonding process of rear surface electrode 9, the wire bonding process or the soldering process of front surface electrode 15, or the like.
In order to solve the above problems, an object of the invention is to remove an electrode that blocks solar light on top cell T of a multi-junction compound solar cell, to provide a multi-junction compound solar cell having a structure that is not easily damaged in a production process, and to reduce a production lead time of a multi-junction compound solar battery.
In order to achieve the above object, the following configurations of the invention are provided.
[1] According to a first aspect of the invention, there is provided a multi-junction compound solar cell including: a multi-junction cell laminate that includes a top cell and a bottom cell; a transparent electrode that is disposed on a light incident surface of the top cell; a lower electrode that has an electric potential of the bottom cell; and a side surface electrode that is disposed on a side surface of the cell laminate through an insulating layer and is conducted to the transparent electrode, wherein the side surface electrode is extended to the lower electrode.
[2] According to a second aspect of the invention, there is provided a multi-junction compound solar battery including: the multi-junction compound solar cell according to [1]; and an external member that is connected to each of the lower electrode and the side surface electrode, wherein conductive members that respectively connect the lower electrode and the side surface electrode with the external member include a stress absorption layer.
[3] According to a third aspect of the invention, there is provided a multi-junction compound solar battery including: the multi-junction compound solar cell according to [1]; and an external member that is connected to each of the lower electrode and the side surface electrode, wherein connection sections that respectively connect the lower electrode and the side surface electrode with the external member are not overlapped with the cell laminate in a pressing direction for connection of the multi-junction compound solar cell with the external member.
[4] According to a fourth aspect of the invention, there is provided a method for manufacturing the multi-junction compound solar battery according to [2], including: pressing and joining the lower electrode and the side surface electrode of the multi-junction compound solar cell, and the external member through a conductive member, wherein the shape of a side surface of the conductive member is a tapered shape, and the tapered shape is crushed and deformed by the joining.
According to the multi-junction compound solar cell of the invention, since an electrode other than the transparent electrode is not provided on a solar light receiving surface, usage efficiency of solar light is enhanced. Further, according to the multi-junction compound solar cell of the invention, since the electrodes (an electrode having an electric potential of the top cell and an electrode having an electric potential of the bottom cell) connected to the outside are extended on one surface, a production process for mounting of an external electrode is performed only once. Thus, a production lead time is reduced.
Further, according to the invention, in a mounting process of the multi-junction compound solar cell to the external member, by positively deforming the stress relaxation layer disposed between the multi-junction compound solar cell and the external member, stress applied to the solar cell is reduced. Alternatively, by regulating the positional relationship between the solar cell and the electrode connected to the outside, stress applied to the solar cell is reduced. Thus, damage of the solar cell is suppressed.
Further, by adjusting the relationship between the thickness of the solar cell and the thickness of the electrode connected to the outside, it is possible to suppress damage of the solar cell.
Hereinafter, a compound solar battery according to an embodiment of the invention will be described with reference to the accompanying drawings. The same reference numerals are given to substantially the same members in the drawings, and description thereof will be omitted.
Solar cell 10 of the multi-junction compound solar battery shown in
Solar cell 10 includes transparent electrode (ZnO) 12 provided on an upper surface of upper contact layer 2a of the cell laminate. Transparent electrode 12 extracts an electric potential of top cell T. Upper electrode 9b is connected to transparent electrode 12. Side surface electrode 16a is connected to upper electrode 9b. Insulating layer 17 is present between side surface electrode 16a and the cell laminate to insulate side surface electrode 16a from the cell laminate. Insulating layer 17 is composed of a silicon nitride film or the like.
On the other band, solar cell 10 includes lower electrode 9a provided on a lower surface of lower contact layer 2b of the cell laminate. Central electrode 16b is provided on a lower surface of lower electrode 9a.
Here, it is preferable that a lower surface of side surface electrode 16a and a lower surface of central electrode 16b are aligned with each other on a broken line LL. When interposer substrate 24 joins with solar cell 10 which will be described later referring to
The lower surface of side surface electrode 16a and the lower surface of central electrode 16b that are arranged on the same plane are electrically connected to element-sided electrodes 25a and 25b of interposer substrate 24 that is the external member through a conductive member, respectively. Side surface electrode 16a and central electrode 16b are electrically arranged independently of each other. Similarly, element-sided electrode 25a and element-sided electrode 25b, through electrode 27a and through electrode 27b, and external extraction electrode 26a and external extraction electrode 26b are electrically arranged independently of each other.
Interposer substrate 24 includes element-sided electrode 25 that is arranged on an upper surface thereof (surface that faces solar cell 10), external extraction electrode 26 that is arranged on a lower surface thereof, and through electrode 27 that passes through the inside of interposer substrate 24 to connect element-sided electrode 25 with external extraction electrode 26.
The conductive member includes protrusion electrode 23 having stress absorption layer 23a. Protrusion electrode 23 is connected to element-sided electrode 25 of interposer substrate 24.
A gap between interposer substrate 24 and solar cell 10 is sealed by sealing resin 22 in order to reinforce mechanical strength and to improve chemical resistance. In this way, an overall configuration of a single multi-junction compound solar battery is achieved as a package.
The epitaxial growth of each metallic layer may be performed by a normal technique. For example, an environment temperature may be set to about 700° C. TMG (trimethylgallium) and AsH3 (arshin) may be used as a material for growth of the GaAs layer. TMI (trimethylindium), TMG and PH3 (phosphine) may be used as a material for growth of an InGaP layer. Further, SiH4 (monosilane) may be used as an impurity for formation of an n-type GaAs layer, an n-type InGaP layer and an n-type InGaAs layer. On the other hand, DEZn (diethyl zinc) may be used as an impurity for formation of a p-type GaAs layer, a p-type InGaP layer and a p-type InGaAs layer.
First, an AlAs layer having a thickness of about 100 nm is grown on GaAs substrate 1 as sacrifice layer 4. Then, an n-type InGaP layer having a thickness of about 0.1 μm is grown as upper contact layer 2a.
Next, top cell T is formed. An n-type InAlP layer having a thickness of about 25 nm that is a window, an n-type InGaP layer having a thickness of about 0.1 μm that is an emitter, a p-type InGaP layer having a thickness of about 0.9 μm that is a base, and a p-type InGaP layer having a thickness of about 0.1 μm that is a BSF are respectively formed by the epitaxial growth method. As a result, top cell T having a thickness of about 1 μm is formed.
After top cell T is formed, a p-type AlGaAs layer having a thickness of about 12 nm and an n-type GaAs layer having a thickness of about 20 nm are grown as tunnel layer 19. As a result, tunnel layer 19 having a thickness of about 30 nm is formed.
Then, middle cell M is formed. An n-type InGaP layer having a thickness of about 0.1 μm that is a window, an n-type GaAs layer having a thickness of about 0.1 μm that is an emitter, a p-type GaAs layer having a thickness of about 2.5 μm that is a base, and a p-type InGaP layer having a thickness of about 50 nm that is a BSF are respectively formed by the epitaxial growth method. As a result, middle cell M having a thickness of about 3 μm is formed.
After middle cell M is formed, a p-type AlGaAs layer having a thickness of about 12 nm and an n-type GaAs layer having a thickness of about 20 nm are grown as tunnel layer 19. As a result, tunnel layer 19 having a thickness of about 30 nm is formed.
Next, grid layer 20 is formed. Grid layer 20 suppresses occurrence of dislocation, defects or the like due to mismatch of grating constants. An n-type InGaP layer having a thickness of about 0.25 μm is provided to form eight layers, and grid layer 20 having a thickness of about 2 μm is formed. Further, an n-type InGaP layer having a thickness of about 1 μm is formed as buffer layer 21.
Next, bottom cell B is formed. An n-type InGaP layer having a thickness of about 50 nm that is a passivation film, an n-type InGaAs layer having a thickness of about 0.1 μm that is an emitter, a p-type InGaAs layer having a thickness of about 2.9 μm that is a base, and a p-type InGaP layer having a thickness of about 50 nm that is a passivation film are respectively formed by the epitaxial growth method. As a result, bottom cell B having a thickness of about 3 μm is formed. Finally, a p-type InGaAs layer having a thickness of about 0.1 μm is formed as lower contact layer 2b.
The manufacturing flow of the compound solar battery will be described with reference to
In a process of
In a process of
In a process of
In a process of
In a process of
In a process of
In a process of
In a process of
Central electrode 16b and side surface electrode 16a are formed by electrolytic Au plating. The thicknesses of central electrode 16b and side surface electrode 16a made of the Au plated film can be larger than 10 μm that corresponds to a thickness of the cell laminate of the solar cell, which is about 10 μm to about 50 μm.
In a process of
In a process of
In a process of
As shown in
One characteristic of the invention is that GaAs substrate 1 is separated to obtain a solar battery without causing damage to the cell laminate, in spite of a reduced thickness (for example, 10 μm or less) of the cell laminate of solar cell 10.
Interposer substrate 24 can be composed of silicon, ceramic, glass epoxy, glass or the like, and includes through electrode 27 passing through the inside thereof. Further, interposer substrate 24 includes element-sided electrode 25 on a surface thereof where solar cell 10 is to be arranged, and external extraction electrode 26 on an opposite surface thereof. The outermost surfaces of element-sided electrode 25 and external extraction electrode 26 are covered by an Au film. The Au film is formed by flash Au plating or electrolytic Au plating, and has a maximum thickness of 0.5 μm.
In a process of
As shown in
The material of protrusion electrode 23 is generally Au, but may be a single metal such as Ti, Cu, Al, Sn, Ag, Pd, Bi, Pb, Ni or Cr, or may be a composite metal thereof. Protrusion electrode 23 made of a metallic material may be formed by a technique such as a stud bump method using a wire bonding process. For example, the diameter of column portion 23b is set to 20 μm to 50 μm, and the thickness of column portion 23b (length in a conducting direction) is set to 6 μm to 10 μm, and the thickness of the stress absorption layer is set to 20 μm or more.
In this way, protrusion electrode 23 is composed of two conductive members (column portion 23b and stress absorption layer 23a) having different shapes. Further, the cross-section of stress absorption layer 23a connected to solar cell 10 is set to be smaller than the cross-section of column portion 23b. Stress absorption layer 23a is deformed due to stress applied when solar cell 10 and interposer substrate 24 are joined to each other to relieve stress (see
In a process of
In a process of
Further, column portion 23b and stress absorption layer 23a of protrusion electrode 23 may be composed of metals having different Young's modulus. Specifically, column portion 23b is composed of a metal having a high Young's modulus, and stress absorption layer 23a is composed of a metal having a low Young's modulus. Two metallic materials are selected from Au, Al, Cu, Ag, Sn, Bi or the like, respectively.
Junctions of central electrode 16b and side surface electrode 16a of solar cell 10 with protrusion electrodes 23 are performed by ultrasonic metal junction using a heating ultrasonic head, for example. In a case where the ultrasonic metal junction is performed, surfaces of side surface electrode 16a and central electrode 16b are formed by Au, Al, Cu, Ag, Sn or the like. The ultrasonic metal junction is a junction method of breaking oxide films of the metal surfaces with heating and ultrasonic energy so as to for an alloy layer between metals.
In this way, by arranging stress absorption layer 23a that is in contact with solar cell 10 composed of a metal having a low Young's modulus, upon preforming junction, the stress absorption layer is easily deformed, and thus, stress is further easily relieved.
Protrusion electrode 23 that is arranged over interposer substrate 24 may be formed of conductive paste. The conductive paste includes a resin component such as epoxy resin or silicone resin, and a conductive metal such as Ag, Pd, Au, Cu, Al, Ni, Cr or Ti. Protrusion electrode 23 that is composed of the conductive paste may be formed by a coating method or a printing method. Protrusion electrode 23 that is composed of the conductive paste may not include stress absorption layer 23a, that is, do not necessarily have a tapered shape. Solar cell 10 is in contact with the conductive paste that constitutes protrusion electrode 23, and then cures the conductive paste. Thus, excessive stress is not applied to solar cell 10.
In order to join central electrode 16b and side surface electrode 16a of solar cell 10 with the protrusion electrode formed of the conductive paste, central electrode 16b and side surface electrode 16a of solar cell 10 may be in contact with protrusion electrode 23 to cure the conductive paste contained in protrusion electrode 23.
Protrusion electrode 23 may be formed of a flexible material (conductive resin or the like). Protrusion electrode 23 composed of the conductive resin may be formed by dispenser coating or mask printing. It is preferable that the viscosity of the conductive resin be 2000 cps to 500000 cps. The conductive resin is a liquid resin including metallic fillers made of Ag, Pd, Au, Cu or the like.
If central electrode 16b and side surface electrode 16a of solar cell 10 are joined to protrusion electrode 23 composed of the flexible material, stress applied to solar cell 10 may be absorbed by protrusion electrode 23.
Central electrode 16b and side surface electrode 16a shown in
The solar cell shown in
As described above, after interposer substrate 24 (see
As described above, the size of GaAs substrate 1 is a 4-inch diameter, and the size of interposer substrate 24 is 20 mm×20 mm or a 4-inch diameter. In a case where the size of interposer substrate 24 is a square of 20 mm×20 mm, a plurality of interposer substrates is mounted on GaAs substrate 1 that is a 4-inch wafer. Sealing resin 22 is flow from a gap between the plurality of interposer substrates to a gap between GaAs substrate 1 and interposer substrate 24 using the capillary phenomenon. As a result the gaps are filled with the sealing resin 22.
On the other hand, in a case where interposer substrate 24 is the 4-inch diameter, similarly, the gaps are filled with sealing resin 22 using the capillary phenomenon. In this case, it is preferable to employ sealing resin 22 with a low viscosity.
After the gap between GaAs substrate 1 and interposer substrate 24 is filled with sealing resin 22, sealing resin 22 is heated at about 150° C. to 200° C. for about 15 minutes to about 1 hour to be cured.
In a process of
In a process of
Further, since the grating constant of GaAs that constitutes substrate 1 is 5.653 angstrom, and the grating constant of AlAs that constitutes sacrifice layer 4 is 5.661 angstrom, both of them approximately matches with each other. Thus, sacrifice layer 4 forms a stable film, and may be stably internally broken.
In a process of
In a process of
Solar cell 10 obtained in this way does not have an electrode that blocks solar light, on the incidence surface of the solar light. Accordingly, the amount of solar light that is incident on solar cell 10 is increased, and power generation efficiency of solar cell 10 is improved.
In a process of
In a process of
The appearance size of the solar battery is 500 μm×500 μm, and the appearance size of the cell laminate of solar cell 10 is 470 μm×470 μm. Further, the extension length of side surface electrode 16a is 15 μm. That is, the occupied area of solar cell 10 (the appearance size of the cell laminate of solar cell 10 with respect to the appearance size of the solar battery) is 88%.
Since an electrode other than transparent electrode 12 is not provided on a light receiving surface of solar cell 10, it is possible to use the overall solar light that is irradiated.
In the solar battery shown in
The present application claims priority based on Japanese Patent Application No. 2011-113643, filed May 20, 2011, the content of which is incorporated herein by reference.
The multi-junction compound solar battery of the invention may be applied to a concentrating solar battery used on the Earth in addition to existing usage in space. Further, it is possible to remarkably enhance conversion efficiency of solar light compared with a silicon solar cell in the related art. Thus, the multi-junction compound solar battery of the invention is particularly suitable for a large-scale power generation system in an area with a large amount of daylight.
Number | Date | Country | Kind |
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2011-113643 | May 2011 | JP | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/JP2012/003010 | 5/8/2012 | WO | 00 | 7/26/2013 |