JP4216059B2 - III / V compound semiconductor solar cell manufacturing method - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a group III / V compound semiconductor solar cell and a method for producing the same.
[0002]
[Prior art]
Various types of solar cells using group III / V compound semiconductors composed of group III and group V elements have been studied. For example, a III / V group compound semiconductor solar cell that prevents a decrease in parallel resistance of a pn junction diode that is a main operating region of the solar cell and has high conversion efficiency has been studied (see Patent Document 1).
[0003]
FIG. 4 shows the structure of a multi-junction solar cell as an example of a III / V group compound semiconductor solar cell. 4A is a plan view, and FIG. 4B is a cross-sectional view taken along line AA in FIG. 4A. FIG. 4C is an enlarged view of the light receiving surface side electrode and its surroundings. In FIG. 4B, an epitaxial layer 41 composed of a GaAs middle cell and a GaInP top cell is provided on a Ge substrate 40 which is a bottom cell, on which a comb-shaped contact layer 42 and a light receiving surface side electrode 43, and a Ge substrate 40 are formed. A back electrode 44 is formed on the back surface of the substrate. As shown in FIG. 4C, the structure of the light receiving surface side electrode 43 is as follows: an AuGe film 43w having a thickness of 100 nm, a Ni film 43x having a thickness of 20 nm, an Au film 43y having a thickness of 70 nm, and a light receiving surface electrode 43. It consists of a 1 μm thick Au plating film 43z occupying the majority.
[0004]
The conventional manufacturing method for III / V group compound semiconductor solar cells is as follows. First, a pn junction is formed on the surface of the Ge substrate by diffusion, and an epitaxial layer made of GaAs or the like is formed on the surface by MOCVD or the like. The structure of the epi layer includes a buffer layer for absorbing the difference in crystal lattice between the Ge substrate and the GaAs epi layer, a tunnel junction layer for electrically overlapping the bottom cell and the middle cell, a middle cell layer by a GaAs pn junction, and a middle cell. It consists of a tunnel junction layer for electrically overlapping the top cell and a top cell layer by GaInP pn junction. A contact layer, which is a semiconductor layer for electrically connecting the top cell and the light receiving surface side electrode, is formed on the epi layer.
[0005]
Subsequently, a comb-shaped electrode pattern made of a photoresist is formed on the contact layer, and an electrode material film made of an AuGe film, an Ni film, and an Au film is formed on the entire surface of the electrode pattern by vapor deposition. After vapor deposition, the electrode pattern made of the resist is removed with an organic solvent or the like in which the resist is soluble, and unnecessary electrode material films formed on the electrode pattern are also removed. Next, a resist pattern is formed by photolithography so that a window is opened only in the portion of the electrode material film made of AuGe film, Ni film, and Au film formed on the surface, and an Au plating film having a thickness of 1 μm is formed. Form. After forming the Au plating film, the contact layer in the region where the light receiving surface side electrode is not formed is removed by etching using the light receiving surface side electrode made of the electrode material film and the Au plating film as a mask.
[0006]
Etching is performed by immersing the substrate in an etching solution using a mixed solution of ammonia water, hydrogen peroxide solution and water as an etching solution. At the time of etching, since Au is not etched by the etching solution, the contact layer (GaAs) under the light receiving surface side electrode and the light receiving surface side electrode is left, and the other contact layers are removed. Next, a photoresist layer is formed, and a cell pattern is formed by photolithography so that a space is formed between the cell region and the cell region on the substrate surface. After the cell pattern is formed, mesa etching is performed in the space so that the cell region has a mesa shape, and a groove is formed. Further, after a back electrode made of Au is formed on the back side of the substrate, an ARC film is formed on the light receiving surface, and finally cut by dicing along a groove formed by mesa etching to manufacture a solar cell. be able to.
[0007]
On the other hand, research is being carried out on the theme of making a low-cost solar cell by using Ag as the main material of the light-receiving surface side electrode instead of expensive Au. An example of a III / V group compound semiconductor solar cell using Ag as the main material of the light-receiving surface side electrode is shown in FIG. Fig.5 (a) is a top view, FIG.5 (b) is AA sectional drawing in Fig.5 (a). FIG. 5C is an enlarged view of the light receiving surface side electrode and its surroundings. In FIG. 5B, an epi layer 51 composed of a GaAs middle cell and a GaInP top cell is formed on a Ge substrate 50 as a bottom cell, and a comb-shaped contact layer 52 and a light-receiving surface side electrode 53 are formed thereon, and Ge A back electrode 54 is formed on the back surface of the substrate 50. As shown in FIG. 5C, the structure of the light receiving surface side electrode 53 is as follows: the AuGe film 53w having a thickness of 100 nm, the Ni film 53x having a thickness of 20 nm, the Au film 53y having a thickness of 70 nm, and the light receiving surface electrode 53. It consists of a 5 μm thick Ag plating film 53z occupying the majority.
[0008]
A solar cell in which Ag is the main material of the light-receiving surface side electrode can be manufactured by the same method as described above. That is, after an epi layer and a contact layer are formed on a Ge substrate, an electrode pattern made of a comb-shaped photoresist is formed on the contact layer, and an electrode material film made of an AuGe film, Ni film, and Au film is formed thereon Then, the resist is removed with an organic solvent or the like in which the resist is soluble, and an electrode pattern made of the resist and an unnecessary electrode material film formed thereon are removed. Next, using this electrode material film as a mask, the contact layer in the region where the electrode material film is not formed is removed by etching. The etching solution is an acidic or alkaline aqueous solution, and Au is not etched, but Ag is etched because it has a high etching rate unlike Au. Therefore, when Ag is used as an electrode material, it is necessary to form an Ag film in a step after etching. That is, after etching the contact layer, alignment is made with the electrode pattern on the substrate surface to form a new photoresist pattern with a window in the electrode pattern portion. An Ag film having a thickness of 5 μm is formed, the resist is removed with an organic solvent or the like in which the resist is soluble, and an unnecessary Ag film formed on the resist is removed. By such a method, a low-cost solar cell having a light-receiving surface side electrode composed of an AuGe film, a Ni film, an Au film, and an Ag film can be manufactured from the lower layer. Also, the cost of the back electrode can be reduced by using Ag instead of expensive Au.
[0009]
[Patent Document 1]
JP-A-8-274358 (pages 2 to 3)
[0010]
[Problems to be solved by the invention]
As shown in FIG. 4, a solar cell using Au as a main material for an electrode uses a large amount of expensive Au, so that the solar cell becomes expensive.
[0011]
On the other hand, in a solar cell in which Ag is the main electrode material instead of expensive Au as shown in FIG. 5, the pattern of the electrode material film composed of the AuGe film, Ni film and Au film on the contact layer, and the electrode material Alignment with the pattern of the Ag film formed on the film is difficult. That is, since Ag is easier to etch than Au, it is necessary to form the Ag film after etching the contact layer. However, the pattern of the electrode material film composed of the AuGe film, Ni film, and Au film, and the electrode material film Since the pattern of the Ag film to be formed is separately formed, the pattern is not exactly the same, and a margin as shown in FIG. 5C is taken in consideration of an alignment error between the two patterns. There must be. Therefore, the electrode area on the light receiving surface of the solar cell is increased accordingly, the light receiving area is reduced, and the photoelectric conversion efficiency is lowered. In addition, since Ag is more easily etched than Au, when Ag is used as an electrode material, even if the upper surface of the Ag electrode is protected by Au or the like, the side wall of the Ag electrode is exposed. Inevitable etching.
[0012]
[Means for Solving the Problems]
In the III / V group compound semiconductor solar battery of the present invention, the light-receiving surface side electrode formed on the contact layer is an electrode mainly composed of Ag and covered with a metal protective film, and the light-receiving surface side electrode is formed. The contact layer that has not been removed is removed. An inexpensive solar cell can be provided by using Ag as the main material of the light-receiving surface side electrode instead of expensive Au.
[0013]
By using an electrode covered with a metal protective film as the light-receiving surface side electrode, even when the main material of the light-receiving surface side electrode is Ag, the resistance of the light-receiving surface side electrode is enhanced when etching the contact layer Can do. Further, since etching of the electrode (wiring) can be prevented, a high-performance and highly reliable solar cell can be manufactured. The metal protective film is preferably made of Au, Pt, Ni, or an alloy thereof in terms of being able to impart great resistance to the light-receiving surface side electrode. Among these, in terms of excellent conductivity, Au is more preferable. The thickness of the metal protective film is preferably 20 nm or more, and more preferably 30 nm or more in terms of imparting sufficient resistance. Further, by removing the contact layer on which the light receiving surface side electrode is not formed, the light transmittance can be increased and the photoelectric conversion efficiency can be increased.
[0014]
The metal protective film preferably contains 50% by volume or more of the electrode material film material covered with the metal protective film when the solar battery is connected to the wiring. The metal protective film is formed of Au or the like and exhibits excellent resistance to the etching solution of the contact layer. However, since Au is not suitable for welding or soldering, a plurality of solar cells are formed after the solar cell is completed. When wiring, it is difficult to connect. On the other hand, Ag has low resistance but is relatively easy to weld or solder. Therefore, the metal protective film is preferably made of Au or the like and has resistance until the etching of the contact layer is completed. However, in the subsequent wiring, a highly reliable connection made of Ag or the like is preferable. preferable. Specifically, since the electrode material film of the present invention contains Ag as a main material, the metal protective film contains about 50% by volume or more of Ag at the time of wiring connection. This is preferable for connecting an interconnector or the like to the light receiving surface side electrode.
[0015]
The method for producing a group III / V compound semiconductor solar cell of the present invention comprises a step of forming a light-receiving surface side electrode on a contact layer, which is made of Ag as a main material and covered with a metal protective film, and a light-receiving surface-side electrode. And a step of removing a contact layer on which the light receiving surface side electrode is not formed as a protective mask. With this method, the above-described inexpensive and efficient solar cell can be manufactured.
[0016]
The metal protective film is preferably formed by a plating method from the viewpoint of obtaining a good thin film, and the electroless plating method is preferable as the plating method. In electroplating, the film thickness changes depending on the applied voltage. Therefore, in a solar cell having a large area compared to other semiconductor devices, the applied voltage tends to vary due to the long wiring length, and the variation in film thickness increases. Tend. On the other hand, electroless plating is a surface reaction. By uniformizing the plating solution by stirring or the like, a uniform thin film can be obtained even in a large area, and the film thickness can be easily controlled. When electroless plating is performed, the plating solution is preferably 60 ° C. or higher, more preferably 70 ° C. or higher, from the viewpoint of easily obtaining a more uniform thin film. The immersion in the plating solution is preferably 20 minutes or longer, and more preferably 30 minutes or longer.
[0017]
In the step of removing the contact layer on which the light receiving surface side electrode is not formed, the contact layer formed under the light receiving surface side electrode can be protected from etching by using the light receiving surface side electrode as a protective mask. . The contact layer on which the light-receiving surface side electrode is not formed is preferably removed with an acidic or alkaline etching solution containing hydrogen peroxide in that the etching rate of the contact layer made of GaAs is high.
[0018]
The step of forming the light receiving surface side electrode on the contact layer includes a step of forming an electrode pattern made of a photoresist, a step of forming an electrode material film on the electrode pattern, and a metal protective film by plating. Preferably, the method includes a step of lifting off unnecessary light-receiving surface side electrodes by removing the photoresist. An electrode pattern is formed by photolithography, an electrode material film is formed on the obtained mask by plating, and then the photoresist is removed, thereby simultaneously removing unnecessary light receiving surface side electrodes and having a desired patterning. A light receiving surface side electrode is formed on the contact layer. By employing such a lift-off method, it is possible to pattern a thick electrode material, reduce the wiring resistance, suppress the electrode area, and increase the photoelectric efficiency.
[0019]
FIG. 2 is a process diagram showing the method for manufacturing a solar cell of the present invention. FIG. 2A shows a state after the epitaxial layer 21 and the contact layer 22 are sequentially formed on the substrate 20. In FIG. 2B, a photoresist layer is formed on the contact layer 22, and an electrode pattern 25 is formed by photolithography. Next, as shown in FIG. 2C, an electrode film material layer 23a is formed on the electrode pattern 25 made of photoresist. Further, in FIG. 2D, when the metal protective film 23b is formed and the electrode pattern 25 is lifted off together with the electrode film material, the structure shown in FIG. 2E is obtained. The portion is removed by etching (FIG. 2F).
[0020]
In the solar cell manufacturing method of the present invention, as shown in FIG. 2C, a gap is formed between the electrode film material layer 23a and the electrode patterns 25 on both sides of the electrode film material layer 23a. In the plating step shown in FIG. 2 (d), an embodiment in which the metal protective film 23b is formed on the upper surface and the side surface of the electrode film material layer 23a by allowing the plating solution to enter the gap is preferable. According to this aspect, the electroless plating can be uniformly applied to the upper surface and the side surface of the electrode film material layer without forming a new mask and without using electroplating.
[0021]
In such an embodiment, as shown in FIG. 2 (c), a gap is formed between the electrode film material layer 23a and the electrode patterns 25 on both sides of the electrode film material layer 23a to facilitate the penetration of the plating solution. Therefore, the thickness of the electrode film material layer 23a is made sufficiently thinner than the thickness of the electrode pattern 25, and the shape of the side surface of the electrode pattern 25 is an inversely tapered shape. As a method of making the shape of the side surface of the electrode pattern 25 into an inversely tapered shape, for example, when the electrode pattern 25 is formed by photolithography, exposure is performed by providing a certain interval between the exposure mask and the photoresist. There is a method using diffracted light generated through a mask for use. If there is a gap between the electrode film material layer 23a and the electrode pattern 25, it is also preferable in that lift-off can be facilitated in a later step. Further, if the thickness of the electrode film material layer 23a is sufficiently smaller than the thickness of the electrode pattern 25, the electrode film material layer 23a is likely to be disconnected when the electrode film material layer 23a is formed due to the step of the electrode pattern 25. It is also preferable from the viewpoint of easy lift-off.
[0022]
After the step of removing the contact layer, it is preferable to anneal the metal protective film at a temperature of 350 ° C. to 450 ° C. for 5 minutes or more. The metal protective film plays an important role in protecting the electrode material film mainly composed of Ag having low resistance in the process of removing the contact layer. However, even after the contact layer is removed, if a protective film made of Au or the like exists on the electrode material film, wiring connection such as welding or soldering is difficult as described above. When a metal protective film is annealed at a temperature of 350 ° C. to 450 ° C. for 5 minutes or longer, a material such as Ag in the internal electrode material film and a material such as Au in the protective film on the surface layer mix. As a result, the material of the electrode material film can be changed to a protective film containing 50% by volume or more. For this reason, at the time of wiring connection, attachment of an interconnector etc. by welding or soldering can be made easy. The annealing treatment is preferably performed in an atmosphere of nitrogen, argon, helium, etc. to avoid oxidation.
[0023]
For example, if a protective film made of Au is formed on the entire surface of the electrode material film made of Ag, and annealing is performed at 380 ° C. for 30 minutes in a nitrogen atmosphere before the interconnector is welded, a gap between Au and Ag is obtained. The mutual movement occurs, and a layer in which Au and Ag are mixed or a layer mainly composed of Ag appears on the surface of the electrode, and welding of the interconnector is facilitated. At this time, the protective film on the electrode surface contains 50% by volume or more of Ag.
[0024]
In the manufacturing process of a III / V group compound semiconductor solar cell having a shallow junction, the annealing treatment is preferably performed at 450 ° C. or lower, and more preferably 430 ° C. or lower, in order to maintain a good junction structure. On the other hand, in order to mix refractory metals such as Au and Ag with each other by heat treatment, the treatment is preferably performed at 350 ° C. or higher, more preferably 380 ° C. or higher. Further, the thickness of the metal protective film is preferably 100 nm or less, and more preferably 50 nm or less so that Au, Ag, and the like are sufficiently mixed at a temperature of 350 ° C. to 450 ° C. for 5 minutes or more.
[0025]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 shows the structure of a III / V group compound semiconductor solar cell according to the present invention. Fig.1 (a) is a top view, FIG.1 (b) is AA sectional drawing in Fig.1 (a). FIG. 1C is an enlarged view of the light receiving surface side electrode and its surroundings. In FIG. 1B, an epi layer 11 composed of a GaAs middle cell and a GaInP top cell on a Ge substrate 10 which is a bottom cell, a comb-shaped contact layer 12 and a light receiving surface side electrode 13, and a light receiving surface of the Ge substrate 10 A back electrode 14 is formed on the surface opposite to the surface. As shown in FIG.1 (b), as for the solar cell of this invention, the contact layer is removed in the area | region in which the light-receiving surface side electrode 13 is not formed. As shown in FIG. 1C, the light receiving surface side electrode 13 includes an electrode material film 13a and an Au protective film 13b, and the Au protective film 13b covers the upper surface and side surfaces of the electrode material film 13a. . The electrode material film 13a includes an AuGe film 13w having a thickness of 100 nm, an Ni film 13x having a thickness of 20 nm, an Au film 13y having a thickness of 70 nm, and an Ag film 13z having a thickness of 5 μm. Therefore, the light-receiving surface side electrode of the solar cell of this invention uses Ag as a main material.
[0026]
About the manufacturing method of the solar cell of this invention, one Embodiment is roughly shown to FIG. 2 (a)-(f) and FIG. 3 (a)-(c). As shown in FIG. 2A, an epi layer 21 and a contact layer 22 are formed on a Ge substrate 20 by a method similar to the conventional method. After a relatively thick photoresist layer is formed thereon, a comb-shaped electrode (wiring) pattern 25 is formed by photolithography (FIG. 2B). An electrode material film 23a is formed on the entire surface by vapor deposition (FIG. 2C). The electrode material film 23a is composed of a 100 nm thick AuGe film, a 20 nm thick Ni film, a 70 nm thick Au film, and a 5 μm thick Ag film. Therefore, the light-receiving surface side electrode formed in the present invention is mainly composed of Ag.
[0027]
Next, the electrode material film 23a is subjected to electroless plating and covered with a 30 nm Au protective film 23b (FIG. 2D). The electroless plating is performed, for example, by immersing a non-cyanide type Au plating solution composed of gold sodium sulfite, sodium sulfite, sodium thiosulfate, glycine, cuperone and ascorbic acid at 70 ° C. and immersing in that for 30 minutes. Since it is substitutional electroless plating, the process margin with respect to the processing time is large, and the thickness of the protective film 23b made of Au hardly changes even if it is processed for a long time. When the specific resistance of the back surface of the Ge substrate 20 is small and there is a risk of Au plating, it is preferable to protect the back surface of the Ge substrate 20 with a resist in advance.
[0028]
Subsequently, the electrode pattern 25 made of the resist is lifted off with an organic solvent or the like in which the resist is soluble. By removing the electrode pattern 25, the unnecessary electrode material film formed on the electrode pattern 25 and the Au protective film thereon are also removed. As a result, the structure shown in FIG. This structure has a light receiving surface side electrode 23 on a contact layer 22, and the light receiving surface side electrode 23 includes an electrode material film 23a made of an AuGe film, a Ni film, an Au film, and an Ag film, and an electrode material film 23a. It is comprised by the protective film 23b made from Au which coat | covers a side surface and an upper surface.
[0029]
Next, an etching solution mixed at a ratio of ammonia water: hydrogen peroxide solution: water = 1: 1: 10 is kept at room temperature, and the structure shown in FIG. 2E is immersed in the etching solution for 1 minute. As a result, the contact layer 22a under the light receiving surface side electrode 23 remains unetched as a result of the light receiving surface side electrode 23 functioning as a protective mask, but the contact layer in the region where the light receiving surface side electrode is not formed It is etched and removed (FIG. 2 (f)). The electrode material film 23a is covered with an Au protective film 23b on the side surface and the upper surface, and the Au protective film 23b exhibits a function of protecting the electrode material film 23a from an etching solution. It remains without being. Subsequently, annealing is performed at 380 ° C. for 30 minutes in a nitrogen atmosphere, so that the Ag content of the surface layer of the electrode is 50% by volume or more.
[0030]
Thereafter, after forming a relatively thick photoresist layer, a cell pattern 35 is formed by photolithography so that a space 36 is formed between the cell region and the cell region (FIG. 3A). After forming the cell pattern 35, the Ge substrate 30 in the portion where the space 36 is provided is mesa-etched so that the cell region has a mesa shape, and a groove 37 is formed (FIG. 3B). Next, a back electrode 34 made of Ag is formed on the back surface side of the Ge substrate 30 and an ARC film (not shown) is formed on the light receiving surface side, and then cut along the groove 37 by dicing or the like. In this way, the cell-shaped solar cell shown in FIG. 3C can be manufactured.
[0031]
The embodiment disclosed this time should be considered as illustrative in all points and not restrictive. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.
[0032]
【The invention's effect】
According to the present invention, it is possible to produce a III / V group compound semiconductor solar cell having the same cell size and a small electrode area by using Ag instead of Au as the main material of the electrode. This III / V compound semiconductor solar cell is inexpensive, has a large output, and is highly reliable.
[Brief description of the drawings]
FIG. 1 is a schematic view showing the structure of a III / V compound semiconductor solar cell according to the present invention.
FIG. 2 is a process diagram showing a method for producing a III / V compound semiconductor solar cell according to the present invention.
FIG. 3 is a process chart showing a method for producing a III / V compound semiconductor solar cell according to the present invention.
FIG. 4 is a schematic view showing the structure of a conventional III / V group compound semiconductor solar cell using Au as the main material of the light-receiving surface side electrode.
FIG. 5 is a schematic view showing the structure of a conventional III / V group compound semiconductor solar cell using Ag as a main material of the light-receiving surface side electrode.
[Explanation of symbols]
10 Ge substrate, 11 epi layer, 12 contact layer, 13 light receiving surface side electrode, 14 back surface electrode, 23a electrode material film, 23b Au protective film, 25 electrode pattern, 35 cell pattern.

Claims (8)

In the method for producing a group III / V compound semiconductor solar cell,
A step of forming on the contact layer a light-receiving surface side electrode made of Ag as a main material and covered with a metal protective film;
Using the light receiving surface side electrode as a protective mask, removing the contact layer on which the light receiving surface side electrode is not formed ;
Annealing the metal protective film at a temperature of 350 ° C. to 450 ° C. for 5 minutes or more,
The material of the metal protective film is Au, Pt, Ni or an alloy thereof,
The method for producing a group III / V compound semiconductor solar cell, wherein the annealing treatment causes an Ag content in the metal protective film to be 50% by volume or more . The metal protective film is formed immediately after, Au, Pt, Ni or claim 1 III · V compound semiconductor solar manufacturing method of battery, wherein the consisting of alloys. The metal protective film is formed immediately claim 1 or 2 method for producing a III · V compound semiconductor solar cell, wherein the a thickness of 20 nm to 100 nm. The said metal protective film is formed by the plating method, The manufacturing method of the III * V group compound semiconductor solar cell in any one of Claims 1-3 characterized by the above-mentioned. 5. The method for producing a group III / V compound semiconductor solar cell according to claim 4 , wherein the plating method is an electroless plating method. 6. The method for producing a III / V group compound semiconductor solar cell according to claim 5, wherein the electroless plating method is performed by immersing in a plating solution at 60 [deg.] C. or more for 20 minutes or more. 5. The method according to claim 1, wherein an acidic or alkaline etching solution containing hydrogen peroxide is used in the step of removing the contact layer on which the light receiving surface side electrode is not formed. A method for producing a group V compound semiconductor solar cell. The step of forming the light receiving surface side electrode on the contact layer,
Forming an electrode pattern made of a photoresist;
Forming an electrode material film from above the electrode pattern;
Forming the metal protective film by a plating method;
Claim 1 III · V compound semiconductor solar manufacturing method of battery, wherein the comprising the step of lifting off the unwanted light-receiving surface side electrode by removing the photoresist.

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