JP4674063B2 - Photoelectric conversion element and manufacturing method thereof - Google Patents

Description

  The present invention relates to a photoelectric conversion element used for, for example, a silicon solar cell and a manufacturing method thereof, and more specifically to a photoelectric conversion element having a passivation film formed on the back surface side and a manufacturing method thereof.

  Conventionally, for the purpose of improving photoelectric conversion efficiency, recombination loss of photoexcited carriers (electrons or holes) is prevented at the interface between the ohmic electrode on the back surface (non-light incident side) of the photoelectric conversion element and the semiconductor substrate, and Solar cells with improved collection efficiency are known. In this solar cell, a silicon oxide film or the like is formed on the back side of a p-type silicon semiconductor substrate, and backside passivation is performed so that minority carriers (electrons) that diffuse inside the p-type semiconductor substrate recombine with majority carriers. Thus, the rate of disappearance is reduced to improve carrier recovery efficiency and open circuit voltage.

  This conventional solar cell will be described with reference to FIG. Here, FIG. 7 schematically shows a cross-sectional structure of a conventional solar cell. As shown in the figure, in this solar cell, an n-type semiconductor layer 52 and a silicon oxide film layer 53 containing phosphorus as an impurity (donor) are formed on the light incident surface (front surface) side of a p-type semiconductor substrate 51. Yes. A light-receiving surface electrode 58 is formed so as to penetrate the silicon oxide film layer 53 and connect to the n-type semiconductor layer 52.

On the other hand, the back surface structure described below is formed on the side opposite to the light incident surface (back surface). That is, in FIG. 7, an opening 54 is provided on the back side to form a silicon oxide film layer 55, and a p ⁺ layer 56 in which boron or aluminum or the like is diffused is formed in the p-type semiconductor substrate 51 in the opening 54. Yes. Further, a back electrode 57 is formed so as to cover the silicon oxide film layer 55 and the p ⁺ layer 56.

  Hereinafter, the function of each layer of the conventional solar cell thus formed will be described focusing on the back surface structure that contributes to the improvement of photoelectric conversion efficiency. First, when light enters the p-type semiconductor substrate 51 and the n-type semiconductor layer 52, each of the p-type semiconductor and the n-type semiconductor layers 51 and 52 is excited by light to generate electron / hole pairs. This electron-hole pair is subjected to the action of an interface electric field in the junction region between the p-type semiconductor substrate 51 and the n-type semiconductor layer 52, so that electrons are on the n-type semiconductor layer 52 side and holes are on the p-type semiconductor substrate 51 side. As a result, a photovoltaic force is generated between the p-type semiconductor substrate 51 and the n-type semiconductor layer 52.

  At this time, the silicon oxide film layer 55 inactivates dangling bonds (recombination centers) of silicon atoms on the back surface layer portion of the p-type semiconductor substrate 51 and has a wider forbidden band than the p-type semiconductor substrate. As a result, a potential barrier is formed with the p-type semiconductor substrate 51. As a result, minority carriers are not captured by the recombination center of the back surface layer portion, and minority carriers do not flow out from the back surface layer portion into the silicon oxide film 55 due to the potential barrier. The recombination loss is suppressed (back surface passivation effect).

In addition, the p ⁺ layer 56 forms an internal electric field between the p ⁺ layer 56 and the p-type semiconductor substrate 51 because the forbidden band is wider than that of the p-type semiconductor substrate 51. As a result, minority carriers (electrons) trying to flow to the back electrode side are pushed back into the p-type semiconductor substrate 51 by this internal electric field, and recombination loss at the interface with the back electrode 57 is suppressed (back surface field effect). ).

  As described above, the back surface of the conventional solar cell has a back surface passivation effect and a back surface field effect in order to reduce the rate at which the photoexcited carriers are recombined and disappear, thereby improving the photoelectric conversion efficiency. A back surface structure is formed. Silicon solar cells that have recorded the world's highest conversion efficiency to date have this backside structure (see, for example, Non-Patent Documents 1 and 2).

  Here, a method for forming a back surface structure of a conventional solar cell will be briefly described.

First, the silicon oxide film 55 is formed on the back surface of the p-type semiconductor substrate 51 by using a thermal oxidation method, a chemical vapor deposition method, or a method of applying and baking a silicon oxide-containing coating solution. Thereafter, an opening 54 is provided in the silicon oxide film 55. Recently, in addition to the conventionally used photolithography method, a plasma etching method (for example, see Non-Patent Document 3) or a laser ablation method (for example, a non-type) is used. Patent Document 4) or mechanical ablation method (for example, see Non-Patent Document 5) has been proposed. Subsequently, the back electrode 57 is formed by vapor deposition of aluminum or the like, or printing / heat treatment of an aluminum paste.
Prog. Photovolt: Res. Appl. 2003; 11: 39-45 Appl. Phys. Lettes 1998; 73: 1991-1993. Proc. of 16th European PVSEC p. 1443 Proc. of 16th European PVSEC p. 1181 Proc. of 16th European PVSEC p. 1091

  The above-described methods for opening the silicon oxide film such as the photolithography method, the plasma etching method, the laser ablation method, or the mechanical ablation method are large and expensive processes, and there is a problem in terms of cost reduction.

  This invention is made | formed in view of such a problem, and makes it a subject to provide the manufacturing method of the highly efficient photoelectric conversion element which has a back surface passivation effect at low cost.

The method for producing a photoelectric conversion element of the present invention includes a step of forming a passivation film on the back surface of a substrate having irregularities on the back surface, and a step of applying a back electrode material so that the thickness at the concave portion is selectively increased. by firing the material, to form a back surface electrode, and forming a back electrode by forming an opening through the passivation film only in the recess, Ru back electrode and the substrate is electrically connected Is provided.
The photoelectric conversion element of the present invention is a photoelectric conversion element using a semiconductor substrate having irregularities on the back surface. A passivation film and a back electrode are formed in this order on the back surface of the semiconductor substrate. Is in contact with the semiconductor substrate through an opening provided in the passivation film, and in the region other than the concave portion of the semiconductor substrate, the back electrode is interposed through the passivation film with respect to the semiconductor substrate.

  According to the present invention, an opening is formed in the passivation film on the back surface of the substrate by selectively applying a thick material for the back electrode to the concave portion of the substrate having unevenness and baking the material. Therefore, a highly efficient photoelectric conversion element having a back surface passivation film can be manufactured easily and at low cost without using an expensive apparatus and without providing a complicated process.

1. Method for Manufacturing Photoelectric Conversion Element The method for manufacturing a photoelectric conversion element according to the present invention includes a step of forming a passivation film on the back surface of a semiconductor substrate having irregularities on the back surface, and a back electrode so that the thickness at the recess is selectively increased. And a step of forming a back electrode that is electrically connected to the substrate through the passivation film in the recess by firing the material.

1-1. Step of forming a passivation film on the back surface of a semiconductor substrate having irregularities on the back surface The semiconductor substrate is composed of an elemental semiconductor substrate such as silicon or a compound semiconductor substrate such as GaAs. The unevenness on the back surface of the substrate can be produced, for example, by growing a semiconductor layer on a substrate on which unevenness is formed, as disclosed in JP-A-2001-223172. Further, unevenness may be formed on the back surface of the substrate having a flat back surface by using mechanical or chemical means. The mechanical means is, for example, cutting with a dicing apparatus, and the chemical means is, for example, a wet etching method. The irregularities are preferably formed substantially periodically. The unevenness may be formed on at least a part of the back surface of the substrate. The unevenness preferably has a bottom pitch of 1 to 3 mm. The unevenness is preferably such that the height difference between the bottom of the recess and the apex of the protrusion is 20 to 200 μm, and more preferably 50 to 150 μm. This is because when such irregularities are formed, openings described later are periodically formed at an appropriate pitch, and a highly efficient photoelectric conversion element can be formed. The “substrate having irregularities on the back surface” includes the case where a recess is formed on the back surface of a flat substrate. In this case, the flat part on the back surface of the substrate becomes a convex part. Further, the shape of the recess may be either a groove shape or a non-through hole shape, and the side surface of the recess may be substantially vertical or tapered.

  As the passivation film, various films having a function of inactivating the recombination center on the back surface of the substrate can be used. The passivation film is made of, for example, silicon oxide or silicon nitride. The passivation film can be formed by thermal oxidation or CVD of the substrate.

1-2. The step of applying the back electrode material so that the thickness of the recess is selectively increased. The back electrode material can be applied by, for example, a printing method. Any other method may be used as long as the back electrode material can be applied so that the thickness of the recess is selectively increased. “The thickness at the concave portion is selectively increased” means that the thickness of the back electrode material applied to the concave portion is larger than the thickness of the back electrode material applied to other portions (for example, convex portions). Means thicker. The back electrode material is made of a material that generates heat in a subsequent firing step. When such a material is thickly applied to the recess and the material is fired in this state, an opening is formed in the recess. The back electrode material is preferably a material containing a dopant of the same conductivity type as that of the substrate. In this case, a high concentration impurity region having the same conductivity type as that of the substrate can be formed in the vicinity of the opening as it is after the opening is formed in the subsequent baking step. As the back electrode material, an aluminum paste is typically used.

1-3. By firing this material, the step of forming a back electrode that penetrates the passivation film in the recess and is electrically connected to the substrate is performed, for example, at a peak temperature of the temperature profile of 700 to 800 ° C. At this time, the thicker the back electrode material is applied, the more combustion heat is released during firing, and an opening is formed in the passivation film only where the back electrode material is applied to a predetermined thickness or more. Is done. Further, the back electrode material is baked and hardened by baking, and becomes a back electrode electrically connected to the substrate through the opening.

2. Photoelectric conversion element The photoelectric conversion element of the present invention is a photoelectric conversion element using a semiconductor substrate having irregularities on the back surface. The photoelectric conversion element has a back electrode through a passivation film on the back surface of the substrate, and has an opening in the passivation film at the recess. The back electrode and the substrate are in contact with each other at the opening.

  The description of the method for manufacturing a photoelectric conversion element is also applicable to the photoelectric conversion element unless it is contrary to its purpose.

  A method for manufacturing a photoelectric conversion element according to Example 1 of the present invention will be described with reference to FIGS. FIG. 1 is a cross-sectional view showing the manufacturing process of the photoelectric conversion element of this example, and FIG. 2 is a process flow diagram showing the manufacturing process of the photoelectric conversion element of this example.

  First, in S21, the n-type semiconductor layer 2 was formed on both surfaces of the p-type semiconductor substrate 1. As shown in FIG. 1A, the p-type semiconductor substrate 1 has substantially uniform size irregularities arranged in a lattice pattern on the back surface. For example, as shown in Japanese Patent Application Laid-Open No. 2001-223172, a substrate having such irregularities is grown on a substrate on which irregularities are formed, and the size of the irregularities is such that the peak-valley height is 100 μm, The pitch is 2.5 mm. The substrate size is 50 mm × 50 mm, and the average thickness of the substrate is 300 μm. The shape and pitch of the unevenness can be changed by changing the uneven pattern of the substrate. The n-type semiconductor layer 2 is formed on both surfaces of the substrate by diffusing phosphorus as an impurity into the p-type semiconductor substrate 1 by thermal diffusion using phosphorus oxychloride as a source.

  Next, in S22, the surface side of the substrate 1 was protected with a resist or the like, and the n-type semiconductor layer on the back surface side was etched away with a hydrofluoric acid solution, leaving the n-type semiconductor layer 2 only on the surface side.

  Next, in S23, the substrate 1 was thermally oxidized to form silicon oxide films 3 and 5 on the front and back surfaces of the substrate. At this time, if the silicon oxide film 5 on the back side is too thick, an aluminum paste to be printed and fired later penetrates the silicon oxide film 5 and does not come into contact with the p-type silicon substrate 1. Then, a silicon oxide film 5 having a thickness corresponding to the thickness of the aluminum paste transferred and formed is formed. According to our study, there is a positive correlation between the thickness of the aluminum paste immediately after printing and the thickness of the silicon oxide film that can be baked through. Specifically, the aluminum paste is baked at 800 ° C. for 2 minutes. The relationship shown in FIG. 3 was obtained. For example, when the thickness of the aluminum paste immediately after printing is 50 μm, the firing penetration is possible if the thickness of the silicon oxide film is 40 nm or less. In addition, the formation of the silicon oxide film is not limited to thermal oxidation, but may be performed by plasma CVD. In this case, the relationship between the thickness of the aluminum paste and the thickness of the silicon oxide film that can be baked through is hot. Care must be taken because it is different from the oxidized silicon oxide film. In this embodiment, a silicon oxide film having a thickness of 50 nm is formed by thermal oxidation.

  Next, in S24, the aluminum paste 7 was printed on the back surface. Through the steps so far, the structure shown in FIG. At this time, the printing conditions are set so that the concave portion on the back surface of the substrate is about 70 μm and the convex portion is about 20 μm, and the thickness of the paste 7 changes according to the concave and convex shape as shown in FIG. I did it.

Next, when this paste 7 is baked, as shown in FIG. 1B, the combustion heat of the paste increases only in the concave portion where the paste 7 becomes thick, and an opening 4 is formed through the silicon oxide film 5. As a result, the p ⁺ layer 6 was formed. At the same time, the paste 7 was baked and hardened to form a back electrode. Baking was performed at 800 ° C. for 2 minutes, but the oxide passivation film 5 was penetrated only in a portion having a paste thickness of 60 μm or more.

  Subsequently, in S25, the light-receiving surface electrode 8 was formed by printing a silver paste on the surface side in a grid shape, firing it, and penetrating the silicon oxide film 3. Next, in order to reduce the interface state between the p-type silicon substrate 1 and the silicon oxide film 5 on the back surface, forming gas annealing is performed in a hydrogen atmosphere (S26), and finally the surface electrode is solder-coated to perform solar coating. The battery was completed (S27).

  As the back electrode, an aluminum paste is used for the p-type substrate, but various pastes can be used in accordance with the substrate conductivity type as long as the paste contains the same conductivity type with respect to silicon as the dopant.

  A method for manufacturing a photoelectric conversion element according to Example 2 of the present invention will be described with reference to FIGS. Although the process flow diagram of FIG. 2 was used in the description of the first embodiment, the manufacturing process of the present embodiment is basically the same as that of the first embodiment, so the process flow diagram of FIG. It is used also for description of an Example. 4 and 5 show the manufacturing process of the photoelectric conversion element of this example, in which (a) is a sectional view and (b) is a bottom view.

  First, the steps S21 and S22 were performed by the same method and material as in Example 1.

  Next, in S23, the substrate 1 was thermally oxidized to form silicon oxide films 3 and 5 on the front and back surfaces of the substrate. At this time, if the silicon oxide film 5 on the back side is too thick, the aluminum paste 7 to be printed and fired later penetrates the silicon oxide film 5 and does not come into contact with the p-type silicon substrate 1 as in the first embodiment. In this embodiment, a silicon oxide film having a thickness of 30 nm is formed by thermal oxidation.

  Next, in S24, the aluminum paste 7 was printed on the back surface. At this time, by setting the printing conditions such that the printing pressure at the time of printing is higher than in Example 1, the paste is not transferred in the vicinity of the convex portion of the substrate surface unevenness as shown in FIG. The paste was transferred. At this time, the paste thickness immediately after printing was about 50 μm at the concave portion of the substrate surface unevenness and almost 0 μm at the convex portion. Therefore, an opening 9 as shown in FIGS. 4A and 4B is formed in the aluminum paste 7, from which the silicon oxide film 5 is exposed.

Next, when this paste 7 is baked, as shown in FIG. 5, the combustion heat of the paste 7 is increased only in the concave portion where the paste 7 is transferred thickly, and an opening 4 is formed through the silicon oxide film 5. As a result, the p ⁺ layer 6 was formed. At the same time, the paste 7 was baked and hardened to form a back electrode. On the other hand, since the paste is hardly transferred in the vicinity of the convex portion, the oxide film 5 remains.

  Subsequently, a silver paste is printed on the surface side in a grid shape, and this is baked to penetrate the silicon oxide film to form a light-receiving surface electrode (S25). Next, forming gas annealing is performed in a hydrogen atmosphere. The step (S26) and finally the step of solder-coating the surface electrode (S27) are the same as in the first embodiment.

(Comparative example)
Here, a solar cell according to the present invention and a conventional solar cell not according to the present invention will be compared. The structure of the conventional solar cell is the same as that according to the present invention except that the back side of the substrate is flat. A conventional method for manufacturing a solar cell will be described below with reference to FIG. The p-type semiconductor substrate used for comparison is the same as in the example.

  First, the step (S61) of forming an n-type semiconductor layer by diffusing impurities into the p-type semiconductor substrate by thermal diffusion is the same as S21 in the embodiment. Next, the step (S63) of forming a silicon oxide film on the front surface and back surface of the substrate by etching away the diffusion layer on the back surface side (S62) and further performing thermal oxidation is the same as S22 and S23 in the embodiment.

  Next, an opening is provided in the silicon oxide film on the back surface of the substrate. As a method for forming the opening, first, a photoresist is spin-coated on the silicon oxide film on the back surface of the substrate and heat-treated (S64). Next, exposure is performed by irradiating the resist with ultraviolet rays through a glass mask in which openings are formed using a mask aligner (S65). Next, after a protective tape is applied to the surface, the resist is immersed in a developer and the resist in the exposed portion is removed by etching (S66). Next, the protective tape on the surface is removed and the resist is heat-treated. Next, after a protective tape is applied to the surface, it is immersed in a hydrofluoric acid aqueous solution, and the silicon oxide film in the opened resist portion is removed by etching (S67). Next, the protective tape on the surface is removed, and the resist is removed in acetone (S68). Thus, the opening process of the silicon oxide film is completed.

Next, when aluminum paste is printed and baked on the back surface, a p ⁺ layer is formed only at the opening of the silicon oxide film, and a back electrode is formed at the same time (S69).

  Next, silver paste is printed on the front side in a grid. The light-receiving surface electrode is formed by baking this and penetrating the silicon oxide film (S70). Next, forming gas annealing is performed in a hydrogen atmosphere in order to reduce the interface state between the p-type silicon substrate on the back surface and the silicon oxide film (S71). Finally, surface electrode solder coating is performed to complete the solar cell (S72).

As described above, the conventional manufacturing method requires a complicated process for forming the opening and an apparatus such as a mask aligner, whereas the manufacturing method according to the present invention does not provide any opening by simply making the substrate back surface uneven. It doesn't require a special process for. Comparing the conventional manufacturing method of FIG. 6 with the manufacturing method of the present invention of FIG. 2, it can be seen that the process is greatly reduced by the present invention (the double-line encircled portion of FIG. 6). Even in other opening methods, a large apparatus such as a plasma CVD apparatus or a laser apparatus is required. Table 1 shows the results of evaluation of the characteristics of the solar cell thus completed at 25 ° C. using a solar simulator of AM 1.5, 100 mW / cm ² .

  As is clear from Table 1, the photoelectric conversion element according to Example 1 or 2 of the present invention has the same characteristics although the process is significantly reduced compared to the photoelectric conversion element according to the process of the comparative example. Have.

It is sectional drawing which shows the manufacturing process of the photoelectric conversion element which concerns on Example 1 of this invention. It is a process flow figure showing a manufacturing process of a photoelectric conversion element concerning Example 1 of the present invention. It is a graph which shows the correlation with the aluminum oxide printing thickness in this invention, and the thermal oxide film thickness in which baking penetration is possible. The manufacturing process of the photoelectric conversion element which concerns on Example 2 of this invention is shown, (a) is sectional drawing, (b) is a bottom view. The manufacturing process of the photoelectric conversion element which concerns on Example 2 of this invention is shown, (a) is sectional drawing, (b) is a bottom view. It is a process flowchart which shows the manufacturing process of the photoelectric conversion element of a comparative example. It is a schematic sectional drawing of the photoelectric conversion element by a prior art.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 p-type semiconductor substrate 2 n-type semiconductor layer 3 Light-receiving surface silicon oxide film 4 Opening part 5 Back surface silicon oxide film 6 p ⁺ layer 7 Aluminum paste 8 Light-receiving surface electrode 9 Opening part

Claims (10)

Forming a passivation film on the back surface of the semiconductor substrate having irregularities on the back surface;
Applying a back electrode material so that the thickness of the recess is selectively increased;
By firing the material, to form a back electrode, forming a back electrode by forming an opening through the passivation film only in the recess, Ru is electrically connected to the substrate and the back electrode The manufacturing method of a photoelectric conversion element provided with these.   The manufacturing method according to claim 1, wherein the back electrode material is a material containing a dopant of the same conductivity type as that of the substrate.   The manufacturing method according to claim 1, wherein the substrate has substantially periodic unevenness on the back surface.   The manufacturing method according to claim 1, wherein the substrate is made of a silicon substrate, and the passivation film is made of a silicon oxide film.   The manufacturing method according to claim 1, wherein the unevenness has a height difference of 20 to 200 μm between the bottom of the recess and the apex of the protrusion. In a photoelectric conversion element using a semiconductor substrate having irregularities on the back surface,
Wherein the rear surface of the semiconductor substrate, the passivation film and the back electrode are formed in this order,
In the recess of the semiconductor substrate , the back electrode is in contact with the semiconductor substrate through an opening provided in the passivation film ,
In the region other than the concave portion of the semiconductor substrate, the back electrode is interposed through the passivation film with respect to the semiconductor substrate .   The photoelectric conversion element according to claim 6, wherein the substrate includes a high-concentration impurity region having the same conductivity type as that of the substrate and having a high impurity concentration in the vicinity of the opening.   The photoelectric conversion element according to claim 6, wherein the substrate has substantially periodic unevenness on the back surface.   The photoelectric conversion element according to claim 6, wherein the substrate is made of a silicon substrate, and the passivation film is made of a silicon oxide film.   The photoelectric conversion element according to claim 6, wherein the unevenness has a height difference of 20 to 200 μm between the bottom of the recess and the apex of the protrusion.

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