JP2008159997A - Manufacturing method for solar cell element, and conductive paste - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a manufacturing method for solar cell element in which aluminum electrodes formed on a non-light receiving surface of a semiconductor substrate are hardly separable, and to provide conductive pastes used for production of solar cell element. <P>SOLUTION: A solar cell element 1 has a semiconductor substrate 10, an antireflection coating 20 formed on a surface 10a of the semiconductor substrate 10, a surface electrode 30 that runs through the antireflection coating 20 to have contact with the surface 10a of the semiconductor substrate 10, and a back electrode 40 formed on the backside 10b of the semiconductor substrate 10. A power collecting electrode 42 of the back electrode 40 is an aluminum electrode mainly having aluminum that is spread in the backside electric field region 12. The power collecting electrode 42 is mainly constituted of aluminum powder, and is formed by applying and calcinating conductive paste containing butyral as an organic binder. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a method for manufacturing a solar cell element in which an aluminum electrode is formed on a non-light-receiving surface of a semiconductor substrate having a photoelectric conversion function, and a conductive paste used for manufacturing the solar cell element.

  FIG. 1 is a cross-sectional view showing a cross-sectional structure of a conventional solar cell element 9.

  As shown in FIG. 1, the solar cell element 9 includes a semiconductor substrate 10, an antireflection film 20 formed on the surface (light receiving surface) 10 a of the semiconductor substrate 10, and the antireflection film 20 so as to penetrate the semiconductor substrate 10. A front surface electrode 30 in contact with the front surface 10a and a back surface electrode 40 formed on the back surface (non-light receiving surface) 10b of the semiconductor substrate 10 are provided. The back electrode 40 includes a linear extraction electrode 41 and a current collecting electrode 42 formed on the remaining portion where the extraction electrode 41 is not formed, and covers the entire back surface 10 b of the semiconductor substrate 10.

  In the solar cell element 9, the extraction electrode 41 of the front electrode 30 and the back electrode 40 is formed by applying and baking a conductive paste mainly composed of silver powder. The collecting electrode 42 of the back electrode 40 is formed by applying and baking a conductive paste containing an aluminum powder as a main component and a cellulose resin such as ethyl cellulose as an organic binder.

  Patent Document 1 is a prior art document relating to a conductive paste containing aluminum powder as a main component and a cellulose resin as an organic binder.

JP 2000-90734 A

  However, in the conventional solar cell element, the collector electrode, which is an aluminum electrode, easily peels from the semiconductor substrate depending on the firing conditions. And in the conventional solar cell element, due to this, the electrical resistance of the current collecting electrode is increased and the output characteristics are deteriorated, or the current collecting electrode peeled off the laminate member formed in the subsequent process breaks down for a long time. Reliability may be reduced.

  For this reason, in the conventional solar cell element, it is necessary to bake the collector electrode on the semiconductor substrate under firing conditions in which the collector electrode is difficult to peel off. However, the firing condition in which the collector electrode is difficult to peel off has the output characteristics of the solar cell element. In some cases, the output characteristics of the solar cell element may be sacrificed, not necessarily in accordance with the firing conditions that improve.

  In addition, in the conventional solar cell element, the firing conditions in which the collector electrode is difficult to peel off do not necessarily match the firing conditions suitable for the front electrode and the back electrode take-out electrode, and the collector electrode, the front electrode and the back electrode are taken out. In some cases, the productivity of the solar cell element cannot be improved by simultaneously firing the electrodes.

  The present invention has been made to solve this problem, and an object of the present invention is to provide a solar cell element manufacturing method and a conductive paste in which an aluminum electrode formed on a non-light-receiving surface of a semiconductor substrate is hardly peeled off. .

  In order to solve the above-mentioned problems, the invention of claim 1 is a method for manufacturing a solar cell element in which an aluminum electrode is formed on a non-light-receiving surface of a semiconductor substrate having a photoelectric conversion function. The aluminum electrode is formed by applying a conductive paste containing butyral as a binder to the non-light-receiving surface and baking it.

  According to a second aspect of the present invention, the aluminum powder has an average particle size D50 of a cumulative particle size distribution on a volume basis of 4.0 μm or more and 8.0 μm or less. Is the method.

  The invention of claim 3 is a conductive paste used for manufacturing a solar cell element in which an aluminum electrode is formed on a non-light-receiving surface of a semiconductor substrate having a photoelectric conversion function, comprising an aluminum powder as a main component, and as an organic binder Including butyral.

  According to the first to third aspects of the invention, the aluminum electrode is difficult to peel off.

<1 Configuration of solar cell element>
1-3 is a figure which shows schematic structure of the solar cell element 1 which concerns on desirable embodiment of this invention, FIG. 1 is also sectional drawing which shows the cross-section of the solar cell element 1, FIG. FIG. 3 is a plan view showing the pattern (planar shape) of the surface electrode 30 viewed from the front surface side (light receiving surface side), and FIG. 3 is a plan view showing the pattern of the back electrode 40 viewed from the back surface side (non-light receiving surface side). It has become.

  As shown in FIGS. 1 to 3, the solar cell element 1 includes a semiconductor substrate 10, an antireflection film 20 formed on the surface 10 a of the semiconductor substrate 10, and the surface of the semiconductor substrate 10 through the antireflection film 20. The front surface electrode 30 in contact with 10a and the back surface electrode 40 formed on the back surface 10b of the semiconductor substrate 10 are provided.

  The semiconductor substrate 10 is a substrate such as single crystal silicon or polycrystalline silicon. As the semiconductor substrate 10, when the material is silicon (Si), a substrate containing an impurity element exhibiting a p-type conductivity such as boron (B) is preferably used. The semiconductor substrate 10 is manufactured by a pulling method or the like when the material is single crystal silicon, and is manufactured by a casting method or the like when the material is polycrystalline silicon.

  The surface layer on the surface 10a side of the semiconductor substrate 10 is a reverse conductivity type diffusion region 11 containing an impurity element having a conductivity type opposite to the inside. When a substrate containing an impurity element exhibiting p-type conductivity such as boron (B) is employed as the semiconductor substrate 10, the reverse conductivity type diffusion region 11 is an impurity exhibiting n-type conductivity such as phosphorus (P). It is formed by diffusing elements. Due to the presence of the reverse conductivity type diffusion region 11, the semiconductor substrate 10 functions as a photoelectric converter having a pn junction.

  The surface layer on the back surface 10b side of the semiconductor substrate 10 is a back surface electric field region 12 having a high carrier concentration. When a silicon substrate containing an impurity element exhibiting a p-type conductivity such as boron is employed as the semiconductor substrate 10, the back surface electric field region 12 diffuses an impurity element exhibiting a p-type conductivity such as aluminum (Al). Is formed. The presence of the back surface electric field region 12 can prevent carriers from recombining in the solar cell element 1.

  The shape of the semiconductor substrate 10 is not particularly limited, but can be a square plate having a thickness of 0.2 to 0.5 mm and a size of 100 mm × 100 mm to 150 mm × 150 mm.

The antireflection film 20 is a thin film such as silicon nitride (SiN _x ) or silicon oxide (SiO _x ). The antireflection film 20 is formed by a film forming method such as an evaporation method, a sputtering method, or a plasma CVD (Chemical Vapor Deposition) method. The antireflection film 20 is formed so as to have a refractive index of 1.8 to 2.3 and a thickness of 500 to 1200 Å in consideration of a difference in refractive index with the semiconductor substrate 10 and the like.

  As shown in FIG. 2, the surface electrode 30 desirably has a lattice pattern in which narrow linear finger electrodes 31 and thick linear bus bar electrodes 32 are orthogonal to each other. The finger electrode 31 mainly plays a role of collecting power generated by the photoelectric conversion function of the semiconductor substrate 10, and the bus bar electrode 32 mainly uses the power collected by the finger electrode 31 for the solar cell element 1. It plays the role of output to outside. As shown in FIG. 2, the bus bar electrode 32 does not need to be formed from one end portion to the other end portion of the semiconductor substrate 10 and may be formed in a range sufficient to have the output function. It ’s fine.

  The surface electrode 30 is a metal electrode whose main material is a metal having good solder wettability such as silver. The surface electrode 30 is formed, for example, by applying and baking a conductive paste containing silver powder as a main component (hereinafter referred to as “silver paste”). As the silver paste, for example, a paste obtained by adding 10 to 30 parts by weight of an organic vehicle and 0.1 to 5 parts by weight of glass frit to 100 parts by weight of silver powder can be used.

  For conduction between the surface electrode 30 and the reverse conductivity type diffusion region 11, the antireflection film 20 in a portion where the surface electrode 30 is to be formed is removed by etching, and a silver paste is applied on the exposed reverse conductivity type diffusion region 11. It may be ensured by performing baking later, or it may be ensured by applying a silver paste on the antireflection film 20 and then baking, and penetrating the antireflection film 20 through the surface electrode 30 by a so-called fire-through method. May be.

  As shown in FIG. 3, the back surface electrode 40 preferably includes a linear extraction electrode 41 and a current collecting electrode 42 formed on the remaining portion where the extraction electrode 41 is not formed. The entire back surface 10b is covered. The extraction electrode 41 and the collecting electrode 42 are overlapped at the boundary portion and are electrically connected. As shown in FIG. 3, the extraction electrode 41 does not have to be formed from one end portion to the other end portion of the semiconductor substrate 10, as long as it is formed in a range sufficient to have the function. good.

  The collecting electrode 42 mainly plays a role of collecting the power generated by the photoelectric conversion function of the semiconductor substrate 10, and the extracting electrode 41 mainly uses the power collected by the collecting electrode 42 as a solar cell. It plays a role of outputting to the outside of the element 1.

  Similarly to the surface electrode 30, the extraction electrode 41 is a metal electrode whose main material is a metal having good solder wettability such as silver. The extraction electrode 41 is also formed by applying and baking a silver paste.

  The current collecting electrode 42 is an aluminum electrode whose main material is aluminum diffused in the back surface electric field region 12. The collecting electrode 42 is formed by applying and baking a conductive paste (hereinafter, “aluminum paste”) containing aluminum powder as a main component. In addition, when forming the current collection electrode 42 using the aluminum paste explained in full detail later, the thickness after baking of the current collection electrode 42 shall be 40 micrometers or less from a viewpoint of suppressing peeling generation | occurrence | production of the current collection electrode 42. In addition, from the viewpoint of uniformly forming the back surface electric field region 12 and suppressing the deterioration of the output characteristics of the solar cell element 1, it is desirable to set it to 10 μm or more.

<2 Aluminum paste>
Next, an aluminum paste used for forming the current collecting electrode 42 will be described.

  The aluminum paste contains aluminum powder as a main component and contains butyral as an organic binder. The organic binder is dissolved in an organic solvent and used as an organic vehicle. The organic solvent for dissolving the organic binder is selected from, for example, butyl carbitol, butyl carbitol acetate, butyl cellosolve, butyl cellosolve acetate, terpineol, hydrogenated terpineol, terpineol, hydrogenated terpineol acetate, methyl ethyl ketone, isobornyl acetate, and nopyrulacetate. can do.

  The composition of the aluminum paste is desirably such that 60% by weight to 80% by weight of the total weight of the aluminum paste is aluminum powder and 1% by weight to 6% by weight is butyral.

  If an aluminum paste containing butyral as such an organic binder is used to form the current collecting electrode 42, the current collecting electrode 42 is unlikely to peel off. This is considered to be because the sintered state of the aluminum powder is improved.

In addition to the aluminum powder, butyral, and the organic solvent, glass frit (for example, SiO ₂ —Pb system, SiO ₂ —B ₂ O ₃ —PbO system, Bi) of 0.1 wt% to 5 wt% of the total weight is used. ₂ O ₃ —SiO ₂ —B ₂ O ₃ glass frit) is preferably contained in the aluminum paste to improve the adhesive strength between the semiconductor substrate 10 and the collector electrode 42. However, if the glass frit content exceeds the above range, the electrical resistance of the collector electrode 42 increases. Therefore, it is desirable that the glass frit content does not exceed the above range.

  Moreover, you may make it add additives, such as a suitable amount of a dispersing agent and an antifoamer, to an aluminum paste.

The aluminum powder desirably has an average particle size D ₅₀ of a cumulative particle size distribution on a volume basis of 4.0 μm or more and 8.0 μm or less. This is because if the average particle diameter D ₅₀ is within this range, the collector electrode 42 is less likely to peel off. If the average particle diameter D ₅₀ is less than this range, the semiconductor substrate 10 is likely to warp. This is because if the average particle diameter D ₅₀ exceeds this range, the collector electrode 42 is easily peeled off.

Furthermore, the aluminum powder desirably has a 10% particle size D ₁₀ of a cumulative particle size distribution on a volume basis of 1.0 μm or more and 2.5 μm or less. This is because if the 10% particle size D ₁₀ is within this range, the sintered state of the aluminum powder can be further improved, and peeling of the current collecting electrode 42 can be further suppressed. This is because if ₁₀ is below this range, the semiconductor substrate 10 is likely to warp, and if the 10% particle size D ₁₀ is above this range, the collector electrode 42 is likely to peel off.

Here, the average particle size D ₅₀ and the 10% particle size D ₁₀ of the cumulative particle size distribution on a volume basis are particles having a particle size smaller than the particle size D (μm) with respect to the particle size D (μm), respectively. When reference is made to the cumulative particle size distribution representing the distribution of the ratio Q (%), Q = 50% and 10% of the particle size D (μm), which is larger than the focused particle size D (μm) This is the particle diameter D (μm) when the total volume of the small particles is 50% (10%) of the total volume of all the particles. Such a cumulative particle size distribution can be measured using a laser diffraction dispersion measuring apparatus.

<3 Manufacturing method of solar cell element>
Below, the manufacturing method of the solar cell element 1 is demonstrated on the assumption that the material of the semiconductor substrate 10 is polycrystalline silicon that can be mass-produced and is more advantageous than single crystal silicon in terms of manufacturing cost. 4 and 5 are diagrams illustrating a method for manufacturing the solar cell element 1. FIG.

In manufacturing the solar cell element 1, first, an ingot of polycrystalline silicon containing an impurity element exhibiting a p-type conductivity such as boron is cut into a quadrangular column having a bottom size of 10 cm × 10 cm to 15 cm × 15 cm. Then, the semiconductor substrate 10 is obtained by slicing the rectangular column to a predetermined thickness, for example, 300 μm or less (FIG. 4A). In order to remove a damage layer or a contamination layer generated by cutting or slicing, an alkali such as sodium hydroxide (NaOH) or potassium hydroxide (KOH), hydrofluoric acid (HF), or hydrofluoric acid (HF-HNO _3). It is desirable that a very small amount of the surface of the semiconductor substrate 10 be etched with an acid such as It is also desirable to form minute protrusions on the surface of the semiconductor substrate 10 using a dry etching method or a wet etching method.

Subsequently, an ion implantation method in which P ⁺ ions are directly diffused into the semiconductor substrate 10, or the semiconductor substrate 10 is placed in a diffusion furnace, and the semiconductor substrate 10 is contained in a gas containing an impurity element such as phosphorus oxychloride (POCl ₃ ). By heat-treating, an impurity element such as phosphorus is diffused in the surface layer of the semiconductor substrate 10 to form the reverse conductivity type diffusion region 11. Further, when the reverse conductivity type diffusion region 11 is formed over the entire surface layer of the semiconductor substrate 10, the remainder of the reverse conductivity type diffusion region 11 formed on the surface 10 a to be the light receiving surface in the solar cell element 1 is left. The reverse conductivity type diffusion region 11 is removed, and the semiconductor substrate 10 is washed with pure water (FIG. 4B).

  The removal of the reverse conductivity type diffusion region 11 is performed by, for example, removing the reverse conductivity type diffusion region 11 formed on the back surface 10b side of the semiconductor substrate 10 with a film resistant to hydrofluoric acid by etching using hydrofluoric acid, Thereafter, it can be carried out by removing the film resistant to hydrofluoric acid covering the surface 10 a of the semiconductor substrate 10.

Subsequently, an antireflection film 20 is formed on the surface 10a of the semiconductor substrate 10 (FIG. 4C). The antireflection film 20 is formed into a plasma by glow discharge decomposition of a source gas such as silane (SiH ₄ ) or ammonia (NH ₃ ) (when the material of the antireflection film 20 is silicon nitride), and is formed on the surface of the semiconductor substrate 10. It is formed by depositing (plasma CVD method). If the antireflection film 20 made of silicon nitride is formed in the presence of hydrogen plasma, the output characteristics of the solar cell element 1 can be improved due to the passivation effect.

  Then, an aluminum paste is applied to the back surface of the semiconductor substrate 10 by a known method such as screen printing, and an organic solvent contained in the aluminum paste is evaporated at a predetermined temperature to dry the aluminum paste. The collector electrode 42 having the pattern shown in FIG. 4 is formed on the back surface 10b of the semiconductor substrate 10 (FIG. 4D).

  Further, the silver paste is applied to the front surface 10a and the back surface 10b of the semiconductor substrate 10 by a known method such as screen printing, and the organic solvent contained in the silver paste is evaporated at a predetermined temperature to dry the silver paste. 2, the surface electrode 30 having the pattern shown in FIG. 2 is formed on the front surface 10a of the semiconductor substrate 10, and the extraction electrode 41 having the pattern shown in FIG. 3 is formed on the back surface 10b of the semiconductor substrate 10 (FIG. 5A). ). FIG. 5A illustrates a case where the antireflection film 20 is penetrated through the surface electrode 30 by a fire-through method.

  The front electrode 30 and the back electrode 40 applied in this way are baked onto the semiconductor substrate 10 by baking at a baking temperature of 600 to 800 ° C. for several tens of seconds to several tens of minutes. In this firing step, aluminum contained in the current collecting electrode 42 diffuses into the semiconductor substrate 10, and the back surface electric field region 12 is formed on the surface layer on the back surface 10b side of the semiconductor substrate 10 (FIG. 5B).

  In the solar cell element 1 thus manufactured, the current collecting electrode 42 is peeled off from the semiconductor substrate 10 regardless of the firing conditions by forming the current collecting electrode 42 using an aluminum paste containing butyral as an organic binder. It is difficult to do. In the solar cell element 1, it is difficult for the output characteristics to deteriorate and the long-term reliability to decrease due to the separation of the collecting electrode 42.

  In addition, the said embodiment does not limit this invention, In the range which does not change the summary of this invention, it can change suitably and can implement. In particular, in the above-described embodiment, since the degree of freedom of the firing conditions of the current collecting electrode 42 is high, the front electrode 30 and the back electrode 40 are baked on the semiconductor substrate 10 in one firing process. This does not prevent the front electrode 30 and the back electrode 40 from being baked on the semiconductor substrate 10 through the baking process. For example, the back electrode 40 may be baked on the semiconductor substrate 10 in the first baking step, and the front electrode 30 may be baked on the semiconductor substrate 10 in the second baking step, or the current collecting electrode 42 may be baked in the first baking step. May be baked onto the semiconductor substrate 10 and the extraction electrode 41 of the front electrode 30 and the back electrode 40 may be baked onto the semiconductor substrate 10 in the second baking step. Of course, the front electrode 30 and the back electrode 40 may be baked on the semiconductor substrate 10 through three or more baking steps. The order of baking the front electrode 30 (bus bar electrode 32 / finger electrode 31) and the back electrode 40 (collecting electrode 42 / extraction electrode 41) on the semiconductor substrate 10 may be changed. It may be changed.

  Furthermore, in the above embodiment, the applied silver paste is dried and then the next silver paste is applied. However, if the aluminum paste does not adhere to the work table or screen of a printing machine for screen printing, the aluminum paste is applied. The silver paste may be applied without waiting for the paste to dry. That is, in the case of applying the conductive paste in a plurality of times in the production of the solar cell element 1, even when the conductive paste to be applied first is not dried, the application of the conductive paste to be applied later is not affected The conductive paste to be applied later can be applied without drying the conductive paste to be applied first.

It is sectional drawing which shows the cross section of the solar cell element 1 which concerns on desirable embodiment of this invention. It is a top view which illustrates the pattern of the surface electrode 30 seen from the surface side. It is a top view which illustrates the pattern of the back surface electrode 40 seen from the back surface side. 3 is a diagram illustrating a method for manufacturing the solar cell element 1. FIG. 3 is a diagram illustrating a method for manufacturing the solar cell element 1. FIG.

Explanation of symbols

1,9 Solar cell element 10 Semiconductor substrate 10a Surface (light receiving surface)
10b Back surface (non-light-receiving surface)
DESCRIPTION OF SYMBOLS 11 Reverse conductivity type diffusion area | region 12 Back surface electric field area | region 20 Antireflection film 30 Front surface electrode 31 Finger electrode 32 Bus bar electrode 40 Back surface electrode 41 Extraction electrode 42 Current collection electrode

Claims (3)

A method for manufacturing a solar cell element in which an aluminum electrode is formed on a non-light-receiving surface of a semiconductor substrate having a photoelectric conversion function,
A method for producing a solar cell element, wherein the aluminum electrode is formed by applying and baking a conductive paste containing aluminum powder as a main component and butyral as an organic binder on the non-light-receiving surface. 2. The method for manufacturing a solar cell element according to claim 1, wherein the aluminum powder has an average particle size D ₅₀ of a cumulative particle size distribution on a volume basis of 4.0 μm or more and 8.0 μm or less.   A conductive paste used for manufacturing a solar cell element in which an aluminum electrode is formed on a non-light-receiving surface of a semiconductor substrate having a photoelectric conversion function, characterized in that it contains aluminum powder as a main component and butyral as an organic binder. Conductive paste.

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