JP2011061109A - Method of manufacturing solar cell element, and the solar cell element - Google Patents

Abstract

Provided is a solar cell element having a high fill factor provided with a smooth semicircular shape with small irregularities and a low resistance collector.
When a multi-layer electrode is formed by printing a conductive paste on a semiconductor substrate by a plurality of screen printing methods, a first electrode is printed using a screen printing plate making plate, and then the substrate is printed. Alternatively, the second layer is superimposed on the first layer electrode in a state where the position of the screen printing plate making is shifted along the longitudinal direction of the first layer electrode from the substrate or screen printing plate making position when the first layer electrode is printed. The third and subsequent electrodes are sequentially shifted in the same direction as described above to form a multilayer electrode.
[Selection] Figure 2

Description

  The present invention relates to a method for manufacturing a solar cell element and a solar cell element obtained thereby.

  In general, a solar cell element has a structure shown in FIG. In FIG. 1, 1 is a plate shape having a size of 100 to 150 mm square and a thickness of 0.1 to 0.3 mm, and is made of polycrystal, single crystal silicon or the like, and doped with p-type impurities such as boron. It is a p-type semiconductor substrate. The substrate 1 is doped with an n-type impurity such as phosphorus to form an N layer 2, an antireflection film 3 such as SiN is attached, and a BSF (Back Surface Field) layer formed on the back surface using a screen printing method. After the conductive aluminum paste is printed on 4, it is dried and fired to form the back electrode 5, and after the conductive silver paste is printed on the surface, dried and fired to form the collector electrode 6. The Hereinafter, the surface of the substrate that is the light receiving surface side of the solar cell element is referred to as the front surface, and the surface of the substrate that is opposite to the light receiving surface side is referred to as the back surface.

  FIG. 2 is a schematic view showing a screen printing process of a conductive paste used in the method for manufacturing a solar cell element described above. In the plate making 7, the emulsion is extracted in the pattern of the collector electrode, and the conductive paste 8 filled thereon is transferred onto the substrate 10 by moving the squeegee 9 to which the printing pressure is applied, thereby transferring the collector electrode ( Surface electrode) is formed.

  The collector electrode formed in this way is shown in FIG. (I) is a perspective view, and (II) is a cross-sectional view cut along a plane indicated by a dashed line in (I). Moreover, in FIG. 3, 10 is a board | substrate, 11 is a collector electrode, D shows the longitudinal direction of a collector electrode. In the above-described screen printing method, as shown in FIG. 3 (II), the height of the collecting electrode 11 formed by one printing is uneven due to the influence of the mesh. For this reason, the resistance of the collector electrode is increased, which hinders improvement in conversion efficiency.

  In order to keep the resistance of the collector electrode low, a method has been proposed in which the conductive paste is printed a plurality of times to increase the aspect ratio of the collector electrode. However, increasing the number of printings increases the possibility of paste sagging due to misalignment between electrodes, which is a problem because it leads to a decrease in conversion efficiency due to an increase in shadow loss. In order to avoid this problem, there is a method of increasing the viscosity of the conductive paste to be used and reducing the increase in shadow loss due to paste sagging. However, when the viscosity of the conductive paste is increased, there is a problem that the unevenness of the collector electrode due to the above-described plate making mesh remains strongly even after printing a plurality of times, and the thickness of the collector electrode varies greatly. .

  In order to avoid this problem, for example, Patent Document 1 (Japanese Patent Application Laid-Open No. 2007-243230) forms a collector electrode that is flattened by reducing unevenness by printing a conductive paste a plurality of times. A method for improving the characteristics is disclosed.

  In the method of Patent Document 1, it is reported that the characteristics are greatly improved when the conductive paste is printed by overlapping a plurality of times by using different mesh plates. However, plate making with different meshes has different ways of stretching when a printing pressure is applied, and therefore, a gap between collecting electrodes tends to occur as a multilayer electrode is formed. This leads to a shadow loss, which causes deterioration of characteristics.

  Further, in the method of Patent Document 1, it is reported that the characteristics are improved even when the conductive paste is printed by overlapping a plurality of times, even if the same mesh plate making is used. However, there is a problem that the improvement width of the characteristics becomes smaller than when a different mesh is used.

JP 2007-243230 A

  The present invention has been made in view of the above circumstances, and manufacture of a solar cell element having improved conversion efficiency by forming a collector electrode (surface electrode) with reduced surface irregularities, low resistance, and high aspect ratio. It aims at providing the method and the solar cell element obtained by this.

  As a result of intensive studies to achieve the above object, the present inventors have conducted screen printing plate making on forming a multi-layer electrode by printing a conductive paste on a semiconductor substrate a plurality of times by screen printing. After printing the first layer electrode, the position of the substrate or screen printing plate making is shifted in the longitudinal direction of the first layer electrode from the position of the substrate or screen printing plate making position during the first layer electrode printing. In the case of a two-layer electrode, a second layer electrode is printed directly on the second electrode, and if necessary, the third and subsequent electrodes are sequentially shifted in the same direction as described above. The unevenness caused by the mesh marks on the second layer and the second layer cancels each other, reducing the effect of variations in thickness and forming a low-resistance electrode. This electrode is used as the light-receiving surface electrode of the solar cell element. It is used, found that it is possible to obtain high conversion efficiency solar cell element, the present invention has been accomplished.

That is, this invention provides the manufacturing method and solar cell element of the following solar cell element.
[Claim 1]
When forming a multilayer electrode by printing a conductive paste on a semiconductor substrate a plurality of times by screen printing, after printing the first electrode using screen printing plate making, the position of the substrate or screen printing plate making is determined. The second layer electrode is printed directly on the first layer electrode in a state shifted from the substrate or screen printing plate making position of the first layer electrode along the longitudinal direction of the first layer electrode, A method for producing a solar cell element, comprising: a step of forming a multilayer electrode by printing the third and subsequent electrodes sequentially shifted in the same direction as described above, if necessary.
[Claim 2]
The manufacturing method according to claim 1, wherein a distance for shifting the substrate or screen printing plate making is 8 to 15 μm.
[Claim 3]
The conductive paste has a viscosity of 600 to 900 Pa · s as a measured value under conditions of a temperature of 25 ° C. and a No. 14 rotor of 5 rpm using a Brookfield rotational viscometer, and as a measured value under a condition of 50 rpm as 160 to 200 Pa. The production method according to claim 1, which is s.
[Claim 4]
The manufacturing method according to any one of claims 1 to 3, wherein the same mesh plate making is used when forming at least the first layer electrode and the second layer electrode.
[Claim 5]
The solar cell element obtained by the manufacturing method of any one of Claims 1 thru | or 4.

  According to the present invention, when the second and subsequent collector electrodes are printed, it is preferable that the substrate or the substrate or the substrate is intentionally formed by a half (= 14 μm) of the plate diameter (for example, 28 μm in the case of 325 mesh). Printing is performed by moving the screen printing plate along the longitudinal direction of the collector electrode. As a result, the mesh mark generated on the first layer electrode is crushed when the second layer electrode is printed. Therefore, the thickness unevenness generated on the formed collector electrode is smaller than when the second collector electrode is printed as it is on the first collector electrode without moving, and a smooth semicircle is formed. A collector electrode having a low shape resistance can be formed. Therefore, a solar cell element with a high fill factor can be made.

  Also, since screen printing is performed multiple times, usually 2-7 times, using a high-viscosity paste with the same mesh platemaking, the difference in elongation between platemaking that occurs when printing pressure is applied is minimized. It is possible to minimize shadow loss due to displacement between electrodes and sagging of paste, and to accurately increase the aspect ratio. Furthermore, since the collector electrode formed by normal screen printing has unevenness, the light incident on the electrode is reflected. However, the smooth semicircular collector electrode formed according to the present invention can be reflected and incident on the antireflection film of the solar cell element when irradiated with light. Thereby, the short circuit current of a solar cell element also rises, and the conversion efficiency can be further increased.

  Moreover, in the multiple times printing of this collector electrode, the kind of paste used for each printing may differ. In addition, in the multiple printing of the collector electrode, the amount of shift when the first layer and the second layer, the second layer and the third layer, and the subsequent layers are overprinted may be changed little by little. Thereby, generation | occurrence | production of an unevenness | corrugation can be suppressed further.

It is sectional drawing which shows an example of the structure of a solar cell element. It is a schematic diagram explaining a screen printing process, (I) is a figure before printing, (II) is a figure after printing. The conventional collector electrode is shown, (I) is a perspective view, (II) shows sectional drawing cut | disconnected along the surface shown with the dashed-dotted line of (I). A conventional two-layer collector electrode is shown, (I) is a perspective view, and (II) is a sectional view cut along a plane indicated by a dashed line in (I). The double-layer collector electrode of this invention is shown, (I) is a perspective view, (II) shows sectional drawing cut | disconnected along the surface shown with the dashed-dotted line of (I). The other example of the conventional 2 layer collector electrode is shown, (I) is a perspective view, (II) shows sectional drawing cut | disconnected along the surface shown with the dashed-dotted line of (I). The other example of the two-layer collector electrode of this invention is shown, (I) is a perspective view, (II) shows sectional drawing cut | disconnected along the surface shown with the dashed-dotted line of (I). It is a graph showing the relationship between the distance which shifted the board | substrate, and a fill factor (FF).

  In the method for producing a solar cell element of the present invention, when forming a multilayer electrode by printing a conductive paste on a semiconductor substrate multiple times by screen printing, the first electrode was printed using screen printing plate making. After that, the position of the substrate or screen printing plate making is shifted from the substrate or screen printing plate making position at the time of printing the first electrode along the longitudinal direction of the first layer electrode, and is superimposed on the first layer electrode. The second layer electrode is printed, and if necessary, the third and subsequent electrodes are sequentially shifted in the same direction as described above to form a multilayer electrode.

  As shown in FIG. 1, a solar cell element produced by the method for manufacturing a solar cell element according to the present invention has a semiconductor substrate 1 that is a solar cell body and a different conductivity type formed on one side of the substrate. The impurity layer 2, the antireflection film 3 formed on the impurity layer, the back electrode 5, the BSF (Back Surface Field) layer 4, and the collector electrode 6 on the surface are provided. As the semiconductor substrate, a semiconductor substrate such as a p-type or n-type single crystal silicon substrate, a p-type or n-type polycrystalline silicon substrate, a p-type or n-type thin film silicon substrate, or a non-silicon compound semiconductor substrate can be used. . The impurity layer may be of a conductivity type different from that of the semiconductor substrate.

  As an example, first, a semiconductor substrate such as a p-type silicon substrate doped with a group III element such as boron or gallium is prepared. This may be a polycrystal or a single crystal, and as described above, a plate having a size of 100 to 150 mm square and a thickness of 0.1 to 0.3 mm is preferably used. Then, a concavo-convex structure called a texture is formed by immersing the substrate in the acidic solution, for example, in the surface of the solar cell element, followed by chemical etching with an alkaline solution, followed by washing and drying. The concavo-convex structure causes multiple reflection of light on the light receiving surface of the solar cell element. Therefore, by forming the concavo-convex structure, the reflectance is effectively reduced and the conversion efficiency is improved.

  The texture is formed by heating in an alkali solution (concentration of several mass% to several tens mass%, temperature 60-100 ° C.) such as heated sodium hydroxide, potassium hydroxide, potassium carbonate, sodium carbonate, sodium hydrogen carbonate, etc. It is easily produced by soaking for about a minute. A predetermined amount of 2-propanol or the like is often dissolved in the solution to promote the reaction. In order to form a uniform texture, it is preferable to use a solution obtained by mixing several mass% of 2-propanol in a sodium hydroxide or potassium hydroxide solution having a concentration of several mass% heated to 60 to 70 ° C.

  After texture formation, washing is carried out in an acidic aqueous solution of a mixed solution using inorganic acids such as hydrochloric acid, sulfuric acid, nitric acid and hydrofluoric acid alone or in combination of two or more. From an economic and efficient standpoint, washing in hydrochloric acid is preferred. In order to improve the cleanliness, several mass% hydrogen peroxide may be mixed in a hydrochloric acid solution and heated at 60 to 90 ° C. for 1 to 30 minutes for washing.

Next, a p-type silicon substrate is placed in a high-temperature gas at about 800 ° C. or higher containing, for example, POCl _{3 and the} like, and a thermal diffusion method in which an n-type impurity element such as phosphorus is diffused over the entire surface of the p-type silicon substrate, An n-type diffusion layer is formed on the surface. The depth of the n-type diffusion layer is preferably 0.2 to 1.0 μm, and the sheet resistance is preferably 40 to 150Ω / □. When the n-type diffusion layer is formed by thermal diffusion, the n-type diffusion layer may be formed on both surfaces and end surfaces of the p-type silicon substrate. In this case, the necessary n-type diffusion layer is formed. By coating the surface of the substrate with an acid resistant resin and immersing the p-type silicon substrate in a hydrofluoric acid solution, an unnecessary n-type diffusion layer can be removed.
This step can be performed by a spin coating method using a diffusing agent, a spray method, or the like, in addition to the vapor phase diffusion method.

  Thereafter, an antireflection film on the surface is formed. Antireflection films include silicon oxide, oxides such as cerium oxide, alumina, tin dioxide, titanium dioxide and tantalum oxide, films made of inorganic substances such as silicon nitride and magnesium fluoride, and two of these A two-layered film is used, and any of them can be used without any problem. For the formation of the antireflection film, a PVD method, a CVD method or the like is used, and any method is possible. For example, the antireflection film SiN can be formed on the surface of the substrate by a plasma CVD method using ammonia, silane, nitrogen, hydrogen, or the like.

  Next, a conductive paste containing, for example, aluminum, glass frit, varnish, and the like is screen-printed on the back surface and dried to form a back electrode having a thickness of 20 to 80 μm. The mesh of the plate used for screen printing in this case is not limited. The metal layer may be deposited by a sputtering method or a vacuum evaporation method. Usually, the electrode metal is uniformly formed on the back surface.

  Thereafter, a conductive paste containing, for example, silver, glass frit, and varnish is screen printed on the surface and dried at 130 to 250 ° C. to form a first collector electrode. The mesh structure of the plate used for screen printing in this case is a plain weave, twill weave, etc., preferably having a uniform number of warps and wefts, 200 to 500 mesh (number of meshes (number of yarns / inch)) ), More preferably 230 to 360 mesh. If the number of meshes is too small, the thick wire diameter may straddle the opening of the collector electrode of the plate making, and the discharge amount of the conductive paste may be reduced.If the mesh number is too large, the screen thickness of the plate making itself is reduced. In some cases, the discharge amount decreases, the thickness of the collector electrode decreases, and the resistance increases.

Also, if the mesh wire diameter (line width) is too thick, it will straddle the opening of the collector electrode of the plate making, leaving mesh marks on the collector electrode, but if it is too thin, the durability of the plate making will deteriorate. Therefore, it is preferable that the thickness is 18 to 40 μm, particularly 24 to 30 μm. If the thickness is too thick, it becomes difficult to discharge the conductive paste due to friction with the wall of the pattern opening. From the point that the discharge amount of the conductive paste is reduced, the film thickness of the collector electrode is reduced, and the resistance is increased, the thickness is preferably 42 to 98 μm, particularly 48 to 73 μm.
Specifically, a 325 mesh (wire diameter of 28 μm) and a thickness of 59 μm can be suitably used.

  After that, it is preferable to align the substrate so that it is accurately superimposed directly on the collector electrode using the same mesh plate making as that for forming the first collector electrode. Then, for example, only half the wire diameter of the mesh used for plate making, specifically 8-15 μm, preferably 10-14 μm, further moving the substrate along the longitudinal direction of the collector electrode, A second electrode is formed by printing over the first electrode, and further baked at 650 to 900 ° C.

  In the present invention, the third and subsequent layers may be further stacked on the two-layer electrode formed as described above. In this case, it is preferable to sequentially form the third and subsequent layers by shifting 6 to 12 μm, particularly 7 to 8 μm in the same direction as described above along the longitudinal direction of the already formed two-layer electrode. The resulting electrode has an aspect ratio of about 0.3 to 1, and the light hitting the smooth semicircular collector electrode is reflected and incident on the antireflection film of the solar cell element. This is preferable from the viewpoint of increasing the rate at which it becomes possible.

  Here, the conductive paste used in the first and second layers, and further, if necessary, the third and subsequent layers may be the same or different. It is preferable that the viscosity measured under the condition of No. 5 rotor is 600 to 900 Pa · s and the viscosity measured under the condition of 50 rpm is 160 to 200 Pa · s. If the viscosity is lower than this, the conductive paste discharged during printing may sag and the aspect ratio may be reduced, and if it is too high, the friction between the wall of the plate-making opening and the conductive paste will increase and the discharge amount will be small. There is a case.

  Specifically, the conductive paste used for forming the surface collector electrode is preferably one containing 60 to 95 mass%, particularly 70 to 90 mass%, of metal particles such as silver.

As glass frit, B-Pb-O, B-Si-Pb-O, B-Si-Bi-Pb-O, B-Si-Zn-O, B-Si-Pb-Al-O For example, an oxide such as PbO, B ₂ O ₃ , Al ₂ O ₃ can be used, and the conductive paste contains 0.1 to 15% by mass, particularly 1 to 10% by mass. preferable.

  Examples of the varnish include ethyl cellulose and nitrocellulose, and those containing 1 to 25% by mass, particularly 2 to 20% by mass in the conductive paste are preferable.

  The conductive paste preferably contains an organic solvent such as butyl carbitol, butyl carbitol acetate, and α-terpineol in a proportion of 1 to 15% by mass. In addition to the above components, dibutyl carbitol and hexylene glycol are also included. , Texanol and the like can be contained.

  FIG. 4 shows a collector electrode that can be formed by superposing the second layer electrode by normal printing without intentionally shifting the alignment when forming a multilayer collector electrode in this way. (I) is a perspective view, and (II) is a cross-sectional view cut along a plane indicated by a dashed line in (I). When the same mesh plate making is used for the first layer 13 and the second layer 14, the influence of the mesh mark is amplified, the unevenness is increased, and the resistance of the collector electrode is not decreased.

  A multilayer collector electrode formed by applying the present invention is shown in FIG. The unevenness caused by the mesh marks of the collector electrode composed of the first layer 16 and the second layer 17 cancels each other, and the influence is reduced. The variation in thickness is suppressed, and the formed electrode has a smooth semicircular shape. . Therefore, the improvement width of the fill factor is larger than when the second layer collector electrode is printed directly on the first layer collector electrode. Further, as shown in FIG. 6, the collector electrode 18 formed by normal multi-layer printing reflects light 19 incident on the collector electrode due to the influence of mesh marks (reflected light 20), but cannot be used effectively. As shown in FIG. 7, the multilayer collector electrode 21 formed by applying the present invention is reflected on the antireflection film when the light enters the semicircular collector electrode (incident light 19) (reflected light 20). ) The short circuit current can be improved.

  In addition, as shown in FIG. 5, the length of the multilayer collector electrode formed by applying the present invention is longer than the length of the first-layer collector electrode itself. In anticipation of this, the length of the collector electrode in the plate-making pattern may be shortened in advance by the amount of shift.

  The shape of the electrode of the present invention formed in this manner can be appropriately selected depending on the use application such as a comb shape, a fishbone shape, a spiral shape or the like.

  EXAMPLES Hereinafter, although an Example and a comparative example are shown and this invention is demonstrated more concretely, this invention is not restrict | limited to the following Example.

[Example 1]
First, by performing outer diameter processing on a p-type silicon substrate made of p-type polycrystalline silicon having a specific resistance of about 1 Ω · cm, which is doped with boron and sliced to a thickness of 0.2 mm, a side of 15 cm is formed. The square plate shape. The p-type silicon substrate was immersed in a hydrofluoric acid solution for 15 seconds, further chemically etched with an alkaline solution for 5 minutes, washed with pure water, and dried to form a textured structure on the p-type silicon substrate surface. .
Next, an n layer was formed on the p-type silicon substrate by a thermal diffusion method in a POCl ₃ gas atmosphere at a temperature of 870 ° C. for 30 minutes. Here, the sheet resistance of the n layer was about 40Ω / □. Then, after forming the acid resistant resin on the n layer, the p-type silicon substrate was immersed in a hydrofluoric acid solution for 10 seconds to remove the portion of the n layer where the acid resistant resin was not formed. Thereafter, the acid-resistant resin was removed to form an n layer only on the surface of the p-type silicon substrate. Subsequently, SiN serving as an antireflection film was formed to a thickness of 100 nm on the surface of the p-type silicon substrate on which the n layer was formed by plasma CVD using ammonia gas.

  Next, a conductive aluminum paste was printed on the back surface of the substrate on which the antireflection film was formed, and dried at 150 ° C. Then, using a screen printing method on the surface, with a Brookfield rotational viscometer, the viscosity measured under the conditions of a temperature of 25 ° C. and a No. 14 rotor of 5 rpm was 720 Pa · s, and the viscosity measured under a condition of 50 rpm was 175 Pa · s. Using a silver paste, a first collector electrode was formed using a 325 mesh (wire diameter 28 μm) plate making and dried at 150 ° C. Next, using the same 325 mesh plate-making, the substrate was aligned so as to be accurately overlaid on the collector electrode. Then, the substrate is moved further in the direction parallel to the collector electrode (along the collector electrode) by 14 μm, the conductive paste is overprinted, dried again at 150 ° C., and then baked at a maximum temperature of 800 ° C. Thus, a collector electrode was formed.

  As a result, as shown in FIG. 5, the unevenness due to the electrode mesh marks was eliminated, and the variation in thickness was suppressed, and at the same time, a smoother semicircular electrode could be formed. Table 1 shows the difference in solar cell characteristics and electrode thickness variation when screen printing is performed by the above method. The short-circuit current, fill factor, open-circuit voltage, and conversion efficiency were measured with a cell tester manufactured by NPC, and the electrode thickness was measured with a laser microscope manufactured by Keyence Corporation.

[Comparative Example 1]
A two-layer electrode was formed in the same manner as in Example 1 except that the intentional shift of the first and second layers was not performed. Table 1 shows the solar cell characteristics measured in the same manner as in Example 1 and the difference in variation in electrode thickness.

  As shown in Table 1, when the screen printing method according to the present invention is used, variations in electrode thickness are suppressed, and an increase in solar cell characteristics can be expected.

[Examples 2 to 4]
As shown in Table 2, collector electrodes were formed in the same manner as in Example 1 except that the second layer printing was performed by changing the distance to which the substrate was moved, and the solar cell characteristics were measured. The results are also shown in Table 2. FIG. 8 is a graph showing the relationship between the distance of shifting the substrate and the fill factor (FF).

  As shown in Tables 1 and 2, when the screen printing method according to the present invention was used, the fill factor was 78% or more, which was a sufficient value.

1 Semiconductor substrate (p-type silicon substrate)
2 Impurity layer (N layer)
3 Antireflection film 4 BSF layer 5 Back electrode 6 Front electrode (collecting electrode)
7 Plate Making 8 Conductive Paste 9 Squeegee 10 Substrate 15 Surface Electrode (Multilayer Collector)
16 First layer electrode 17 Second layer electrode 19 Incident light 20 Reflected light 21 Surface electrode (multilayer collector electrode)
D Longitudinal direction

Claims (5)

  When forming a multilayer electrode by printing a conductive paste on a semiconductor substrate a plurality of times by screen printing, after printing the first electrode using screen printing plate making, the position of the substrate or screen printing plate making is determined. A second layer electrode is printed directly on the first layer electrode in a state shifted from the substrate or screen printing plate making position of the first layer electrode along the longitudinal direction of the first layer electrode, A method for producing a solar cell element, comprising: a step of forming a multilayer electrode by printing the third and subsequent electrodes sequentially shifted in the same direction as described above, if necessary.   The manufacturing method according to claim 1, wherein a distance for shifting the substrate or screen printing plate making is 8 to 15 μm.   The viscosity of the conductive paste is 600 to 900 Pa · s as a measured value under conditions of a temperature of 25 ° C. and a rotor No. 14 of 5 rpm by a Brookfield rotational viscometer, and as a measured value under a condition of 50 rpm as 160 to 200 Pa. The production method according to claim 1, which is s.   The manufacturing method according to any one of claims 1 to 3, wherein the same mesh plate making is used when forming at least the first layer electrode and the second layer electrode.   The solar cell element obtained by the manufacturing method of any one of Claims 1 thru | or 4.

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