JP2007234625A - Photoelectric conversion element, conductive paste therefor, and method of manufacturing photoelectric conversion element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a conductive paste that suppresses warpage in a semiconductor substrate following the formation of a back surface electrode in a solar cell, and can form the back surface electrode having a small face resistance and high adhesive strength. <P>SOLUTION: The conductive paste containing Al alloy powder as an electrode constituent is used for forming the back surface electrode 5. An Al alloy having a melt point that is higher than that of Al is used for the Al alloy powder. The higher the melt point is, the higher the sintering start temperature of metal powder is. Then, the sintering is suppressed, thus relaxing a shrinkage difference caused by the difference in a thermal coefficient of expansion when temperature drops after sintering, and suppressing the generation of warpage in a semiconductor substrate 1. An element to be alloyed is preferably one of Ti, V, Zr, Sb, Cr, and B, and the weight ratio for the entire alloy is preferably 0.01 to 10%. Alloy powder is obtained by alloying in advance, thus preventing an eutectic region that has a small presence ratio of Al and weak adhesive strength from being formed, which differs from a case for adding single body metal powder. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

The present invention relates to a conductive paste for a photoelectric conversion element and a photoelectric conversion element produced using the same, and in particular, a conductive paste suitable for forming a back electrode layer and a p ⁺ layer of a solar cell, and using the same The present invention relates to a solar cell to be manufactured.

In recent years, the demand for solar cells for home use tends to increase significantly from the viewpoint of environmental protection. The configuration of the solar cell, the n ⁺ layer provided on the surface side of the p-type Si substrate, the n ⁺ / p / p ⁺ junction is formed by providing a p ⁺ layer on the back side, the more the light-receiving surface side a light receiving surface electrode in the n ⁺ layer side, the p ⁺ layer side opposite manner with a rear surface electrode is widely employed conventionally. It is also common to provide an antireflection film on the light receiving surface side.

  Printing methods are widely used for electrode formation. The printing method has a merit that it is easy to automate and has high productivity. Therefore, the printing method is a general method for forming electrodes of various electronic devices. In the printing method, a conductive paste (conductor paste) obtained by kneading a metal powder responsible for electrical conductivity with an organic binder or an organic solvent is applied to an object by screen printing or the like, and then fired in a heat treatment furnace. In this method, an organic component is evaporated to form an electrode as a sintered body of metal powder.

In the case of a solar cell, a conductive paste (Al paste) containing metal Al powder is applied to the back side of the Si substrate and baked, thereby forming not only the back electrode but also the p ⁺ layer. Yes. Specifically, when an Al electrode layer mainly composed of Al serving as a back electrode is formed by firing, Al diffuses into the Si substrate, thereby forming a p ⁺ layer containing Al as an impurity. The back electrode serves as a current collecting electrode for extracting electricity generated in the solar cell, and the p ⁺ layer increases the current collecting efficiency in the back electrode by causing a so-called BSF (Back Surface Field) effect. Playing a role.

On the other hand, in order to reduce the cost of the solar cell, a reduction in the thickness of the Si substrate to 200 μm or less has been studied. As a problem in realizing such a thin layer, as the Si substrate is made thinner, warpage due to a difference in thermal expansion from the Al electrode layer is more likely to occur in the Si substrate. The thermal expansion coefficient of Si is 2.5 × 10 ⁻⁶ / deg, whereas Al is 23.25 × 10 ⁻⁶ / deg, both being about 10 times different. A technique intended to solve this problem is already known (see, for example, Patent Document 1).

  Also disclosed is a method for preventing oxidation of Al when an electrode is formed by firing in an oxidizing atmosphere by using a paste in which a single metal powder that forms a high melting point alloy with Al is added to the Al powder. (For example, see Patent Document 2).

Japanese Patent Laid-Open No. 2003-223813 JP 59-168669 A

  The warpage of the Si substrate described above is caused by a difference in thermal expansion between the Al electrode layer and the Si substrate when the temperature is lowered after the Al paste is printed on the Si substrate and baked. When such a warp occurs, there is a problem that a handling error to an automatic machine is likely to occur in the subsequent process, and the solar cell element is cracked or chipped, resulting in a decrease in manufacturing yield.

As a solution to such a problem, a method of physically reducing the warping force by reducing the amount of Al paste applied and thinning the Al electrode layer is assumed. However, this method has a problem that the amount of Al diffusion into the Si substrate is reduced, the p ⁺ layer is hardly formed, and the power generation efficiency is lowered.

Patent Document 1 discloses an Al powder, an organic vehicle, an inorganic compound having a smaller coefficient of thermal expansion than Al and a melting temperature, softening temperature, or decomposition temperature higher than the melting point of Al, specifically SiO _2. And a method of forming a back electrode using a paste to which Al ₂ O ₃ or the like is added.

  However, in the method disclosed in Patent Document 1, although the warpage of the Si substrate can be reduced, since the inorganic compound added to the paste exists as it is after firing, bonding between Al particles in the back electrode Are weak and have low adhesion strength.

  Further, the problem in Patent Document 2 is prevention of Al oxidation, and it has not been recognized as a problem to prevent the warpage associated with the thinning of the Si substrate. There is no suggestion about the means to solve this problem.

  Further, in the method disclosed in Patent Document 2, other single metal is added to Al, specifically, at least one of Co, Cr, Mn, Mo, Ti, Zr, B, W, and Sb. Therefore, the aggregation of trace added components causes non-uniformity in the sinterability of Al, resulting in a problem that the peel strength is lowered.

  The present invention has been made in view of the above problems, and is a conductive paste used in the manufacture of solar cells and other photoelectric conversion elements, which suppresses warpage associated with the formation of a back electrode of a semiconductor substrate and has a sheet resistance. It aims at providing the electrically conductive paste which can form a back electrode with small and high adhesive strength, and a photoelectric conversion element produced using this.

  In order to solve the above problems, the invention of claim 1 is a conductive paste for forming an electrode on a semiconductor substrate for a photoelectric conversion element, wherein an Al-containing alloy powder having a melting point higher than that of Al is used as a conductive material. It is characterized by including.

  The invention according to claim 2 is the conductive paste for photoelectric conversion element according to claim 1, wherein any element other than Al constituting the Al-containing alloy is any one of titanium, vanadium, zirconium, antimony, chromium, and boron. It is characterized by that.

  Invention of Claim 3 is the conductive paste for photoelectric conversion elements of Claim 2, Comprising: The weight ratio which occupies for the whole said Al containing alloy of elements other than said Al is 0.01% or more and 10% or less It is characterized by that.

  According to a fourth aspect of the present invention, an electrode layer made of the Al-containing alloy is formed on a substantially entire surface of one main surface of the semiconductor substrate using the conductive paste for a photoelectric conversion element according to any one of the first to third aspects. It is characterized by.

  The invention of claim 5 is a photoelectric conversion element comprising a semiconductor substrate for a photoelectric conversion element and an electrode layer formed on substantially the entire surface of one main surface of the semiconductor substrate, wherein the electrode layer is made of Al. Is formed using an Al-containing alloy having a high melting point, and the Al-containing alloy does not include a eutectic structure in which the abundance ratio of elements other than Al is higher than the abundance ratio of Al.

  The invention of claim 6 is a method for producing a photoelectric conversion element, comprising using Al and a predetermined single metal to produce an Al-containing alloy powder having a melting point higher than that of Al, and the Al-containing alloy powder. A step of producing a conductive paste including a conductive material, and a step of forming an electrode layer on substantially the entire main surface of one of the semiconductor substrates for a photoelectric conversion element by a printing method using the conductive paste. It is characterized by.

  According to the invention of claim 1 to claim 6, by using a conductive paste containing, as a conductive material, an Al-containing alloy powder having a melting point higher than that of Al, the sintering start temperature is higher than when using an Al paste. can do. This makes it possible to suppress the sintering of the metal powder as compared with the case of using the Al paste under the same firing conditions, so that the semiconductor substrate generated due to the difference in thermal expansion coefficient between the Al and the semiconductor substrate when the temperature is lowered. Can be suppressed.

<Conductive paste>
The conductive paste according to this embodiment is mainly used when an electrode is formed by a printing method on a semiconductor substrate used for forming a photoelectric conversion element such as a solar cell. For example, in the case of producing a solar cell, a preferred example of the usage mode is to use a back electrode formed on the back side of a p-type semiconductor substrate such as a Si substrate by a printing method.

Such a conductive paste includes an Al alloy powder, a glass powder, an organic binder, and an organic solvent. Here, the Al alloy powder is a conductive material in the conductive paste, and is a metal powder made of an alloy (Al alloy) of Al and a metal element (alloy target element) other than Al. The conductive paste containing the Al alloy powder is also referred to as an alloy paste. By applying and baking the alloy paste by a printing method, a back electrode and a p ⁺ layer made of an Al alloy can be formed on the back side of the photoelectric conversion element. The alloy paste may contain Al powder. Even in such an aspect, the effect of the present invention is not disturbed.

  As the Al alloy powder, an Al alloy having a melting point higher than that of Al (melting point 660 ° C. under normal pressure) is used. The higher the melting point, the higher the sintering start temperature of the metal powder. If the firing conditions are the same, the higher the sintering start temperature, the lower the sintering of the metal powder contained in the applied conductive paste. As a result, the shrinkage difference due to the difference in thermal expansion coefficient is alleviated when the temperature is lowered after firing, and warpage of the semiconductor substrate is suppressed.

  As an Al alloy, an alloy target element is any one of Ti, V, Zr, Sb, Cr, and B, and a weight ratio of these alloy target elements to the entire alloy is preferably 0.01 to 10%. . This is because the melting point of the Al alloy of these alloy target elements is higher than that of Al at least in the range of this weight ratio. Especially, it is more preferable that it is 0.1%-5%.

  Even when the weight ratio of the alloy target element is smaller than 0.01%, the effect of increasing the sintering start temperature is not necessarily obtained. However, the smaller the weight ratio, the lower the melting point of the Al alloy and the melting point of Al. Therefore, in order to effectively obtain the effect of increasing the sintering start temperature, the weight ratio of the alloy target element is preferably set to 0.01% or more.

  Further, the reason why the weight ratio of the alloy target element is desirably 10% or less is that if it is within such a range, the back electrode can be formed with low resistance, and the power generation efficiency of the solar cell can be maintained high. It is.

  Further, an Al alloy whose alloy target elements are Mn and Co and the weight ratio of these alloy target elements to the entire alloy is 1% to 10% may be used for the Al alloy powder. Since the Al alloys of these elements have eutectic points with Al when the weight ratio is less than 1%, they have a melting point lower than that of Al within this weight ratio range. That is, it is not preferable because the sintering start temperature is lower than Al and promotes the sintering, but if the weight ratio is in the range of 1% to 10%, the effect of increasing the sintering start temperature is obtained. Can do.

  In the formed electrode, an alloy paste in which an element other than Al has a rich eutectic structure is also not preferable because the adhesion strength of the electrode becomes weak.

  As an Al alloy powder, a preferred embodiment is to use a powder having an average particle diameter of 3 to 20 μm. Such an Al alloy powder can be produced by a known atomizing method after melting a desired alloy target element in an Al ingot.

  About the organic binder and the organic solvent, the thing equivalent to what is used with the conventional Al paste can be used. As the organic binder, it is preferable to use a cellulose-based or acrylic-based binder from the viewpoint of printability. As an organic solvent, α terpineol or phthalate is a preferred embodiment.

  After mixing these Al alloy powder, organic binder, and organic solvent with a ball mill or a stirrer, the conductive paste (alloy paste) according to the present embodiment can be obtained by kneading with a three roll. In addition, the conductive paste may contain 10% by weight or less of a metal oxide or a glass component. Even in such a case, the effects of the present invention can be obtained.

<Solar cell>
Next, a solar cell as one embodiment of the photoelectric conversion element according to this embodiment, which is manufactured using the above-described conductive paste, will be described. FIG. 1 is a schematic cross-sectional view schematically showing a configuration of a solar cell 10 according to the present embodiment.

The solar cell 10 is formed on the semiconductor substrate 1, the front surface side (light receiving surface side) of the semiconductor substrate 1, the n ⁺ layer 2 having n-type impurities, and the back surface side of the semiconductor substrate 1, and p ⁺ layer 3 having a p-type impurity, formed by formed (on the n ⁺ layer 2 in FIG. 1) on the surface of n ⁺ layer 2, the light-receiving surface electrode 4 made of Ag or the like, the semiconductor substrate 1 It is mainly composed of a back electrode 5 made of the above-mentioned Al alloy or the like formed by interposing a p ⁺ layer 3 on the back side (under the p ⁺ layer 3 in FIG. 1). The solar cell 10 is configured such that a current can be taken out in response to the incidence of light in a predetermined wavelength range on the light receiving surface. That is, solar cell 10 has an n ⁺ / p / p ⁺ junction formed by n ⁺ layer 2, semiconductor substrate 1, and p ⁺ layer 3. It can also be said that the back electrode 5 has a structure formed respectively.

  As the semiconductor substrate 1, for example, it is preferable to use a Si-based group IV semiconductor. For example, an Si substrate obtained by slicing a polycrystalline Si ingot having an outer shape of 150 mm □ and adding B (boron) or the like as a p-type dopant to an arbitrary thickness within a range of 150 to 200 μm, It can be used as the semiconductor substrate 1. A suitable example of the specific resistance of the Si substrate is about 1.5 Ω · cm. Note that it is desirable to slightly etch the surface with NaOH, KOH, hydrofluoric acid, hydrofluoric acid, or the like in order to remove a damage layer or a contamination layer generated by processing. In order to increase the confinement efficiency of the received light, it is desirable to form minute irregularities on the surface of the semiconductor substrate 1 by a dry etching method or a wet etching method.

  Moreover, the material of the semiconductor substrate 1 is not limited to the above-mentioned material, and single crystal Si may be used. Alternatively, another semiconductor may be used as long as it can form a back electrode made of an Al alloy using the above-described alloy paste.

The n ⁺ layer 2 is a so-called reverse conductivity type diffusion region. The n ⁺ layer 2 is formed by implanting P (phosphorus) on one main surface side of the semiconductor substrate 1 by a known ion implantation method. The side on which the n ⁺ layer 2 is formed is the light receiving surface side of the solar cell. For example, the n ⁺ layer 2 is preferably formed to have a specific resistance of about 1.5 × 10 ⁻³ Ω · cm and a thickness of about 0.5 μm. Alternatively, the n ⁺ layer 2 may be formed by a so-called vapor phase diffusion method in which heat treatment is performed in a gas such as POCl ₃ (phosphorus oxychloride).

The p ⁺ layer 3 and the back electrode 5 are formed by a printing method using an alloy paste containing the Al alloy powder described above. For example, an alloy paste is applied to almost the entire surface of the semiconductor substrate 1 after the n ⁺ layer 2 is formed by screen printing, dried at 150 ° C. for several minutes, and then higher than the melting point of Al in the air. By firing at a firing temperature of 700 to 850 ° C. for several seconds to several tens of minutes, the Al alloy powder in the alloy paste is sintered to form the back electrode 5 made of an Al alloy, and Al and the alloy elements are semiconductors. The p ⁺ layer 3 is formed by diffusing toward the substrate 1.

The light receiving surface electrode 4 is formed by printing using Ag paste. For example, after forming the p ⁺ layer 3 and the back electrode 5, an Ag paste is applied on the n ⁺ layer 2 by screen printing in a comb-like shape, dried at 150 ° C. for several minutes, and then in the air. The comb-shaped light-receiving surface electrode 4 made of Ag is formed by firing at a firing temperature of 600 to 850 ° C. for several seconds to several tens of minutes.

  In the present embodiment, since the solar cell 10 is configured in this manner, the shrinkage difference caused by the difference in thermal expansion coefficient between the back electrode and the semiconductor substrate can be reduced as compared with the conventional case. While using a thinned semiconductor substrate, it is possible to realize a solar cell having a back electrode with reduced warpage, low surface resistance, and high adhesion strength.

  It should be noted that the embodiment of the present invention is not limited to the above-described example, and it is needless to say that various modifications can be made without departing from the gist of the present invention. For example, it is preferable to provide an antireflection film (not shown) made of a silicon nitride film or a silicon oxide film on the light receiving surface side of the semiconductor substrate.

  Further, as the back surface electrode, it is preferable to further form an electrode mainly composed of Ag for taking out the output in addition to the electrode mainly composed of Al formed on almost the entire back surface as described above.

(Example)
As an example, six types of alloy target elements of Ti, V, Zr, Sb, Cr, and B were used, and the weight ratios were set to three levels, respectively. As the conductive paste according to the above-described embodiment, a total of 18 types An alloy paste was prepared. Furthermore, 18 types of solar cells were produced by forming a back surface electrode using each alloy paste.

  In producing each alloy paste, first, an alloy target element was melted in an Al ingot at a predetermined weight ratio, and an Al alloy powder having an approximately spherical average particle diameter of 10 μm was obtained as a mother powder by an atomizing method. Next, glass powder having an average particle diameter of 5 μm and a softening point of 380 ° C. was prepared. After mixing an Al alloy powder and glass powder so that the latter weight ratio is 1% to obtain an inorganic raw material, 5 parts by weight of nitrocellulose as an organic binder with respect to 100 parts by weight of the inorganic raw material is organic. 20 parts by weight of α-terpineol was added as a solvent and mixed with a stirrer. Three rolls of this were processed to obtain an alloy paste. At that time, the viscosity of the alloy paste was adjusted to about 200 poise.

  18 types of solar cells were produced using each of the total 18 types of conductive pastes obtained.

In the production of each solar cell, a Si substrate obtained by slicing a polycrystalline Si ingot having an outer diameter of 150 mm □ to a thickness of 200 μm was used as the semiconductor substrate 1. An n ⁺ layer 2 having a depth of 0.5 μm was formed by implanting P on the surface of the Si substrate by ion implantation.

After that, an alloy paste is applied on the entire back surface of the Si substrate by screen printing, and after drying at 150 ° C. for 10 minutes, the maximum temperature is heated to 750 ° C. in air and baked for 10 minutes. The back electrode 5 and the p ⁺ layer 3 were formed.

Further, Ag paste is applied to the surface of the n ⁺ layer 2 in a comb-like shape by a screen printing method, and after drying at 150 ° C. for 10 minutes, the maximum temperature is heated to 600 ° C. in air. The light-receiving surface electrode 4 was formed by baking for a minute. Thereby, 18 types of solar cells were obtained.

  For each of the solar cells thus fabricated, the amount of warpage of the semiconductor substrate, the surface resistance of the back electrode, and the peel strength of the back electrode were measured. FIG. 2 is a diagram for explaining the evaluation method of the warpage amount of the semiconductor substrate according to this example. In the present embodiment, the amount of warpage was evaluated using a value including the thickness of the semiconductor substrate 1. Specifically, as shown in FIG. 2, the amount of warpage was evaluated by the difference in height between the lowest part (horizontal plane) and the highest part when placed on a horizontal plane. At that time, 2 mm or more was made impossible. In addition, the surface resistance of the Al electrode portion is not allowed to be 15 mΩ / □ or more. The peel strength of the Al electrode part was evaluated by a peeling test using a cellophane tape, and those with peeling were regarded as impossible because the adhesion strength was not sufficient.

  Moreover, the obtained solar cell was embedded in resin, polished, and the back electrode was observed by a reflection electron image by SEM to confirm the presence or absence of a eutectic structure.

(Comparative example)
On the other hand, as a comparative example, in addition to Al paste containing only Al powder as a metal powder, and four kinds of metal additions each containing four kinds of single metal powders of Ti, V, Sb, and Cr in addition to Al powder A paste was prepared, and a solar cell was prepared and evaluated in the same manner as in the above-described example. Here, the latter four types of metal-added pastes correspond to the conductor paste disclosed in Patent Document 2.

  In the preparation of each paste, an Al powder having an average particle size of 10 μm was prepared by the atomizing method in the same manner as in the examples to obtain a mother powder. Next, for the metal-added paste, metal powder having an average particle diameter of 1 μm was prepared in an amount corresponding to a predetermined weight ratio. These were mixed with glass powder, an organic binder, and an organic solvent in the same manner as in Example to prepare an Al paste and a metal-added paste.

  Further, as in the examples, the amount of warpage, surface resistance, and peel strength were evaluated, and the presence or absence of a eutectic structure was also confirmed.

(Comparison of Example and Comparative Example)
Table 1 shows the evaluation results of the 18 types of solar cells according to the examples and the 5 types of solar cells according to the comparative examples obtained as described above. In Table 1, the results of numbers 1 to 18 are for the examples, and numbers 19 to 23 are for the comparative examples.

  As shown in Table 1, in the solar cells of numbers 19 to 23, which are comparative examples using an Al paste and a metal-added paste, the warp value exceeded 2 mm, and peeling occurred in the peel strength test (in Table 1). △ mark). That is, sufficient adhesion strength could not be obtained.

  Moreover, in the solar cells of Nos. 19 to 23 using the metal-added paste corresponding to the conductive paste disclosed in Patent Document 2 among the comparative examples, as a result of the structure observation, the portion where the added metal element is rich, That is, formation of a eutectic structure was confirmed in a portion where the Al content ratio was small. The presence of such a eutectic structure is a factor that decreases the adhesion strength.

  On the other hand, in the solar cells of Nos. 1 to 18, which are examples using an alloy paste, it was found that the warpage was good at 2 mm or less and the peel strength of the back electrode was also good (Table 1). Inside circle). The surface resistance of the back electrode was almost the same as that of the comparative example. Furthermore, no eutectic structure was observed in the structure observation.

  Based on the above results, the back electrode of a solar cell using a thinned semiconductor substrate is made of an alloy paste containing an Al alloy powder having a melting point higher than that of Al as a conductive material, thereby warping the semiconductor substrate. It was confirmed that a solar cell with a low surface resistance and a high adhesion strength of the back electrode can be obtained.

It is a cross-sectional schematic diagram which shows schematically the structure of the solar cell 10 which concerns on this Embodiment. It is a figure for demonstrating the evaluation method of the curvature amount of a semiconductor substrate.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Semiconductor substrate 2 n ⁺ layer 3 p ⁺ layer 4 Light-receiving surface electrode 5 Back surface electrode 10 Solar cell

Claims (6)

A conductive paste for forming an electrode on a semiconductor substrate for a photoelectric conversion element,
Including an Al-containing alloy powder having a melting point higher than that of Al as a conductive material,
A conductive paste for a photoelectric conversion element. It is the electrically conductive paste for photoelectric conversion elements of Claim 1, Comprising:
Elements other than Al constituting the Al-containing alloy are any of titanium, vanadium, zirconium, antimony, chromium, and boron.
A conductive paste for a photoelectric conversion element. The conductive paste for a photoelectric conversion element according to claim 2,
The weight ratio of elements other than Al to the entire Al-containing alloy is 0.01% or more and 10% or less.
A conductive paste for a photoelectric conversion element. Using the conductive paste for photoelectric conversion elements according to any one of claims 1 to 3, an electrode layer made of the Al-containing alloy is formed on substantially the entire main surface of the semiconductor substrate.
The photoelectric conversion element characterized by the above-mentioned. A semiconductor substrate for a photoelectric conversion element;
An electrode layer formed on substantially the entire surface of one main surface of the semiconductor substrate;
A photoelectric conversion element comprising:
The electrode layer is formed using an Al-containing alloy having a higher melting point than Al,
The Al-containing alloy does not contain a eutectic structure in which the abundance ratio of elements other than Al is higher than the abundance ratio of Al,
The photoelectric conversion element characterized by the above-mentioned. A method for producing a photoelectric conversion element, comprising:
A step of producing an Al-containing alloy powder having a melting point higher than that of Al using Al and a predetermined simple metal,
Producing a conductive paste containing the Al-containing alloy powder as a conductive material;
Forming an electrode layer on substantially the entire main surface of one of the semiconductor substrates for photoelectric conversion elements by a printing method using the conductive paste;
A process for producing a photoelectric conversion element, comprising:

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