JP5289705B2 - Conductive paste for photoelectric conversion element, photoelectric conversion element, and method for producing photoelectric conversion element - Google Patents

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.

  A printing method (coating method) is 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).

Japanese Patent Laid-Open No. 2003-223813

  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.

  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 the conductive material is an Al powder having a purity of 99.9% by mass or more. And having been heat-treated at a heating temperature of 300 ° C. or higher and 500 ° C. or lower in an inert gas atmosphere.

According to a second aspect of the present invention, in the photoelectric conversion element, an electrode layer containing Al as a main component and a glass component on one main surface of the semiconductor substrate using the conductive paste for the photoelectric conversion element according to the first aspect. It is characterized by forming.

Invention of Claim 3 is a photoelectric conversion element provided with the semiconductor substrate for photoelectric conversion elements, and the electrode layer containing the glass component formed on one main surface of the said semiconductor substrate, Comprising: The said electrode layer is, becomes the the Al powder or purity of 99.9% by weight obtained by heat treatment at 300 ° C. or higher 500 ° C. or less of the heating temperature in the inert gas atmosphere is formed by using a starting material, containing the metal or metalloid other than Al The ratio is 1% by mass or less, and the Young's modulus is 40 GPa or less.

The invention of claim 4 is a method for producing a photoelectric conversion element, wherein an Al powder having a purity of 99.9% by mass or more as a conductive material is heat-treated at a heating temperature of 300 ° C. or more and 500 ° C. or less in an inert gas atmosphere. A step of producing a conductive paste, and a step of forming an electrode layer containing a glass component on one main surface of a semiconductor substrate for a photoelectric conversion element by a coating method using the conductive paste, The electrode layer is formed so that the main component is Al having a purity of 99.9% by mass or more, and the content ratio of a metal or a semimetal other than Al is 1% by mass or less.

According to the first to fourth aspects of the present invention, a soft electrode layer having a small Young's modulus containing high-purity Al that has undergone heat treatment as a main component is formed. The shrinkage difference between the electrode layer and the semiconductor substrate caused by the difference in coefficient of thermal expansion is alleviated as compared with the conventional case of forming and in the case of not performing the heat treatment, and as a result, the semiconductor substrate It is possible to realize a photoelectric conversion element in which the warpage is suppressed.

<Conductive paste>
The conductive paste according to the present embodiment is mainly used when an electrode is formed by a printing method (coating 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 contains high-purity Al powder, glass frit powder, an organic binder, and an organic solvent. Here, the high-purity Al powder is a particle responsible for conductivity in the conductive paste, and means an Al powder having a purity of 99.9% by mass or more. Furthermore, the purity is preferably 99.99% by mass or more. As a high-purity Al powder, a preferred embodiment is to use a powder having an average particle diameter of 15 μm or less. By applying and baking the conductive paste according to the printing method, the back electrode and the p ⁺ layer mainly composed of Al can be formed on the back surface side of the photoelectric conversion element. In addition, it is not inherently preferable that the conductive paste contains Al powder other than the above-described high-purity Al powder, but contamination caused by unintentional factors is so small that the effect of the present invention is not hindered. Percentages are acceptable.

  In the present embodiment, the conductive paste containing the above-described high-purity Al powder is used for forming the back electrode, as compared with the case where the conventional non-high-purity Al powder is used, the hardness is small, that is, the soft back surface. This is because an electrode can be formed. Specifically, a back electrode having a Young's modulus of 40 GPa or less can be formed. Furthermore, a back electrode having a Young's modulus of 30 GPa or less can be formed by more suitably determining the conditions. In a solar cell with such a soft back electrode formed, the shrinkage difference between the electrode layer and the semiconductor substrate due to the difference in thermal expansion coefficient is reduced when the temperature is lowered after firing, and the semiconductor substrate may be warped. Effectively suppressed. In other words, a solar cell in which the back electrode is formed so that the Young's modulus is 40 GPa or less can be handled as having practically suppressed warpage.

  In the present embodiment, the Young's modulus of the back electrode is calculated by performing a Martens hardness test (ISO14577) on the back electrode after firing.

From the viewpoint of suppressing the warpage of the semiconductor substrate, the high-purity Al powder is heat-treated in an inert gas atmosphere such as N _{2 at} a temperature of 300 to 500 ° C. and then supplied to the preparation of the conductive paste. Is more preferable. This is because by performing such heat treatment, the back electrode to be formed becomes softer (its Young's modulus becomes smaller).

As the glass frit powder, the organic binder, and the organic solvent, those equivalent to those used in the conventional Al paste can be used. A preferred example of the glass frit powder is to use, for example, a B ₂ O ₃ —SiO ₂ —PbO glass material. As the organic binder, it is preferable to use a cellulose-based or polymethacrylate-based one from the viewpoint of printability. For example, ethyl cellulose is an example. As the organic solvent, it is preferable to use a polyhydric alcohol. For example, α-terpineol is an example.

  These high-purity Al powder, glass frit powder, organic binder, and organic solvent are added in predetermined weights, for example, high-purity Al powder is about 70% by weight, glass frit powder is about 0.1% by weight, and organic binder is 4% by weight. The conductive paste according to the present embodiment can be obtained by weighing about 25% and about 26% by weight of the organic solvent, mixing them with a ball mill or a stirrer, and then kneading them with a three roll.

<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 to 5 Ω · cm. Note that it is desirable to slightly etch the surface with NaOH or KOH, or hydrofluoric acid or hydrofluoric acid, in order to remove a damage layer or a contamination layer generated by processing. Further, 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 conductive 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 0.3 to 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 the conductive paste containing the above-described high-purity Al powder. For example, a conductive 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 the maximum temperature is 700 to 700 in air. When baking is performed at a baking temperature of 850 ° C. for about 1 to 30 minutes, a back electrode 5 containing Al as a main component and including a glass component is formed, and Al diffuses toward the semiconductor substrate 1 to cause a p ⁺ layer. 3 is formed.

  As described above, in this embodiment, since the conductive paste containing high-purity Al powder is used for forming the back electrode, the Young's modulus of the back electrode 5 obtained is 40 GPa or less, which is the conventional high purity. It is softer than the back electrode formed with a conductive paste using non-Al powder. In the solar cell 10, the back electrode 5 is formed so softly that the shrinkage difference due to the difference in the coefficient of thermal expansion between the back electrode 5 and the semiconductor substrate 1 is relaxed compared to the conventional case. Warpage of the semiconductor substrate 1 is effectively suppressed.

  Further, the surface resistance of the back electrode 5 is about the same as or lower than that of the prior art. That is, it can be said that the solar cell 10 has electrode characteristics that are as good as or better than the conventional one. On the other hand, in the back electrode 5, the abundance ratio of metals or metalloids other than Al is 1% by mass or less, and an oxide layer is formed on the surface of Al particles, so that Al is in a nonconductive state. It is considered that these contribute to the realization of the above-mentioned good electrical characteristics. Further, the peel strength of the back electrode 5 is at least about the same as the conventional one.

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. Thus, the comb-shaped light-receiving surface electrode 4 made of Ag is formed by baking at a baking temperature of about 600 ° C. for about 1 minute to 10 minutes.

  In the present embodiment, the solar cell 10 is configured in this manner, so that the shrinkage difference caused by the difference in thermal expansion coefficient between the back electrode and the semiconductor substrate is reduced as compared with the case of using conventional non-pure Al powder. Therefore, it is possible to realize a solar cell having a back electrode with reduced surface resistance and high adhesion strength while using a semiconductor substrate that is thinner than the conventional one, while reducing its warpage. .

  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.

  Two types of Al powders with a purity of 99.998% by mass, 99.9% by mass, and 99.85% by mass were prepared in two levels with or without heat treatment, and a conductive paste was prepared for each. . That is, six levels of conductive pastes with different combinations of Al powder purity and heat treatment were prepared.

In the preparation of each conductive paste, first, an Al powder having an approximately spherical average particle diameter of 5 μm was obtained by an atomizing method. For those subjected to a heat treatment, in an N ₂ gas atmosphere, and then heated for 30 minutes at a maximum temperature of 500 ° C..

  Next, after preparing a glass powder having an average particle size of 5 μm and a softening point of 380 ° C., mixing the Al powder and the glass powder so that the weight ratio of the latter is 0.1%, an inorganic raw material is obtained, Furthermore, 5 parts by weight of ethyl cellulose as an organic binder with respect to 100 parts by weight of an inorganic raw material and 20 parts by weight of α-terpineol as an organic solvent were added and mixed with a stirrer. This was roll-treated to obtain a conductive paste.

  Six types of solar cells were produced using each of the obtained six types of conductive pastes.

In the production of each solar cell, a Si substrate obtained by slicing a polycrystalline Si ingot having an outer shape of 150 mm □ to a thickness of 180 μm was used as the semiconductor substrate 1. By implanting P on the surface of the Si substrate by ion implantation, an n ⁺ layer 2 having a depth of 0.5 μm and a specific resistance of about 1.5 × 10 ⁻³ Ω · cm was formed.

After that, a conductive paste is applied on the entire back surface of the Si substrate by screen printing and dried at 150 ° C. for 10 minutes, and then heated in air to a maximum temperature of 750 ° C. 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, dried at 150 ° C. for 10 minutes, and then heated in air to a maximum temperature of 600 ° C. for 10 minutes. The light-receiving surface electrode 4 was formed by baking. Thereby, six types of solar cells were obtained.

  About each solar cell produced in this way, the warpage amount of the semiconductor substrate, the surface resistance of the back electrode, the peel strength of the back electrode, and the Young's modulus 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. In that case, it was decided that 2 mm or more was impossible. Further, the surface resistance of the Al electrode portion was measured by a four-terminal method, and it was determined that 15 mΩ / □ or more was not possible. The peel strength of the Al electrode portion was evaluated by a peeling test using a cellophane tape, and it was determined that a peeled material was judged to be unacceptable if the adhesion strength was not sufficient. Young's modulus was measured by a Martens hardness test. The Martens hardness test is an evaluation by a scratch test using a nanoindenter, and can be obtained by converting the Young's modulus from the measurement result of the depth of the scratch. A nanoindenter is a device that can perform an indentation test with ultra-low load (maximum load 20 mN, load resolution 1 nN) and high accuracy (displacement resolution 0.01 nm) using a small triangular pyramid indenter made of a diamond chip.

The evaluation results for the six types of solar cells thus obtained are shown in Table 1. Sample No. Reference numerals 2 and 4 are reference samples.

As shown in Table 1, none of the Al surface resistance and peel strength were determined to be unacceptable. On the other hand, with regard to the amount of warpage and Young's modulus, the results showed that the higher the purity of Al, and the higher the purity of the heat-treated, the smaller the amount of warpage and the Young's modulus. And the purity of Al is 99.85 mass%, and No. which does not heat-process. Only about the solar cell of 6, the curvature amount exceeded 2 mm, and it was determined that it was impossible. Focusing on the Young's modulus, this No. Only 6 solar cells were above 40 GPa and others were below 40 GPa. In particular, the purity of Al is 99.998% by mass. 1 and no. For the solar cell of 2, the Young's modulus was less than 30 GPa. Moreover, it was confirmed that the Young's modulus of the back electrode formed becomes smaller (the back electrode becomes softer) by performing the heat treatment.

  The above results show that by using high-purity Al powder for the conductive paste, a soft back electrode with a small Young's modulus can be formed while maintaining sufficient adhesion strength and small sheet resistance, and this It shows that the warp of the semiconductor substrate can be reduced by forming such a soft back electrode.

  Also, from the results in Table 1, it can be said that the higher the purity of the Al powder, the smaller the total amount of surface resistance and impurities other than Al, and it is confirmed that there is a positive correlation between the surface resistance of the back electrode. Is done. That is, it can be said that the higher the purity of the Al powder, the better the electrode characteristics of the back electrode.

  From the above results, it is possible to reduce the warpage of the semiconductor substrate by producing the back electrode of the solar cell using the thinned semiconductor substrate with a conductive paste containing high-purity Al powder as a conductive material. It was confirmed that a solar cell with high adhesion strength of the back electrode and low surface resistance was obtained. It has also been confirmed that it is more effective to heat-treat the high-purity Al powder.

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 (4)

A conductive paste for forming an electrode on a semiconductor substrate for a photoelectric conversion element,
The conductive material is Al powder having a purity of 99.9% by mass or more, and is heat-treated at a heating temperature of 300 ° C. or more and 500 ° C. or less in an inert gas atmosphere.
A conductive paste for a photoelectric conversion element. Using the conductive paste for photoelectric conversion element according to claim 1, an electrode layer containing Al as a main component and a glass component is formed on one 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 containing a glass component formed on one main surface of the semiconductor substrate;
A photoelectric conversion element comprising:
The electrode layer is formed by using, as a starting material, an Al powder having a purity of 99.9% by mass or more and heat-treated at 300 ° C. or more and 500 ° C. or less in an inert gas atmosphere, and a metal other than Al Alternatively, the metalloid content ratio is 1% by mass or less, and the Young's modulus is 40 GPa or less.
The photoelectric conversion element characterized by the above-mentioned. A method for producing a photoelectric conversion element, comprising:
A step of producing a conductive paste in which the conductive material is a heat treatment of Al powder having a purity of 99.9% by mass or higher at a heating temperature of 300 ° C. or higher and 500 ° C. or lower;
Forming an electrode layer containing a glass component on one main surface of a semiconductor substrate for a photoelectric conversion element by a coating method using the conductive paste;
With
The electrode layer is formed so that the main component is Al having a purity of 99.9% by mass or more, and the content ratio of the metal or semimetal other than Al is 1% by mass or less.
A method for manufacturing a photoelectric conversion element.

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