JP2005317898A - Paste composition and solar cell element using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a paste composition preventing warpage or cracks from occurring even if using a thinner silicon semiconductor substrate and achieving high BSF effect and energy conversion efficiency, and also to provide a solar cell element having an electrode formed by using the composition. <P>SOLUTION: The paste composition is used for forming an aluminum electrode on a p-type silicon semiconductor substrate, and contains aluminum powder, an organic vehicle, and carbon powder. The solar cell element has an aluminum electrode layer 8 formed by applying the paste composition having the above feature onto the p-type silicon semiconductor substrate 1 for burning. The aluminum electrode layer 8 contains a carbon particle. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention generally relates to a paste composition and a solar cell element using the same, and more specifically, when a back surface aluminum electrode is formed on a p-type silicon semiconductor substrate constituting a crystalline silicon solar cell. The present invention relates to a paste composition used and a solar cell element using the same.

  Solar cells are desired to be used in a wider range as clean energy that is safe and has low environmental impact. In particular, it is necessary to promote weight reduction and cost reduction in order to popularize solar cells that use silicon semiconductor substrates to form elements. In response to this requirement, research and development have been repeated in order to reduce the thickness of the solar cell element.

  FIG. 1 is a diagram schematically showing a general cross-sectional structure of a solar cell element.

  As shown in FIG. 1, on the light-receiving surface side of a p-type silicon semiconductor substrate 1 having a thickness of 300 to 600 μm, an n-type impurity layer 2, a reflective film layer 3 and a grid electrode 4 having a thickness of 0.3 to 0.5 μm. Are formed in order.

On the back side of the p-type silicon semiconductor substrate 1, an aluminum electrode layer 8 is formed as a back electrode. The aluminum electrode layer 8 includes an aluminum sintered layer 5 and an aluminum silicon mixed layer 6. By applying and baking a paste containing aluminum on the back surface of the p-type silicon semiconductor substrate 1, the aluminum sintered layer 5 and the aluminum silicon mixed layer 6 are formed, and at the same time, aluminum is contained in the p-type silicon semiconductor substrate 1. A p ⁺ layer (or p ⁺⁺ layer) 7 is formed by diffusion. By the presence of the p ⁺ layer 7, a so-called BSF (Back Surface Field) effect can be obtained, and the collection efficiency of carriers generated in the p-type semiconductor substrate 1 can be increased. That is, among the minority carriers generated in the p-type silicon semiconductor substrate 1, carriers toward the back electrode are repelled in the surface direction by the p ⁺ layer 7 forming an internal electric field and serving as a barrier, and the photocurrent is generated at the surface electrode. As a result, the photovoltaic power and the photocurrent are increased and the conversion efficiency can be increased.

  The application of the aluminum-containing paste for forming the aluminum electrode layer is generally performed using a screen printing method, and the firing is generally performed in an oxidizing atmosphere. In order to obtain a desired BSF effect, it is necessary to apply a thick aluminum-containing paste and fire it. Specifically, unless the aluminum-containing paste is applied to the entire surface of the silicon semiconductor substrate to a thickness of 40 to 70 μm and fired, the BSF effect is not sufficient and the conversion efficiency cannot be increased.

However, simultaneously with the formation of the p ⁺ layer that brings about the BSF effect, the aluminum electrode layer is formed to be thicker than necessary. For this reason, when the thickness of the silicon semiconductor substrate is reduced to 300 μm or less in order to reduce the thickness of the solar cell element, the ratio of the thickness of the aluminum electrode layer to the thickness of the semiconductor substrate increases. As a result, when cooling from the firing temperature for forming the aluminum electrode layer to room temperature, an internal stress is generated due to the difference in the thermal expansion coefficient between the aluminum or aluminum silicon mixture and the silicon substrate. There was a problem that warping or cracking occurred.

  Therefore, in the conventional method for manufacturing a solar cell element, there is a problem when the thickness of the silicon semiconductor substrate is reduced to 300 μm or less in response to a request for thinning the solar cell element.

  In order to solve such a problem, several methods have been proposed.

  For example, Japanese Patent Laid-Open No. 2002-353476 (Patent Document 1) proposes a method of partially etching the surface of the back electrode in the thickness direction after the back electrode is formed on the semiconductor substrate. However, this method has a problem that the manufacturing process is complicated and warpage or cracking during firing cannot be prevented.

In addition, the aluminum-containing paste is first thinly applied to the entire back surface of the semiconductor substrate, and then the aluminum-containing paste is applied again to the portion to be thickened, and then baked, so that the back surface of the semiconductor substrate has two or more thicknesses. A method for forming an electrode is proposed in, for example, Japanese Patent Laid-Open No. 2002-217435 (Patent Document 2). For example, Japanese Laid-Open Patent Publication No. 2002-141533 (Patent Document 3) proposes a method of forming backside electrodes in a lattice pattern on the backside of a semiconductor substrate. For example, Japanese Patent Application Laid-Open No. 2002-141534 (Patent Document 4) proposes a method of forming back electrodes in stripes having different thicknesses on the back surface of a semiconductor substrate. For example, Japanese Patent Application Laid-Open No. 2002-141546 (Patent Document 5) proposes a method of forming a back electrode in a rectangular shape on the back surface of a semiconductor substrate. However, there is a problem that the p ⁺ layer that produces the BSF effect formed by any of these methods becomes non-uniform, leading to a decrease in conversion efficiency.

  Further, as a conductive paste composition capable of thinning an aluminum electrode layer while ensuring a desired BSF effect, a composition containing aluminum powder, glass frit, an organic vehicle, and an aluminum-containing organic compound is disclosed in, for example, This is proposed in Japanese Patent Laid-Open No. 2000-90734 (Patent Document 6). However, the use of this conductive paste reduces the amount of warpage generated in the p-type silicon semiconductor substrate by reducing the coating amount and thinning the back electrode layer while ensuring certain solar cell characteristics. In order to obtain a good BSF effect, the amount of warp cannot be reduced without reducing the amount of conductive paste applied.

Therefore, a paste composition containing an inorganic compound having a coefficient of thermal expansion smaller than that of aluminum and having a melting temperature, a softening temperature, or a decomposition temperature higher than the melting point of aluminum, an aluminum powder, and an organic vehicle is, for example, This is proposed in Japanese Laid-Open Patent Publication No. 2003-223813 (Patent Document 7). However, increasing the addition amount of the inorganic compound can further suppress the occurrence of warping or cracking during firing, but the surface resistance of the aluminum electrode layer increases, the ohmic resistance between the electrodes increases, and the sunlight resistance increases. There was a problem that energy generated by irradiation could not be taken out effectively, resulting in a decrease in energy conversion efficiency.
JP 2002-353476 A JP 2002-217435 A JP 2002-141533 A JP 2002-141534 A JP 2002-141546 A JP 2000-90734 A Japanese Patent Laid-Open No. 2003-223813

  SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to solve the above-described problem, and even when a thinner silicon semiconductor substrate is used, it is possible to prevent warping or cracking, and a high BSF effect and energy conversion efficiency. It is providing the solar cell element provided with the paste composition which can achieve these, and the electrode formed using the composition.

  As a result of intensive studies to solve the problems of the prior art, the present inventors have found that the above object can be achieved by using a paste composition having a specific composition. Based on this finding, the paste composition according to the present invention has the following characteristics.

  A paste composition according to the present invention is a paste composition for forming an electrode on a p-type silicon semiconductor substrate, and includes aluminum powder, an organic vehicle, and carbon powder.

  Preferably, the paste composition of the present invention further includes glass frit.

  Moreover, preferably, the paste composition of this invention contains 0.1 mass% or more and 18 mass% or less of carbon powder.

  More preferably, the paste composition of the present invention contains 60% to 75% by weight of aluminum powder, 20% to 35% by weight of organic vehicle, and 0.1% to 18% by weight of carbon powder. .

  When the glass frit is contained, preferably, the paste composition of the present invention has an aluminum powder of 60% by mass to 75% by mass, an organic vehicle of 20% by mass to 30% by mass, and a carbon powder of 0.1% by mass or more. 18 mass% or less and glass frit are contained 5.0 mass% or less.

  In the paste composition of the present invention, the carbon powder is preferably carbon black.

  Furthermore, in the paste composition of the present invention, the average particle size of the carbon powder is preferably 10 μm or less.

  Furthermore, in the paste composition of the present invention, the average particle size of the carbon powder is preferably 0.5 μm or less.

  In the paste composition of the present invention, when the average particle size of the carbon powder is 0.5 μm or less, it is preferable to contain the carbon powder in an amount of 0.3% by mass to 6% by mass.

  The solar cell element according to the present invention includes an electrode formed by applying a paste composition having the above-described characteristics onto a p-type silicon semiconductor substrate and then baking it.

  A solar cell element according to another aspect of the present invention includes an aluminum electrode layer, and the aluminum electrode layer includes carbon particles.

  As described above, according to the present invention, it is possible to prevent the p-type silicon semiconductor substrate from being warped or cracked by firing the p-type silicon semiconductor substrate coated with the paste composition containing carbon powder. In addition, a solar battery element including a paste composition capable of achieving a high BSF effect and energy conversion efficiency and an electrode formed using the composition can be obtained.

The paste composition of the present invention is characterized by further containing carbon powder in addition to aluminum powder and organic vehicle. The thermal expansion coefficient of the carbon powder is approximately 0.5 to 10 × 10 ⁻⁶ / ° C. although it varies depending on the crystal structure and the like, and is smaller than the aluminum thermal expansion coefficient 23.5 × 10 ⁻⁶ / ° C. Moreover, the electrical conductivity of carbon powder is larger than inorganic compound powder or aluminum-containing organic compound powder.

  By including such a carbon powder in the paste composition, the difference in thermal expansion coefficient between the carbon powder and silicon is relatively small, so that the deformation of the p-type silicon semiconductor after the paste is applied and baked can be reduced. Can be suppressed. Moreover, since the electrical conductivity of the carbon powder is relatively high, an increase in the surface resistance of the aluminum electrode layer can be suppressed.

  Conventionally, in order to suppress deformation of the p-type silicon semiconductor after firing, a certain solar cell characteristic is secured by reducing the coating thickness of the paste and using a paste to which an aluminum-containing organic compound powder is added. However, there was substantially no effective means other than thinning the coating film thickness of the paste to make the back electrode layer thin, or using a paste to which inorganic compound powder was added.

  If the coating thickness of the paste is reduced, the diffusion amount of aluminum from the surface of the p-type silicon semiconductor substrate to the inside becomes insufficient, and a sufficient BSF effect cannot be obtained, resulting in deterioration of the characteristics of the solar cell element. .

  When a paste to which an inorganic compound powder or an aluminum-containing organic compound powder is added is used, the surface resistance of the aluminum electrode layer increases, the energy conversion efficiency decreases, and as a result, the characteristics of the solar cell element decrease.

  However, in this invention, since the surface resistance of the aluminum electrode layer does not increase, there is no possibility of causing a decrease in energy conversion efficiency. Further, since the deformation of the p-type silicon semiconductor substrate after firing can be suppressed without reducing the paste coating thickness, a sufficient BSF effect can be obtained.

  As the carbon powder included in the paste composition of the present invention, any material such as artificial graphite powder, activated carbon or carbon black, carbon fiber, glassy carbon and the like can be suitably used.

  The content of the carbon powder included in the paste composition of the present invention is preferably 0.1% by mass or more and 18% by mass. When the content of the carbon powder is less than 0.1% by mass, it is not possible to obtain an effect of addition sufficient to suppress deformation of the p-type silicon semiconductor substrate after firing. When the content of the carbon powder exceeds 18% by mass, the applicability of the paste in screen printing or the like decreases.

  The average particle size of the carbon powder included in the paste composition of the present invention is preferably 10 μm or less, more preferably 1 μm or less, more preferably 0.5 μm or less. If the average particle diameter of the carbon powder exceeds 10 μm, the number of carbon particles present in the aluminum electrode layer formed during firing of the paste is reduced, so that the deformation of the p-type silicon semiconductor substrate after firing is suppressed. It is not possible to obtain a sufficient addition effect.

  In order to reduce the deformation (warp) and electrode layer surface resistance of the p-type silicon semiconductor substrate after firing in a well-balanced manner, the average particle size of the carbon powder is 0.5 μm or less, and the content of the carbon powder is 0.3 mass% or more. What is necessary is just to make it 6 mass% or less. By doing so, fine carbon powder is effectively dispersed and contained in the aluminum electrode layer.

The lower limit of the average particle size of the carbon powder is not particularly limited, but when the specific surface area that increases with the decrease of the average particle size exceeds 20000 m ² / g, the viscosity of the paste increases, and the applicability of the paste during screen printing decreases. To do.

  Moreover, it is preferable that content of the aluminum powder included in the paste composition of this invention is 60 mass% or more and 75 mass% or less. When the content of the aluminum powder is less than 60% by mass, the surface resistance of the aluminum electrode layer after firing increases, which may cause a decrease in energy conversion efficiency. When the content of the aluminum powder exceeds 75% by mass, the applicability of the paste during screen printing is lowered.

  As the organic vehicle included in the paste composition of the present invention, one obtained by dissolving ethyl cellulose, acrylic resin, alkyd resin or the like in a solvent is used. The content of the organic vehicle is preferably 20% by mass or more and 35% by mass or less. When the content of the organic vehicle is less than 20% by mass or exceeds 35% by mass, the printability of the paste is deteriorated.

  Further, the paste composition of the present invention may contain glass frit. The glass frit content is preferably 5.0% by mass or less. Glass frit is not directly related to the deformation of the p-type silicon semiconductor, the BSF effect and the energy conversion efficiency, but is added to improve the adhesion between the baked aluminum electrode layer and the p-type silicon semiconductor substrate. is there. If the glass frit content exceeds 5.0% by mass, segregation of the glass may occur.

As the glass frit included in the paste composition of the present invention, in addition to the SiO ₂ —Bi ₂ O ₃ —PbO system, B ₂ O ₃ —SiO ₂ —Bi ₂ O ₃ system, B ₂ O ₃ —SiO ₂ — Examples thereof include a ZnO system and a B ₂ O ₃ —SiO ₂ —PbO system.

  The solar cell element as one embodiment of the present invention has the cross-sectional structure shown in FIG. 1, and an aluminum electrode layer 8 as a back electrode formed on the back side of the p-type silicon semiconductor substrate 1 is Contains carbon particles.

  Hereinafter, one embodiment of the present invention will be described.

  First, the aluminum powder is contained in the range of 60 to 75% by mass, the glass frit in the range of 0.3 to 5.0% by mass, the organic vehicle in the range of 20 to 30% by mass, and the carbon powder is contained in the ratio shown in Table 1. Various paste compositions were prepared.

Specifically, aluminum powder and B ₂ O ₃ —SiO ₂ —PbO glass frit are added to an organic vehicle in which ethyl cellulose is dissolved in a glycol ether organic solvent, and various carbon powders shown in Table 1 are added. The mixture was mixed with a known mixer to obtain a paste composition.

  Here, the aluminum powder is formed from particles having an average particle diameter of 2 to 20 μm or a shape close to a sphere from the viewpoint of ensuring reactivity with the p-type silicon semiconductor substrate, coating properties, and uniformity of the coating film. This powder was used.

  The various paste compositions described above were applied to a p-type silicon semiconductor substrate having a size of 2 inches (50.8 mm) × 2 inches (50.8 mm) and a thickness of 280 μm using a 180 mesh screen printing plate. Printed. The coating amount was set so that the thickness of the fired aluminum electrode layer was 40 to 50 μm.

  After drying the p-type silicon semiconductor substrate on which the paste has been printed, it is heated in an infrared baking furnace at a heating rate of 400 ° C./min in an air atmosphere and held at a temperature of 710 to 720 ° C. for 30 seconds. did. After firing, cooling was performed to obtain each sample in which the aluminum electrode layer 8 serving as the back electrode was formed on the p-type silicon semiconductor substrate 1 as shown in FIG.

  The amount of warpage of the p-type silicon semiconductor substrate after the baking with the aluminum electrode layer formed is determined by pressing the ends of the four corners of the substrate as shown by arrows with the aluminum electrode layer facing upward as shown in FIG. Evaluation was made by measuring the amount of lift (including the thickness of the substrate) x at one end located at the diagonal. The lift amount x is shown in “Warpage (mm)” in Table 2.

  The surface resistance of the aluminum electrode layer was measured with a four-terminal surface resistance measuring instrument (RG-5 type sheet resistance measuring instrument manufactured by Napson). The measurement conditions were a voltage of 4 mV, a current of 100 mA, and a load applied to the surface of 200 grf (1.96 N). The measured value is shown in the electrode layer surface resistance (mΩ / □) in Table 1.

Thereafter, the p-type silicon semiconductor substrate on which the aluminum electrode layer is formed is immersed in an aqueous hydrochloric acid solution to dissolve and remove the aluminum electrode layer 8, and then the surface of the p-type silicon semiconductor substrate 1 on which the p ⁺ layer 7 is formed. Resistance was measured with a 4-terminal surface resistance measuring instrument. The measured value is shown in p ⁺ layer surface resistance (Ω / □) in Table 1.

表１の結果から、実施例は従来例に比べて、ｐ⁺層表面抵抗（Ω／□）を維持した上で、反りの量を低く押さえることができるとともに電極層表面抵抗（ｍΩ／□）も小さくできることがわかる。

From the results shown in Table 1, the example can maintain the p ⁺ layer surface resistance (Ω / □) and the electrode layer surface resistance (mΩ / □) while maintaining the p ⁺ layer surface resistance (Ω / □) lower than the conventional example. It can be seen that it can be made smaller.

  It should be considered that the embodiments and examples disclosed above are illustrative and non-restrictive in every respect. The scope of the present invention is shown not by the above embodiments and examples but by the scope of claims, and includes all modifications and variations within the scope and meaning equivalent to the scope of claims.

It is a figure which shows typically the general cross-section of the solar cell element to which this invention is applied as one embodiment. It is a figure which shows typically the method of measuring the curvature amount of the p-type silicon semiconductor substrate after baking which formed the aluminum electrode layer in the Example and the prior art example.

Explanation of symbols

1: p-type silicon semiconductor substrate, 2: n-type impurity layer, 3: antireflection film, 4: grid electrode, 5: aluminum sintered layer, 6: aluminum silicon mixed layer, 7: p ⁺ layer, 8: aluminum electrode layer.

Claims (11)

  A paste composition for forming an electrode on a p-type silicon semiconductor substrate, the paste composition comprising an aluminum powder, an organic vehicle, and a carbon powder.   The paste composition of claim 1 further comprising a glass frit.   The paste composition of Claim 1 or Claim 2 containing 0.1 mass% or more and 18 mass% or less of carbon powder.   The paste composition according to claim 1, comprising 60% by mass to 75% by mass of aluminum powder, 20% by mass to 35% by mass of organic vehicle, and 0.1% by mass to 18% by mass of carbon powder.   60% by mass to 75% by mass of aluminum powder, 20% by mass to 30% by mass of organic vehicle, 0.1% by mass to 18% by mass of carbon powder, and 5.0% by mass or less of glass frit Item 3. The paste composition according to Item 2.   The paste composition according to any one of claims 1 to 5, wherein the carbon powder is carbon black.   The paste composition according to any one of claims 1 to 6, wherein an average particle diameter of the carbon powder is 10 µm or less.   The paste composition according to any one of claims 1 to 6, wherein an average particle size of the carbon powder is 0.5 µm or less.   The paste composition of Claim 8 which contains the said carbon powder 0.3 mass% or more and 6 mass% or less.   The solar cell element provided with the electrode formed by apply | coating the paste composition of any one of Claim 1- Claim 9 on a p-type silicon-semiconductor substrate, and baking.   A solar cell element comprising an aluminum electrode layer, wherein the aluminum electrode layer contains carbon particles.

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