WO2007004281A1 - Paste composition and solar cell element using same - Google Patents

Abstract

A paste composition which can prevent warpage or breakage from being generated even when a thinner silicon semiconductor substrate is used and can achieve high BSF effects and a high energy conversion efficiency, and a solar cell element provided with an electrode formed by using such composition are provided. The paste composition is provided for forming an aluminum electrode layer (8) on a p-type silicon semiconductor substrate (1) and includes an aluminum powder, organic vehicle and a carbon powder. The solar cell element is provided with the aluminum electrode layer (8), which is formed by applying the paste composition having the above mentioned features on the p-type silicon semiconductor substrate (1) and then burning the paste. The aluminum electrode layer (8) includes carbon particles.

Description

 Specification

 Paste composition and solar cell element using the same

 Technical field

 [0001] The present invention generally relates to a paste composition and a solar cell element using the same.

Specifically, the present invention relates to a paste composition used for forming a back surface aluminum electrode on a p-type silicon semiconductor substrate constituting a crystalline silicon solar cell, and a solar cell element using the paste composition.

 Background art

 [0002] Solar cells are desired to be used in a wider range as clean energy that is safe and has a low environmental impact. In particular, it is necessary to promote light weight and low cost in order to popularize solar cells that use silicon semiconductor substrates to form elements. In response to this demand, research and development have been repeated in order to reduce the thickness of solar cell elements.

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

 [0004] As shown in FIG. 1, an n-type impurity layer 2 having a thickness of 0.3 to 0.5 m is provided on the light-receiving surface side of a ρ-type silicon semiconductor substrate 1 having a thickness of 00 to 600 μm. A film 3 and a grid electrode 4 are formed in this order.

[0005] 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, an aluminum sintered layer 5 and an aluminum silicon mixed layer 6 are formed, and at the same time, aluminum is contained in the p-type silicon semiconductor substrate 1. Diffuses to form a p + layer (or p ⁺⁺ layer) 7. The presence of the p + layer 7 provides a so-called BSF (Back Surface Field) effect, and can improve the collection efficiency of carriers generated in the p-type semiconductor substrate 1. That is, out of the minority carriers generated in the p-type silicon semiconductor substrate 1, carriers that are directed to 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 light is emitted from the surface electrode. It can be effectively collected as current, and as a result, photovoltaic power and photocurrent can be increased and conversion efficiency can be increased. [0006] Application of an aluminum-containing paste for forming an aluminum electrode layer is generally performed using a screen printing method, and the firing is generally performed in an acidic atmosphere. In order to obtain the 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 / ζ πι and baked, the conversion efficiency without sufficient BSF effect cannot be improved.

 [0007] However, simultaneously with the formation of the ρ + layer that brings about the BSF effect, the aluminum electrode layer is formed to be thicker than necessary. Therefore, if the thickness of the silicon semiconductor substrate is reduced to 300 m 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, internal stress is generated due to the difference in thermal expansion coefficient between the aluminum or aluminum silicon mixture and silicon. There has been a problem that the semiconductor substrate is warped or cracked.

 [0008] Therefore, the conventional method for manufacturing a solar cell element has 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.

 [0009] In order to solve such a problem, several methods have been proposed.

[0010] A method of etching a part of the surface of the back electrode in the thickness direction after forming the back electrode on the semiconductor substrate has been proposed in, for example, Japanese Unexamined Patent Application Publication No. 2002-353476 (Patent Document 1). However, this method has a problem in that the manufacturing process is complicated and warpage or cracking during firing cannot be prevented.

[0011] Also, two or more types of aluminum-containing paste are applied to the entire back surface of the semiconductor substrate by first thinly applying it and then applying the aluminum-containing paste again to the portion to be thickened, followed by firing. For example, Japanese Patent Laid-Open No. 2002-217435 (Patent Document 2) proposes a method of forming a back electrode with a thickness of 10 mm. For example, Japanese Patent Laid-Open No. 2002-141533 (Patent Document 3) proposes a method of forming backside electrodes in a lattice pattern on the backside of a semiconductor substrate. A method of forming a back electrode on the back surface of a semiconductor substrate in a strip shape having a different thickness is proposed, for example, in Japanese Patent Laid-Open No. 2002-141534 (Patent Document 4). It has been proposed. A method of forming a back electrode in a rectangular shape on the back surface of a semiconductor substrate is, for example,

JP-A-2002-141546 (Patent Document 5). However, the p + layer that produces the BSF effect formed by any of these methods becomes non-uniform, resulting in a decrease in conversion efficiency.

[0012] Furthermore, as a composition of a conductive paste capable of thinning the 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. JP 2000-90734 (Patent Document 6). However, by using this conductive paste, the back electrode layer is thinned by reducing the coating amount while ensuring certain solar cell characteristics, and the amount of warpage generated in the p-type silicon semiconductor substrate is reduced. However, the amount of warpage cannot be reduced without reducing the amount of conductive paste applied in order to obtain a sufficient BSF effect.

[0013] Therefore, the coefficient of thermal expansion is smaller than that of aluminum, and the melting temperature, softening temperature and decomposition temperature are higher than the melting point of aluminum, and include an inorganic compound, aluminum powder, and organic vehicle. Paste composition strength For example, it has been proposed in JP 2003-223813 A (Patent Document 7). However, when the amount of the inorganic compound added is increased, the occurrence of warpage or cracking during firing can be further suppressed, but the surface resistance of the aluminum electrode layer is increased and the ohmic resistance between the electrodes is increased. There was a problem that the energy generated by the irradiation of the material could not be extracted effectively, leading to a decrease in energy conversion efficiency.

 Patent Document 1: Japanese Patent Laid-Open No. 2002-353476

 Patent Document 2: JP 2002-217435 A

 Patent Document 3: Japanese Patent Laid-Open No. 2002-141533

 Patent Document 4: Japanese Patent Laid-Open No. 2002-141534

 Patent Document 5: JP 2002-141546 A

 Patent Document 6: Japanese Unexamined Patent Publication No. 2000-90734

 Patent Document 7: Japanese Patent Laid-Open No. 2003-223813

 Disclosure of the invention

Problems to be solved by the invention [0014] Therefore, an object of the present invention is to solve the above-mentioned problem, and even if a thinner silicon semiconductor substrate is used, warpage or cracking can be prevented, and high, A paste composition capable of achieving a BSF effect, high energy conversion efficiency, and a solar cell element including an electrode formed using the composition.

 Means for solving the problem

[0015] 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.

 [0016] 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 an aluminum powder, an organic vehicle, and a carbon powder.

 [0017] Preferably, the paste composition of the present invention further includes glass frit.

 [0018] Preferably, the paste composition of the present invention includes 0.1% by mass or more and 18% by mass or less of carbon powder.

 More preferably, in the paste composition of the present invention, the aluminum powder is 60% by mass or more and 75% by mass or less, the organic vehicle is 20% by mass or more and 35% by mass or less, and the carbon powder is 0.1% by mass or more 18% by mass. Including mass% or less.

 [0020] When the glass frit is included, preferably, the paste composition of the present invention contains aluminum powder in an amount of 60% by mass to 75% by mass, an organic vehicle in an amount of 20% by mass to 30% by mass, and a carbon powder of 0. 1% to 18% by mass and glass frit to 5.0% by mass or less.

 [0021] In the paste composition of the present invention, the carbon powder is preferably carbon black.

 [0022] Further, in the paste yarn according to the present invention, the average particle size of the carbon powder is preferably 10 μm or less.

[0023] Further, in the paste yarn according to the present invention, the average particle size of the carbon powder is 0.5. It is preferable that it is below μm.

[0024] In the paste yarn composition according to the present invention, when the average particle size of the carbon powder is 0.5 µm or less, the carbon powder is preferably contained in an amount of 0.3 mass% to 6 mass%.

[0025] A solar cell element according to the present invention includes an electrode formed by applying a paste thread and composition having the above-described characteristics onto a p-type silicon semiconductor substrate and then firing it.

[0026] 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.

 The invention's effect

 As described above, according to the present invention, when a p-type silicon semiconductor substrate coated with a paste composition containing carbon powder is baked to form an electrode, the p-type silicon semiconductor substrate is warped or cracked. In addition to being able to prevent generation, a solar cell element having an electrode formed using the composition can achieve a high BSF effect and a high energy conversion efficiency.

 Brief Description of Drawings

 FIG. 1 is a diagram schematically showing a general cross-sectional structure of a solar cell element to which the present invention is applied as one embodiment.

 FIG. 2 is a diagram schematically showing a method of measuring a warpage amount of a p-type silicon semiconductor substrate after firing in which an aluminum electrode layer is formed in Examples and Conventional Examples.

 Explanation of symbols

 [0029] l: 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

BEST MODE FOR CARRYING OUT THE INVENTION

[0030] The paste composition of the present invention is characterized by further containing carbon powder in addition to the aluminum powder and the organic vehicle. The thermal expansion coefficient of carbon powder varies depending on the crystal structure and the like, but is approximately 0.5 to: LO X 10 ^_6 / ° C, which is smaller than the aluminum thermal expansion coefficient 23.5 X 10 ^_6 Z ° C. The electrical conductivity of the carbon powder is determined by the inorganic compound powder or the key. Larger than lumium-containing organic compound powder.

 [0031] By including such carbon powder in the paste yarn and the composition, the difference in thermal expansion coefficient between the carbon powder and silicon is relatively small. Therefore, the p-type after applying and baking the paste Deformation of the silicon semiconductor can be suppressed. In addition, 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.

 [0032] Conventionally, in order to suppress deformation of the p-type silicon semiconductor after firing, certain solar cell characteristics can be obtained by reducing the thickness of the paste coating film and using a paste added with an aluminum-containing organic compound powder. In addition to reducing the coating thickness of the paste and reducing the thickness of the back electrode layer while securing the thickness, or using a paste to which inorganic compound powder was added, there was no effective means.

 [0033] If the coating thickness of the paste is reduced, the amount of diffusion of aluminum from the surface of the p-type silicon semiconductor substrate becomes insufficient, and a sufficient BSF effect cannot be obtained, resulting in a solar cell element. The characteristics of the decrease.

 [0034] 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 is increased, the energy conversion efficiency is lowered, and as a result, the characteristics of the solar cell element are lowered.

 However, in the present 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 baking can be suppressed without reducing the paste coating thickness, a sufficient BSF effect can be obtained.

 [0036] 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. .

[0037] The content of the carbon powder included in the paste yarn according to the present invention is preferably 0.1% by mass or more and 18% by mass. If the content of the carbon powder is less than 0.1% by mass, it is not possible to obtain a sufficient addition effect to suppress deformation of the p-type silicon semiconductor substrate after firing. If the content of the carbon powder exceeds 18 mass _^0/0, the application of the paste in screen printing or the like is reduced. [0038] 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, and even more preferably 0.5 m or less. If the average particle size of the carbon powder exceeds 10 m, the number of carbon particles present in the aluminum electrode layer formed when the paste is baked is reduced, so that deformation of the p-type silicon semiconductor substrate after baking is suppressed. The sufficient addition effect cannot be obtained.

 [0039] In order to reduce the deformation (warpage) 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 not more than 0.3 and the content of the carbon powder is 0.3 mass. % To 6% by mass. By doing so, fine carbon powder is effectively dispersed and contained in the aluminum electrode layer.

[0040] 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 in the average particle size exceeds 20000 m ² / g, the viscosity of the paste increases, and the paste does not have a good effect during screen printing. The applicability decreases.

 [0041] The content of the aluminum powder included in the paste composition of the present invention is preferably 60% by mass or more and 75% by mass or less. If the content of aluminum powder is less than 60% by mass, the surface resistance of the aluminum electrode layer after firing increases, which may lead to a decrease in energy conversion efficiency. If the aluminum powder content exceeds 75% by mass, the applicability of the paste during screen printing will decrease.

 [0042] As the organic vehicle included in the paste composition of the present invention, one obtained by dissolving ethyl cellulose, acrylic resin, alkyd resin, etc. 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 decreases.

[0043] Furthermore, 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, BSF effect and energy conversion efficiency of p-type silicon semiconductor, but is added to improve the adhesion between the sintered aluminum electrode layer and p-type silicon semiconductor substrate. It is a thing. If the content of glass frit exceeds 5.0% by mass, there is a risk of glass praying. [0044] Examples of the glass frit included in the paste composition of the present invention include SiO-BiO-PbO.

 In addition to 2 2 3 system, B O -SiO- Bi O system, B O-SiO- ZnO system, B 2 O-SiO-PbO

 Examples include the 2 3 2 2 3 2 3 2 2 3 2 system.

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

 Example

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

[0047] First, the aluminum powder 60-75 wt _^0/0, the glass frit 0.3 to 5.0 mass _^0/0, with containing in the range of organic vehicle 20-30 wt%, a carbon powder Various paste compositions containing the proportions shown in Table 1 were prepared.

 [0048] Specifically, an organic vehicle in which ethyl cellulose is dissolved in a glycol ether organic solvent is coated with aluminum powder and a B 2 O—SiO—PbO glass frit.

 2 3 2

 Various carbon powders shown in Table 1 were added to and mixed with a known mixer to obtain a paste composition.

 Here, the aluminum powder has a spherical shape with an average particle diameter of 2 to 20 / ζ πι, or a spherical shape from the viewpoint of ensuring reactivity with the p-type silicon semiconductor substrate, coating properties, and uniformity of the coating film. A powder composed of particles having a close shape was used.

 [0050] The above-mentioned various paste compositions are screen-printed on a ρ-type silicon semiconductor substrate having a size of 2 inches (50. 8 mm) x 2 inches (50.8 mm) and a thickness of 280 μm. The plate was applied and printed. The coating amount was set so that the thickness force of the aluminum electrode layer after firing was 0 to 50 μm.

 [0051] After drying the p-type silicon semiconductor substrate on which the paste has been printed, it is heated in an infrared firing furnace at a temperature increase rate of 400 ° CZ in an air atmosphere and maintained at a temperature of 710 to 720 ° C for 30 seconds. Firing was carried out under the conditions. After firing, cooling was performed to obtain each sample in which an aluminum electrode layer 8 serving as a back electrode was formed on a p-type silicon semiconductor substrate 1 as shown in FIG.

[0052] The amount of warpage of the p-type silicon semiconductor substrate after the baking with the aluminum electrode layer formed is indicated by arrows at one end of the four corners of the substrate with the aluminum electrode layer facing upward as shown in FIG. Evaluation was performed by measuring the amount of lifting (including the thickness of the substrate) X of one end located at the opposite corner while holding the substrate as shown. The amount of lift X is shown in “Warpage (mm)” in Table 2.

[0053] The surface resistance of the aluminum electrode layer was measured with a 4-terminal surface resistance measuring device (RG-5 type sheet resistance measuring device manufactured by Nabson). The measurement conditions were a voltage of 4 mV, a current of 100 mA, and a load applied to the surface of 200 grf (l. 96N). The measured values are shown in the electrode layer surface resistance (πιΩ inlet) in Table 1.

 [0054] Thereafter, the ρ-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 ρ-type silicon on which the ρ + layer 7 is formed. The surface resistance of the semiconductor substrate 1 was measured with a 4-terminal surface resistance measuring instrument. The measured values are shown in Table 1 as Ρ + layer surface resistance (Ω port).

[0055] [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（ΩΖ口）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 (πιΩ /口）も小さく できることがわ力 。

From the results shown in Table 1, compared to the conventional example, the example can maintain the p-layer surface resistance (Ω port), while keeping the amount of warpage low, and the electrode layer surface resistance (πιΩ / port) is also small. Power that can be done.

The embodiments and examples disclosed above are illustrative and not restrictive in all respects. It should be considered not. The scope of the present invention is defined by the scope of the claims rather than the embodiments and examples described above, and includes all modifications and variations that are equivalent in scope and scope to the claims.

Industrial applicability

 The paste composition of the present invention can prevent warping or cracking of the p-type silicon semiconductor substrate even when applied on the p-type silicon semiconductor substrate and baked, and the composition In a solar cell element with electrodes formed using, high BSF effect, high V, and energy conversion efficiency can be achieved.

Claims

The scope of the claims  [I] A paste composition for forming an electrode (8) on a p-type silicon semiconductor substrate (1), the paste composition comprising an aluminum powder, an organic vehicle, and a carbon powder.  [2] The paste composition according to claim 1, further comprising glass frit. [3] The paste composition according to claim 1, comprising 0.1% by mass to 18% by mass of carbon powder.  [4] The aluminum powder according to claim 1, comprising 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. Paste composition.  [5] 60% to 75% by weight of aluminum powder, 20% to 30% by weight of organic vehicle, 0.1% to 18% by weight of carbon powder, and 5.0% by weight of glass frit The paste composition according to claim 2, comprising: 6. The paste composition according to claim 1, wherein the carbon powder is carbon black. [7] The paste composition according to claim 1, wherein an average particle size of the carbon powder is 10 μm or less.  [8] The paste yarn composition according to claim 1, wherein the carbon powder has an average particle size of 0.5 μm or less.  [9] The paste composition according to claim 8, comprising 0.3% by mass to 6% by mass of the carbon powder.  [10] An electrode (8) formed by applying a paste composition containing aluminum powder, an organic vehicle, and carbon powder on a ρ-type silicon semiconductor substrate (1) and then firing the paste composition, Solar cell element.  [I I] A solar cell element comprising an aluminum electrode layer (8), wherein the aluminum electrode layer (8) contains carbon particles.