WO2007032151A1 - Aluminum paste composition and solar cell element making use of the same - Google Patents

Abstract

An aluminum paste composition that is capable of inhibiting the occurrence of blister and aluminum beads on a backside electrode layer at firing; and a solar cell element fitted with an electrode formed using the composition. There is provided an aluminum paste composition for forming of electrode (8) on silicon semiconductor substrate (1), comprising powdery aluminum, an organic vehicle and a glass frit wherein the glass frit contains an alkaline earth metal oxide. Further, there is provided a solar cell element fitted with electrode (8) formed by applying the above characterized paste composition onto silicon semiconductor substrate (1) and firing the same.

Description

 Specification

 Aluminum 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, an aluminum paste composition containing aluminum powder, which is used when forming an electrode on a silicon semiconductor substrate constituting a crystalline silicon solar cell, and the aluminum paste composition. It relates to a solar cell element.

 Background art

 [0002] As electronic components having electrodes formed on a silicon semiconductor substrate, as disclosed in JP 2000-90734 A (Patent Document 1), JP 2004-134775 A (Patent Document 2) Solar cell elements are known.

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

As shown in FIG. 1, the solar cell element is configured using a p-type silicon semiconductor substrate 1 having a thickness of 220 to 300 / ζ πι. On the light-receiving surface side of the silicon semiconductor substrate 1, a η-type impurity layer 2 having a thickness of 0.3 to 0.6 / zm, and an antireflection film 3 and a grid electrode 4 are formed thereon.

An aluminum electrode layer 5 is formed on the back side of the ρ-type silicon semiconductor substrate 1. The aluminum electrode layer 5 is formed by applying an aluminum powder, glass frit, and an aluminum paste composition that also has an organic vehicle force by screen printing or the like, drying it, and firing it at a temperature of 660 ° C (melting point of aluminum) or higher for a short time. It is formed by. During this firing, the Al—Si alloy layer 6 is formed between the aluminum electrode layer 5 and the p-type silicon semiconductor substrate 1 by diffusing into the aluminum-powered silicon semiconductor substrate 1 at the same time, A P + layer 7 is formed as an impurity layer by diffusion of aluminum atoms. Due to the presence of the p + layer 7, a back surface field (BSF) effect that prevents recombination of electrons and improves the collection efficiency of generated carriers can be obtained.

[0006] For example, as disclosed in JP-A-5-129640 (Patent Document 3), aluminum is used. A solar cell element in which the back electrode 8 composed of the -um electrode layer 5 and the Al—Si alloy layer 6 is removed with acid or the like and a collector electrode layer is newly formed with silver paste or the like has been put into practical use. . However, it is necessary to dispose of the acid used for removing the back electrode 8, and there is a problem that the process becomes complicated due to the removal process. In order to avoid such a problem, recently, it is often used to constitute a solar cell element by using the collector electrode as it is, leaving the back electrode 8.

、て裏面電極層にブリスターやアルミニウムの玉 が発生しやくなる。このため、太陽電池の製造工程でシリコン半導体基板の割れ等が 発生し、その結果、太陽電池の製造歩留まりが低下するという問題があった。 Recently, in order to reduce the cost of solar cells, it has been studied to reduce the thickness of the silicon semiconductor substrate. However, if the silicon semiconductor substrate is thinned, the silicon semiconductor substrate is formed so that the back surface side on which the back electrode layer is formed becomes concave after firing of the aluminum paste composition due to the difference in thermal expansion coefficient between silicon and aluminum. The substrate deforms and warps. In order to suppress the occurrence of warping, there is a method of reducing the coating amount of the aluminum paste composition and thinning the back electrode layer. While reducing the amount of aluminum paste composition applied,

In addition, blisters and aluminum balls are likely to occur on the back electrode layer. For this reason, there was a problem that the silicon semiconductor substrate was cracked in the manufacturing process of the solar cell, and as a result, the manufacturing yield of the solar cell was lowered.

[0008] Various aluminum paste compositions have been proposed as methods for solving these problems.

 [0009] Japanese Unexamined Patent Application Publication No. 2004-134775 (Patent Document 2) discloses that an aluminum powder, a glass frit, and an organic substance are used as a conductive paste capable of suppressing warpage of a Si wafer. In addition to the vehicle, it is disclosed that the organic vehicle contains particles that are hardly soluble or insoluble, and the particles are at least one of organic compound particles and carbon particles.

 JP-A-2005-191107 (Patent Document 4) discloses a high-performance back electrode in which formation of aluminum balls and protrusions and swelling of the electrode are suppressed in the back electrode. A method of manufacturing a solar cell element having high productivity with reduced warpage of a conductive substrate is disclosed, and an average particle size D force ½ to cumulative particle size distribution based on volume is used as an aluminum paste used in the manufacturing method. 20 m and less than half the average particle size D

 50 50

Including aluminum powder whose diameter accounts for 15% or less of the total particle size distribution. Mu is disclosed.

 [0011] Furthermore, Japanese Patent Laid-Open No. 2000-90733 (Patent Document 5) allows an A1-Si eutectic structure layer to be uniformly formed at the interface between a back electrode and a p-type Si semiconductor substrate without any gaps. As a conductive paste capable of improving the conversion efficiency of the solar battery, it contains A1 powder, glass frit and bikunole, and the glass frit contains BiO: 30 to 70 mol%, BO: 20 ~

 2 3 2 3

Those containing 60 mol% and SiO: 10 to 50 mol% are disclosed.

 2

 [0012] However, even when these aluminum pastes are used, it has not been possible to sufficiently suppress the generation of blisters or aluminum balls in the back electrode layer during firing.

 Patent Document 1: Japanese Unexamined Patent Publication No. 2000-90734

 Patent Document 2: JP 2004-134775 A

 Patent Document 3: Japanese Patent Laid-Open No. 5-129640

 Patent Document 4: JP-A-2005-191107

 Patent Document 5: Japanese Unexamined Patent Publication No. 2000-90733

 Disclosure of the invention

 Problems to be solved by the invention

[0013] Therefore, an object of the present invention is to solve the above-described problem, and an aluminum paste composition capable of suppressing the generation of a prestar or aluminum ball in the back electrode layer during firing. An object of the present invention is to provide a solar cell element provided with an electrode formed using the composition.

 Means for solving the problem

 [0014] 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 an aluminum paste composition having a specific composition. It was. Based on this finding, the aluminum paste composition according to the present invention has the following characteristics.

[0015] An aluminum paste composition according to the present invention is a paste composition for forming an electrode on a silicon semiconductor substrate, and includes aluminum powder, an organic vehicle, and glass frit. Contains alkaline earth metal oxides. [0016] Preferably, in the aluminum paste composition of the present invention, the alkaline earth metal oxide is a group that also includes magnesium oxide (MgO), calcium oxide (CaO), strontium oxide (SrO), and barium oxide (BaO). It is at least one selected from more.

 [0017] Preferably, in the aluminum paste composition of the present invention, the glass frit includes boron oxide (B 2 O 3), zinc oxide (ZnO), bismuth oxide (Bi 2 O 3), silicon oxide (Si

 2 3 2 3

 O), aluminum oxide (Al 2 O 3), oxidized soot (SnO), zirconium oxide (ZrO 2), oxidized oxide

2 2 3 2 Lom (Cr 2 O 3), lithium oxide (Li 0), acid sodium (Na 2 O) and acid potassium (K 2 O

2 3 2 2 2

) At least one selected from the group consisting of:

[0018] Further preferably, in the aluminum paste composition of the present invention, the content of the alkaline earth metal oxide is 5% by mass or more and 75% by mass or less in the glass frit.

 [0019] In the aluminum paste composition of the present invention, the glass frit content is preferably 0.1% by mass or more and 8% by mass or less in the paste composition.

 [0020] A solar cell element according to the present invention includes an electrode formed by applying an aluminum paste composition having any of the above-described characteristics onto a silicon semiconductor substrate and then firing the composition.

 The invention's effect

 [0021] As described above, according to the present invention, an aluminum electrode formed on the back surface of a silicon semiconductor substrate by using an aluminum paste composition containing a glass frit containing an alkaline earth metal oxide. Generation of blisters and aluminum balls in the layer can be suppressed, and the production yield of solar cell elements can be improved.

 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.

 Explanation of symbols

[0023] l: p-type silicon semiconductor substrate, 2: n-type impurity layer, 3: antireflection film, 4: grid electrode, 5: aluminum electrode layer, 6: A1-Si alloy layer, 7: p + layer, 8: Back electrode. BEST MODE FOR CARRYING OUT THE INVENTION

 [0024] The aluminum paste composition of the present invention is characterized in that it contains glass frit in addition to aluminum powder and an organic vehicle, and the glass frit contains an alkaline earth metal oxide. Glass frit is said to have an effect of assisting the reaction between aluminum and silicon and sintering of the aluminum powder itself. However, when glass frit with a conventional composition is used, the amount of Al—Si alloy produced locally increases due to the reaction between aluminum and silicon, and the generation of blisters and aluminum balls increased. The glass frit included in the aluminum paste used to form the back electrode of solar cells is PbO— B as the main component.

 2

O-SiO series, PbO- B O -Al O series, PbO- B O-ZnO series, Bi O- B O-Si

3 2 2 3 2 3 2 3 2 3 2 3

Those containing oxides such as O-based and Bi 2 O—B 2 O—ZnO are known.

 2 2 3 2 3

 [0025] A conventional glass frit mainly composed of PbO and BiO is made of an aluminum paste during firing.

 twenty three

 Promotes the reaction between the aluminum powder inside and the silicon of the substrate. However, in some cases, due to the promoting action, the reaction between aluminum and silicon proceeds rapidly, the amount of A1-Si alloy produced increases, and blisters and aluminum balls are likely to be generated. In the present invention, the reaction between aluminum and silicon can be controlled so as not to proceed excessively by including glass frit containing gallium earth metal oxide in the aluminum paste. Thereby, it is thought that generation | occurrence | production of a blister or an aluminum ball can be suppressed. In general, MgO, CaO, SrO or BaO can be used as the argali earth metal oxide.

 [0026] The glass frit of the present invention contains at least one kind of argali earth metal oxide as an essential component, but as an oxide for constituting a glass having predetermined characteristics, BO, ZnO

 twenty three

, Bi O, SiO, Al O, SnO, ZrO, Cr O, Li 0, Na O or K O at least 1

2 3 2 2 3 2 2 3 2 2 2

 It can be used with more than seeds.

[0027] The content of the argali earth metal oxide in the glass frit of the present invention is not particularly limited, but is preferably 5% by mass or more and 75% by mass or less, more preferably 10% by mass or more. 65% by mass or less. If the content of the argali earth metal acid is less than 5% by mass, the prescribed blister or aluminum ball suppression effect may not be obtained. In addition, if the content of argali earth metal oxide exceeds 75% by mass, There is a risk of exceeding the enclosure, making it difficult to produce glass. The glass frit of the present invention can be used by further containing other oxides and compounds as trace components, if necessary. The content of oxides and compounds as trace components is generally 5% by mass or less.

[0028] The method for producing the glass frit of the present invention is not particularly limited, and various raw materials are prepared, melted, made glassy, and so on so as to have a predetermined glass frit composition by a known glass manufacturing method. The predetermined glass frit is obtained by pulverization, drying and classification.

[0029] If an aluminum paste composition containing a glass frit containing an alkaline earth metal oxide of the present invention is used, blisters or aluminum balls are formed on the aluminum electrode layer formed on the back surface of the silicon semiconductor substrate. Generation | occurrence | production can be suppressed.

[0030] The content of the glass frit in the aluminum paste composition of the present invention is not particularly limited, but is preferably 8% by mass or less. If the glass frit content exceeds 8% by mass, glass prayers occur, the resistance of the aluminum electrode layer increases, and the power generation efficiency of the solar cell may be reduced. The lower limit of the glass frit content is not particularly limited. 1S is usually 0.1% by mass or more. If the lower limit of the glass frit content is less than 0.1% by mass, the reaction between aluminum and silicon becomes insufficient, and the BSF effect may not be sufficiently obtained.

 [0031] The average particle size of the glass frit particles contained in the aluminum paste composition of the present invention is not particularly limited, but is preferably 20 µm or less.

 [0032] The content of the aluminum powder included in the aluminum paste composition of the present invention is preferably 60% by mass or more and 80% by mass or less. If the content of the aluminum powder is less than 60% by mass, the resistance of the aluminum electrode layer after firing becomes high, which may cause a decrease in the energy conversion efficiency of the solar cell. If the aluminum powder content exceeds 80% by mass, the applicability of the aluminum paste in screen printing and the like will be reduced.

In the present invention, a wide range of aluminum powder having an average particle diameter of 1 to 20 μm can be used. When blended in an aluminum paste composition, it is preferably 2 to 15 ^ m, more preferably Use 3 to 10 / ζ πι. If the average particle diameter is less than 1, the specific surface area of the aluminum powder becomes large, which is not preferable. When the average particle diameter exceeds 20 m, the aluminum paste composition is formed by adding aluminum powder. In this case, an appropriate viscosity cannot be obtained. Moreover, the aluminum powder included in the aluminum paste composition of the present invention is not particularly limited to the shape of the powder and the method for producing the powder.

[0034] The components of the organic vehicle included in the aluminum paste composition of the present invention are not particularly limited, but resins such as ethyl cellulose and alkyd, and solvents such as glycol ethers and terbinols can be used. The content of the organic vehicle is preferably 20% by mass or more and 40% by mass or less. If the content of the organic vehicle is less than 20 mass _^0/0, reduced printing of the aluminum paste, it is impossible to form a satisfactory aluminum electrode layer. On the other hand, when the content of the organic vehicle exceeds 40% by mass, there arises a problem that the baking of aluminum is hindered by the presence of excess organic vehicle as well as the viscosity of the aluminum paste being increased.

 [0035] The aluminum paste composition of the present invention can be used by containing various additives such as a dispersant, a plasticizer, an anti-settling agent, and a thixotropic agent that adjust the properties of the aluminum paste as necessary. . The composition of the additive is not particularly limited, but the content is preferably 10% by mass or less.

 Example

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

[0037] First, the aluminum powder 60-80 wt _^0/0, with including organic vehicle prepared by dissolving E chill cellulose glycol ether organic solvent in the range of 20 to 40 wt%, Table

Aluminum paste compositions to which glass frit containing various alkaline earth metal oxides were added at the ratio shown in 1 were prepared.

 [0038] Specifically, an organic vehicle in which ethyl cellulose is dissolved in a glycol ether organic solvent, aluminum powder and glass frit containing various alkaline earth metal oxides in the addition amounts shown in Table 1. Aluminum paste compositions (Examples 1 to 7) were prepared by mixing with a known mixer. Further, in the same manner as described above, aluminum paste compositions (Comparative Examples 1 to 5) containing conventional glass frit without alkaline earth metal oxides were prepared as shown in Table 1. .

Here, the aluminum powder ensures the reactivity with the silicon semiconductor substrate, the coating property, and the From the viewpoint of the uniformity of the coating film, a powder composed of particles having an average particle diameter of 2 to 20 / ζ πι or a shape close to a sphere was used. Glass frit having an average particle diameter of 1 to 12 m was used.

 [0040] The various aluminum paste compositions described above have a thickness of 220 μm and a size of 155 mm.

 The coating was printed on a 155 mm p-type silicon semiconductor substrate using a 165 mesh screen printing plate and dried. The coating amount was set to 1.5 ± 0.lgZ before drying.

[0041] The p-type silicon semiconductor substrate on which the aluminum paste was printed was dried and then fired in an air atmosphere in an infrared continuous firing furnace. The temperature of the firing zone of the firing furnace was set to 760 to 780 ° C, and the residence time (firing time) of the substrate was set to 8 to 12 seconds. After firing, cooling was performed to obtain a structure in which an aluminum electrode layer 5 and an Al—Si alloy layer 6 were formed on a p-type silicon semiconductor substrate 1 as shown in FIG.

[0042] In the aluminum electrode layer 5 formed on the silicon semiconductor substrate, the amount of blisters and aluminum balls generated per measured surface area 150 X 150mm ² of the aluminum electrode layer 5 was visually counted, and the total value is shown in Table 1. . In order to prevent the silicon semiconductor substrate from cracking during the manufacturing process, the target value of the amount of blister and aluminum balls is set to 5.

 [0043] The surface resistance of the back electrode layer that affects the ohmic resistance between the electrodes was measured with a four-probe surface resistance measuring instrument.

 [0044] Thereafter, the p-type silicon semiconductor substrate on which the back electrode layer was formed was immersed in an aqueous hydrochloric acid solution to dissolve and remove the aluminum electrode layer 5 and the Al-Si alloy layer 6, and the p + layer 7 was formed. The surface resistance of the type silicon semiconductor substrate was measured with the above surface resistance measuring instrument.

 [0045] There is a correlation between the surface resistance of the p + layer 7 and the BSF effect, and the smaller the surface resistance, the higher the BSF effect. Here, the target surface resistance value is 18 mΩ or less for the back electrode 8 and 16 Ω or less for the ρ + layer 7.

 [0046] Table 1 shows the surface resistance of the back electrode 8 and the surface resistance of the p + layer 7 measured as described above.

[0047] [Table 1] Glass frit Glass blister Back electrode Si substrate Main component Al force re-frit 1. A1 ball surface resistance p + layer

(Underlined: Earth metal addition amount Amount generated [τη Ω / α] Surface low resistance alkali earth metal oxide) Oxide [mass% 閥 Ω [Ω / D] Content

[Mass ^_0/0]

1 BaO + Bi ₂ 0 ₃ + B ₂ Os + Si02 3 2.0 6 15.5 14.3

2 CaO + Si0 ₂ + Al20 ₃ + ZnO 7 2.0 4 15.1 14.1 Application 3 MeO + SiOs + AlsOa 15 2.0 3 15.8 14.0 Example 4 BaO + CaO + B ₂ 0 ₃ + Si02 45 0.05 5 13.2 16.9

5 BaO + CaO + B _¾ O _{fi +} Si0 ₂ 45 0.2 3 13.5 13.9

6 BaO + CaO + B ₂ 0 ₃ + Si02 45 2.0 0 15.0 13.2

7 BaO + CaO + B¾0.s + SiOs 45 5.0 1 15.3 12.9

8 BaO + CaO + B ₂ Os + Si02 45 7.0 1 16.2 12.7

9 BaO + CaO + B20 ₃₊ Si02 45 10.0 1 18.7 12.3

10 BaO + Si0 ₂ + Al ₂ 0 ₃ 65 2.0 2 15.6 13.7 Ratio 1 PbO + B ₂ 03 + Si0 ₂ 0.2 15 14.9 16.5 Comparison 2 Bi203 + B ₂ 0 ₃ + Si02 2.0 17 15.5 14.3 Example 3 ZnO + B ₂ 0 ₃ + Si02 2.0 20 16.5 14.5

[0048] From the results shown in Table 1, the alkaline earth metal of the present invention was compared with the aluminum paste composition (Comparative Examples 1 to 5) supplemented with a conventional glass frit containing no alkaline earth metal oxides. By using an aluminum paste composition (Examples 1 to 7) using a glass frit containing a metal oxide, the electrode function of the aluminum electrode layer and the BSF effect are not lowered, and the blister in the aluminum electrode layer It can be seen that generation of aluminum balls and aluminum can be suppressed.

 [0049] The embodiments and examples disclosed above are to be considered in all respects as illustrative and not restrictive. The scope of the present invention is defined by the scope of the claims, rather than the embodiments and examples described above, and is intended to include all modifications and variations that are equivalent in scope and scope to the claims.

 Industrial applicability

[0050] According to the present invention, an aluminum paste composition containing glass frit containing an alkaline earth metal oxide is used to form on the back surface of the silicon semiconductor substrate. Generation | occurrence | production of a blister or an aluminum ball can be suppressed in an aluminum electrode layer, and the manufacturing yield of a solar cell element can be improved.

Claims

The scope of the claims  [1] A paste composition for forming an electrode (8) on a silicon semiconductor substrate (1), An aluminum paste composition comprising aluminum powder, an organic vehicle and glass frit, wherein the glass frit comprises an alkaline earth metal oxide. [2] The aluminum paste according to claim 1, wherein the alkaline earth metal oxide is at least one selected from the group consisting of magnesium oxide, calcium oxide, strontium oxide, and barium oxide power. Composition. [3] The glass frit is made of boron oxide, zinc oxide, bismuth oxide, silicon oxide, acid aluminum, tin oxide, acid zirconium, acid chromium, lithium oxide, sodium oxide, 2. The aluminum paste composition according to claim 1, comprising at least one selected from the group consisting of:  [4] The aluminum paste composition according to claim 1, wherein the content of the alkaline earth metal oxide is 5% by mass to 75% by mass in the glass frit.  [5] The aluminum paste composition according to claim 1, wherein a content of the glass frit is 0.1% by mass or more and 8% by mass or less in the paste composition.  [6] A solar cell element comprising an electrode (8) formed by applying the aluminum paste composition according to claim 1 on a silicon semiconductor substrate (1) and then baking the composition.