JP2000114556A - Solar cell and method of manufacturing the same - Google Patents

Abstract

(57) [Summary] [PROBLEMS] To suppress the warpage of a wafer when a back electrode is manufactured. SOLUTION: On a back surface opposite to a light receiving surface of a wafer 1, a back electrode 3 formed by firing a metal paste is provided, and a plurality of slits are formed at positions where the metal paste is not applied. Notches 8 are provided in a distributed manner.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a crystalline solar cell provided with a back electrode.

[0002]

2. Description of the Related Art Conventionally, in a solar cell, in order to extract a current generated by irradiation of sunlight, FIG.
As shown in FIGS. 1 and 12, a surface electrode is formed on a front surface (light receiving surface) 1a of a silicon wafer 1 as a semiconductor substrate on which a pn junction is formed, and a back electrode 3 is formed on almost the entire back surface 1b of the wafer 1. are doing. The back electrode 3 is formed by applying and baking a metal paste such as aluminum (see, for example, JP-A-6-209115 and JP-A-9-172196).

[0003]

However, when the back electrode 3 is formed by firing the metal paste, there is a problem in that the wafer 1 is warped and the strength of the solar cell is reduced. As shown in FIG.
The wafer 1 has a central portion protruding from the light-receiving surface and is curved from the central portion to the periphery to have a bowl shape. This is because the coefficient of thermal expansion of aluminum is larger than the coefficient of thermal expansion of silicon, so when the paste-like aluminum is baked and solidified, the volume of the alloy portion of aluminum and silicon and the volume of aluminum are smaller than that of silicon. This occurs to cause so-called residual stress.

In some cases, a polycrystalline wafer is manufactured by an electromagnetic casting method in which a melt of silicon or the like is gradually cooled and solidified. When a wafer is manufactured by this electromagnetic casting method, a certain regularity is generated in the crystal grains of the wafer, and the growth directions of the crystals are aligned. When the back electrode is similarly formed using this wafer, the wafer may be warped.

FIG. 13 is a diagram showing the grain boundaries of each crystal on the wafer (see the dotted lines in FIG. 13). According to this figure,
The crystal growth direction is radial from one corner of the wafer 1 to the other. If the back electrode 3 is formed on the wafer 1, the wafer 1 bends and warps in a direction substantially perpendicular to the crystal growth direction near the center as shown in FIG. The wafer 1 shown in FIGS. 12 and 14 does not have surface electrodes and the like.

As described above, if the wafer is warped, the strength of the solar cell itself is reduced, and in the next step after the formation of the back surface electrode, the wafer is rattled when the wafer is transported, or the worst case may occur. In some cases, the wafer may be cracked.

[0007] In view of the above problems, an object of the present invention is to provide a solar cell capable of preventing wafer warpage during fabrication.

[0008]

Means for Solving the Problems According to the present invention, there is provided a semiconductor substrate having, on a surface thereof, an electrode formed by firing a conductive material, and a notch for alleviating residual stress in which no electrode is present. A plurality of notches are dispersedly arranged on the surface of the semiconductor substrate, and the notches comprise slit-shaped holes.

That is, the conductive material is made of a metal paste, the electrode is formed on the surface opposite to the light receiving surface, in the region where the metal paste is applied, and the cutout portion is formed in the region where the metal paste is not applied. Formed. Further, instead of the notch, a groove for alleviating residual stress may be formed on the surface of the electrode.

According to the above configuration, since the notch or the groove is provided, it is possible to suppress the warpage of the semiconductor substrate caused when the electrode is formed by firing the metal paste.

On the other hand, in a semiconductor substrate having a uniform crystal growth direction, the semiconductor substrate bends and warps in a direction substantially perpendicular to the crystal growth direction. In order to suppress the warpage of the semiconductor substrate, the notch may be formed along the crystal growth direction. For example, the crystal growth direction is recognized by image recognition processing or the like, and the crystal growth direction and the length of the notch are determined. The relative positional relationship between the direction of the semiconductor substrate and the direction in which the notch should be formed is adjusted so that the directions match.

[0012]

Embodiments of the present invention will be described below in detail with reference to the accompanying drawings.

<First Embodiment> FIG. 2 is a view showing a configuration of a solar cell according to a first embodiment of the present invention. The solar cell includes a polycrystalline wafer 1 as a semiconductor substrate, a front electrode 2 formed on the front surface of the wafer 1, and a back electrode 3 formed on the back surface of the wafer 1.

The wafer 1 is formed by solidifying p-type silicon, and an n ⁺ layer 4 is formed on the surface thereof to form a pn junction. Irregularities 5 are formed on the surface of n ⁺ layer 4 to promote the absorption of sunlight, and an antireflection film 6 is formed on the surface of irregularities 5 to prevent reflection of sunlight. Further, the p ⁺ layer 7 and the back electrode 3
Is formed.

The wafer 1 is manufactured by an electromagnetic casting method, a ribbon method, or the like. The electromagnetic casting method is a method in which a silicon melt is cooled in a crucible and solidified. The wafer 1 is manufactured by thinly slicing an ingot manufactured thereby. On the other hand, the ribbon method is a method in which crystalline silicon is pulled up through a capillary die set in a crucible containing a melt of silicon, and cut into an appropriate size with a laser or the like to produce a wafer 1.

The irregularities 5 are formed by performing dry etching on the wafer 1 using a chlorine gas such as chlorine trifluoride gas (ClF ₃ ). Alternatively, it is formed by performing wet etching using an alkaline aqueous solution such as a sodium hydroxide aqueous solution (NaOH). n
⁺ Layer 4 is made of phosphorus oxychloride (POC
l ₃ ) and the like, and is formed by performing impurity diffusion using an impurity diffusion source.

The antireflection film 6 is made of a silicon nitride film formed by a plasma CVD method using a mixed gas of silane and ammonia as a raw material, or a titanium oxide film formed by a normal pressure CVD method using a alkoxide titanate as a raw material. Become. The surface electrode 2 is formed by applying an Ag paste to the light receiving surface of the wafer 1 and baking it. At this time, the antireflection film 6 where the surface electrode 2 is to be formed is removed.

As shown in FIG. 1, a back electrode 3 and a plurality of notches 8 for relaxing residual stress are formed on the back surface of the wafer 1. The notch 8 is formed of a slit-like hole,
The cutouts 8 are distributed so that the longitudinal direction of the cutouts 8 is directed vertically or horizontally in the drawing. Further, the back surface electrode 3 is formed on almost the entire back surface of the wafer 1 except for the notch 8.

As a specific method for forming the back electrode 3 and the notch 8, a pattern of an aluminum metal paste in which only the notch 8 is removed is printed on the back of the wafer 1 by silk screen printing. Then, it is fired at about 700 ° C. using a belt firing furnace. Thereby, the back electrode 3 is formed in the region where the metal paste is applied,
The notch 8 is formed in a region where the metal paste is not applied.

Alternatively, a resist is dispersed and applied to a position on the back surface of the wafer 1 where the notch 8 is to be formed. Next, a metal paste is applied so as to wrap around the resist, and then fired. The resist is evaporated by heat during baking, and the portion where the resist is applied becomes the cutout 8. If the resist does not evaporate by baking, the resist may be removed by etching or the like.

The back electrode 3 may be formed by a plating method or a vacuum evaporation method. Further, p ⁺ layer 7 is formed by the thermal diffusion of aluminum inside the back surface of wafer 1 when back electrode 3 is formed.

As described above, by providing a plurality of slit-shaped holes in the back surface of the wafer 1 in a dispersed manner, when the metal paste is baked and solidified, the alloy portion of aluminum and silicon and aluminum are formed by the notch 8. Since it is in a divided state, it does not shrink entirely but partially shrinks, so that the residual stress of the wafer 1 is alleviated and the warpage of the wafer 1 can be suppressed. Therefore, it is possible to prevent a decrease in the strength of the solar cell itself and the separation of the back electrode 3, eliminate a transport error of the wafer 1, and prevent the wafer 1 from being damaged or cracked.

The notch 8 may be filled with a plurality of fitting members made of a material different from the back electrode 3 such as glass or ceramic. That is, a plurality of fitting members are mounted on the wafer 1, a metal paste is applied around the fitting members, and firing is performed. Then, since the fitting member is present between the back electrodes 3, the contraction of the back electrodes 3 is suppressed, whereby the residual stress can be reduced, and the warpage of the wafer 1 can be suppressed. That is, the notch 8 can be said to be a space filled with a substance different from that of the back surface electrode 3, which is air or another material. This material needs to have a coefficient of thermal expansion different from the coefficient of thermal expansion of the back electrode 3.

Further, instead of the slit-shaped holes as the cutouts 8, a plurality of grooves 9 may be dispersedly provided in the back electrode 3 as shown in FIG. The groove 9 is formed by applying a metal paste to the back surface of the wafer 1 and pressing and firing a mold corresponding to the shape of the groove 9. Alternatively, the groove 9 may be formed by cutting off a part of the back surface electrode 3 after cooling the metal paste and before cooling.

If too many notches 8 or grooves 9 are provided in the back electrode 3, short-circuit current is reduced and conversion efficiency is disadvantageous. It is desirable not to make the total area of the hologram too large. Further, the shape of the notch 8 or the groove 9 is not limited to an elongated rectangular shape, and may be substantially circular, or may be formed in a square shape as shown in FIG. Further, each notch 8 or groove 9 may be arranged so as to radially spread from the center as shown in FIG. 5, or may be arranged on a plurality of concentric circles as shown in FIG. .

<Second Embodiment> In this embodiment, a notch for alleviating residual stress is formed on the back surface of a polycrystalline wafer 1 having a uniform crystal growth direction along the crystal growth direction. The part 8 is formed. For example, in a polycrystalline wafer 1 manufactured by an electromagnetic casting method in which a silicon melt is gradually cooled to be solidified, and the solidified material is thinly cut to form a wafer 1, as described in the section of the prior art, Thus, the crystal growth directions are aligned. At this time, if the back surface electrode 3 is formed by firing the metal paste on the back surface of the wafer 1, the wafer 1 warps in a direction substantially perpendicular to the crystal growth direction.

Therefore, before forming the back electrode 3, the direction of crystal growth of the wafer 1 is recognized, and the arrangement of the notch 8 is determined based on the direction of growth. Specifically, the crystal growth direction is recognized by, for example, irradiation of X-rays or ultrasonic waves or image recognition (image processing) by a camera. in this case,
As shown in FIG. 13, if the crystal growth direction is radially directed from one corner of the wafer 1 to another corner, as shown in FIG. 7, the longitudinal direction of the notch 8 is along the crystal growth direction. Are formed, that is, the notches 8 are formed so as to be distributed in parallel from one corner to the other. By doing so, the notch 8 suppresses the contraction of the back surface electrode 3 in a direction substantially perpendicular to the direction of crystal growth, thereby alleviating the residual stress and effectively suppressing the warpage of the wafer 1.

Here, as shown in FIG. 8A, after the crystal growth direction is recognized by image recognition, if the crystal growth direction is different from the longitudinal direction of the notch 8 to be formed, The direction of the wafer 1 is adjusted. For example, if the wafer 1 is mounted on a mounting table (not shown), the mounting table is rotated clockwise by 90 °, and the orientation of the wafer 1 is changed as shown in FIG. Then, the back electrode 3 and the cutout 8 are formed on the back surface of the wafer 1 by the method described in the first embodiment. By doing so, the crystal growth direction and the longitudinal direction of the notch 8 can be aligned.

If the crystal growth direction is different from the longitudinal direction of the notch 8 to be formed, the wafer 1
The orientation of the notch 8 may be changed while the arrangement of the notches 8 is maintained, so that the crystal growth direction and the longitudinal direction of the notch 8 are aligned. Other configurations are the same as in the first embodiment.

If the crystal growth direction is radial from one corner to the other (see FIG. 13), the notch 8
May be similarly formed radially along the crystal growth direction, as shown in FIG. According to this, since the crystal growth direction and the longitudinal direction of the notch 8 are more precisely aligned, the warpage of the wafer 1 can be more effectively suppressed. If the crystal growth direction is in the vertical direction in the drawing, the cutouts 8 are similarly dispersed in the vertical direction as shown in FIG.

The present invention is not limited to the above embodiment, and many modifications and changes can be made to the above embodiment within the scope of the present invention.

[0032]

As described above, according to the present invention, an electrode and a cutout made of a different material are mixed on the surface of a semiconductor substrate, so that an electrode is formed by firing a conductive material. The residual stress at the time can be reduced, and the warpage of the semiconductor substrate can be suppressed. Therefore, a decrease in the strength of the solar cell itself can be prevented, and the semiconductor substrate can be prevented from being damaged or cracked by eliminating a mistake in transporting the semiconductor substrate, whereby a high-quality solar cell can be provided.

[Brief description of the drawings]

FIG. 1 is a back view of a solar cell according to a first embodiment of the present invention.

FIG. 2 is a cross-sectional view of the same solar cell.

FIG. 3 is a partial cross-sectional view of a solar cell in which a groove is formed in a back electrode.

FIG. 4 is a back view of a solar cell according to a modified example.

FIG. 5 is a rear view of a solar cell according to a modified example.

FIG. 6 is a rear view of a solar cell according to a modified example.

FIG. 7 is a back view of the solar cell according to the second embodiment.

8A and 8B show a crystal growth direction and a notch forming direction in a polycrystalline wafer, wherein FIG. 8A shows a state before the wafer direction is changed, and FIG. 8B shows a state after the wafer direction is changed. Diagram shown

FIG. 9 is a rear view of a solar cell according to a modified example.

FIG. 10 is a back view of a solar cell according to a modification.

FIG. 11 is a rear view of a conventional solar cell.

12 is a view showing the warpage of the solar cell after the formation of the back electrode at the line aa or bb in FIG. 11;

FIG. 13 is a back view of another conventional solar cell.

FIG. 14 is a diagram showing the warpage of the solar cell after the formation of the back electrode at line cc in FIG. 13;

[Explanation of symbols]

 1 Wafer 3 Back electrode 8 Notch 9 Groove

Claims (7)

[Claims] 1. A solar cell comprising: an electrode formed by sintering a conductive material; and a notch for alleviating residual stress in which no electrode is provided, provided on a surface of a semiconductor substrate. 2. The solar cell according to claim 1, wherein a plurality of notches are distributed on the surface of the semiconductor substrate. 3. The solar cell according to claim 1, wherein the notch comprises a slit-shaped hole. 4. The semiconductor device according to claim 1, wherein the notch is formed along the direction of crystal growth of the semiconductor substrate.
The solar cell according to any one of the above. 5. The conductive material is made of a metal paste, the electrode is formed in a region where the metal paste is applied on a surface of the semiconductor substrate opposite to the light receiving surface, and the cutout portion is formed of the metal paste. The solar cell according to any one of claims 1 to 4, wherein the solar cell is formed in an uncoated region. 6. A solar cell, wherein an electrode formed by sintering a conductive material is provided on a surface of a semiconductor substrate, and a groove for relaxing residual stress is formed on the surface of the electrode. 7. A semiconductor substrate having a crystal whose growth direction is aligned, the growth direction of the crystal is recognized by image processing, and the semiconductor is aligned so that the direction of the notch for relaxing residual stress and the direction of the crystal are aligned. A method for manufacturing a solar cell, comprising: arranging a substrate; and baking a conductive material on the semiconductor substrate to form the notch and the electrode.