JP3209130B2 - Solar cell - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a solar cell, and more particularly to a structure for improving the photoelectric conversion efficiency of a solar cell.

[0002]

2. Description of the Related Art Heretofore, a solar cell has been developed in which an electrode is formed on the back surface opposite to the light receiving surface in order to secure as much area of the light receiving surface as possible. JP-A-3-16557
No. 8 also discloses a solar cell in which a p + type silicon (Si) layer and an n + type Si layer are alternately arranged as a positive electrode and a negative electrode. FIG. 10 shows a partial cross-sectional view of a solar cell according to this conventional example. In FIG. 10, a p + -type Si layer 12 is provided on the back side of a p-type crystalline silicon substrate 10.
And a groove made of the n + -type Si layer 14 are formed. A positive electrode 16 made of an aluminum-silicon alloy layer is provided in a groove made of the p + -type Si layer 12.
Is embedded, and a negative electrode 18 made of silver is embedded in a groove made of the n + -type Si layer 14.

According to this conventional example, the p + type Si layer 12, n
The contact area between the + type Si layer 14 and the positive electrode 16 and the negative electrode 18 embedded therein can be increased, and the contact resistance between them can be reduced. Further, since the thickness of the positive electrode 16 and the negative electrode 18 can be increased, the metal specific resistance can be reduced. For this reason, the internal resistance of the solar cell can be reduced. Further, the p + -type Si layer 12, n
Since the + type Si layer 14 has a groove shape as described above, the distance between the concave portion and the light receiving surface of the crystalline silicon substrate 10 is shortened, and the carrier recombination is reduced, so that the photoelectric conversion efficiency can be improved. .

[0004]

However, in the above-mentioned prior art, there is a problem that the groove formed on the back surface side of the light receiving surface is U-shaped, and it is difficult to form the groove. Also,
Although only the formation of a groove on the back side is disclosed, it is considered that there is an optimum value for the size and arrangement of the groove, and it is necessary to clarify this point.

Further, silver is used as the negative electrode 18, but titanium (Ti) is desirably used in consideration of adhesion and contact resistance with the n + -type Si layer.

Further, in the above conventional example, since the aluminum-silicon alloy is used as the positive electrode 16, aluminum is also used for the bus electrode on the positive electrode side. However, since aluminum cannot be soldered, aluminum is generally plated with nickel and then with gold. However, since the surface of aluminum is oxidized during the pattern formation, there is a problem that the adhesion to nickel plated on aluminum is not sufficient and the aluminum is easily peeled off.

[0007] The present invention has been made in view of the above-mentioned conventional problems, and an object of the present invention is to provide a solar cell having a structure with improved photoelectric conversion efficiency.

[0008]

According to a first aspect of the present invention, a light receiving surface is formed on the front side of a crystalline silicon substrate, and a p + type silicon (Si) layer is formed on the back side. In the case of a solar cell in which n + -type Si layers are alternately arranged and p-type Si is used for the crystalline silicon substrate, the n + -type Si layer uses n-type Si for the crystalline silicon substrate In this case, the p + type Si layer is formed in a V-shaped groove having a triangular cross section provided on the back surface of the crystalline silicon substrate.

According to a second aspect, in the solar cell according to the first aspect, the depth of the groove is 50 to 90% of the thickness of the crystalline silicon substrate, and the pitch of the groove is the thickness of the crystalline silicon substrate. It is characterized by being 50 to 150% of the total.

According to a third aspect of the present invention, a light receiving surface is formed on a front surface of a crystalline silicon substrate, and a p + type Si layer and an n +
A solar cell in which a + type Si layer is alternately arranged, and a groove is formed above the n + type Si layer from the light receiving surface side of the crystalline silicon substrate toward the n + type Si layer. It is characterized by the following.

Further, the fourth invention is the third invention from the first invention.
The solar cell according to any one of the aspects of the present invention is characterized in that titanium, palladium, and silver layers are laminated in this order after aluminum is patterned as a positive electrode side bus electrode.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described below with reference to the drawings.

Embodiment 1 FIG. 1 is a cross-sectional view of Embodiment 1 of the solar battery cell according to the present invention. In FIG. 1, a texture structure 20 for improving light confinement and improving photoelectric conversion efficiency is formed on the light receiving surface side of the crystalline silicon substrate 10. Also, on the back surface opposite to the light receiving surface on which the texture structure 20 is formed, p
+ -Type Si layers 12 and n + -type Si layers 14 are provided alternately.

As shown in FIG. 1, a p + type Si layer 12 is formed.
Accordingly, an aluminum layer as the positive electrode 16 is formed. Further, a metal layer as a negative electrode 18 is formed corresponding to the n + -type Si layer 14. In the case of this embodiment, the n + -type Si layer 14 is formed in a V-shaped groove having a triangular cross section, and the negative electrode 18 is formed on the inner surface of the V-shaped groove. As the metal layer as the negative electrode 18, for example, a layer in which respective layers of titanium, palladium, and silver (Ti / Pd / Ag) are laminated in this order is used.

Titanium is known as a metal having a high adhesion to an n + -type Si layer, a low contact resistance, and a so-called spike phenomenon in which a metal element penetrates the n + -type Si layer. Silver is known as a metal having good soldering characteristics. Palladium has good adhesion to both titanium and silver, and functions as a kind of adhesive. The combination of metals having the same function as the combination of titanium, palladium, and silver and usable as the negative electrode 18 includes titanium, nickel, palladium (Ti / Ni / Pd), titanium, nickel, and gold (Ti). / Ni / Au) and the like.

As described above, in this embodiment,
The negative electrode 18 is formed in a V-shaped groove having a triangular cross section. This is different from the conventional example in which both the positive electrode 16 and the negative electrode 18 are formed in grooves. In the present embodiment, only the negative electrode 18 is formed in the groove,
The formation becomes easier as the number of grooves is reduced. In addition, a decrease in the strength of the crystalline silicon substrate 10 can be prevented by the small number of grooves. Furthermore, since the shape of the formed groove is also V-shaped,
The formation is easier than in the U-shaped groove shown in the conventional example.

In the present embodiment, p-type Si is used as the crystalline silicon substrate 10, and the negative electrode 18, which is an electrode for receiving minority carriers of such a substrate, has a V-groove shape. As a result, the distance between the light receiving surface and the negative electrode 18 is shortened, and the average moving distance of electrons, which are minority carriers, can be significantly reduced. When the average moving distance of the electrons is reduced, the loss of carriers due to Auger recombination in the crystalline silicon substrate 10 can be reduced, and the photoelectric conversion efficiency of the solar cell can be improved.

Further, by forming the negative electrode 18 in a V-groove shape, the area itself of the negative electrode 18 can be increased.
The area in which the positive electrode 16 is formed has a margin by that much, so that the area of both the positive electrode 16 and the negative electrode 18 can be increased. Thereby, the resistance in the electrode can be reduced.

As described above, in order to reduce the average moving distance of electrons, which are minority carriers, and to increase the electrode area, it is better that the depth of the V-groove is deep. There is a certain upper limit because the strength of No. 10 decreases and leads to cracking.

In order to reduce the moving distance of electrons as minority carriers, it is effective to reduce the pitch between V grooves. However, when the pitch is reduced, the area of the positive electrode 16 is reduced, and the power that can be extracted is reduced.
There is also an optimum value for the pitch.

As described above, there are optimum values for the depth and the pitch of the V-groove, respectively, and it is considered that the photoelectric conversion efficiency of the solar cell can be improved by appropriately controlling these values. . Therefore, a plurality of crystalline silicon substrates 10 having different depths and pitches were manufactured, and the short-circuit current density was measured for each of them. FIG. 2 shows the relationship between the depth of the V-groove and the short-circuit current density, and FIG. 3 shows the relationship between the pitch of the V-groove, the short-circuit current density, and the fill factor (ff). Here, the fill factor (ff) is a value obtained by dividing the actual maximum output of the solar cell by the product of the short-circuit current, which is the theoretical maximum output, and the open-circuit voltage.
Thereby, the power generation capacity of the solar cell can be evaluated. The evaluation of the performance of the solar cell shown in FIGS. 2 and 3 was performed in a non-light-collecting state with a short-circuit current density and a light-collecting state with a fill factor of 50 times.

In FIG. 2, the abscissa indicates the ratio between the depth of the V-groove and the thickness of the crystalline silicon substrate 10. This is the same in FIG. As shown in FIG. 2, the depth of the V-shaped groove is 50 to 50 times the thickness of the crystalline silicon substrate 10.
Between 90%, the short circuit current density shown on the vertical axis is improved. That is, when the horizontal axis is less than 50%, the short-circuit current density was about 35 mA / cm ² ,
By weight, about 3mA / cm ² increased by 37.5 to 38.
It is 3 mA / cm ² . However, when the abscissa exceeds 60%, the effect of increasing the short-circuit current density is almost saturated. If it exceeds 90%, the crystalline silicon substrate 1
In this case, cracks occurred in 0, the number of substrates on which solar cells could not be formed increased, and the yield was reduced. From the above results, V
The depth of the groove is 50 times the thickness of the crystalline silicon substrate 10.
A range of ~ 90% is considered desirable. Note that the pitch of the V-groove is 100 times the thickness of the crystalline silicon substrate 10.
%.

Next, as shown in FIG. 3, as the pitch of the V-groove increases with respect to the thickness of the crystalline silicon substrate 10, the short-circuit current density indicated by a circle in the figure decreases. In particular, it rapidly decreases when it exceeds 150%. In this case, the depth of the V-groove is set to the crystal silicon substrate 1
A thickness of 60% of 0 was used.

On the other hand, the fill factor indicated by the symbol “△” in the figure increases with an increase in the pitch of the V-groove, and particularly increases when the thickness exceeds 50% of the thickness of the crystalline silicon substrate 10. I have. This is, as mentioned above,
When the pitch is reduced, the area of the positive electrode 16 is reduced, so that the fill factor is considered to be reduced accordingly.
This is because the resistance increases as the area of the positive electrode 16 decreases. Further, when the pitch is larger than 150%, the average moving distance of the electrons, which are minority carriers, in the crystalline silicon substrate 10 increases, which appears to lower the short-circuit current density. From the above results, it is considered that the range of 50 to 150% is appropriate as the pitch of the V-groove.

In the first embodiment described above, the case where p-type Si is used as the crystalline silicon substrate 10 has been described. However, n-type Si can naturally be used.
In this case, the p + -type Si layer 1 on which the positive electrode 16 is formed
2 will be formed in the V groove. That is, an electrode for receiving minority carriers in the used crystalline silicon substrate 10 is formed in a V-groove shape.

Next, a method for forming electrodes of a solar cell according to this embodiment will be described.

As described above, the negative electrode 18 of this embodiment is
For example, titanium, palladium, and silver are used. This is because the adhesion between titanium and the n + -type Si layer 14 is high, but an appropriate etching solution is not known for titanium. For this reason, in order to form a titanium layer at a predetermined position on the crystalline silicon substrate 10, a lift-off method is generally used. In the lift-off method, as shown as a partial cross-sectional view in FIG. 4, a resist 22 is applied on a portion where a titanium layer is not formed, for example, on a positive electrode 16, and a portion where the resist 22 is applied and a portion where the resist 22 is not applied are applied. A titanium palladium silver layer 24 is deposited all over. next,
Unnecessary portions of the titanium-palladium-silver layer 24 are removed together with the resist 22 using an etchant, and the titanium-palladium-silver layer 24 is left in a predetermined portion, for example, to form the negative electrode 18.

However, since such a conventional lift-off method is performed on the flat surface of the crystalline silicon substrate 10, the stripping solution of the resist 22 does not sufficiently penetrate into the resist 22, and a part of the titanium palladium silver There is a problem that the layer 24 remains in unnecessary portions. For this reason,
A short circuit between the positive electrode 16 and the negative electrode 18 may occur.

On the other hand, in the method of the present embodiment shown in the partial sectional view of FIG. 5, since the n + -type Si layer 14 is formed in the V-groove, the stripping liquid sufficiently permeates the resist 22 and The titanium palladium silver layer 24 on the substrate 16 can be completely peeled off. Thereby, a short circuit between the positive electrode 16 and the negative electrode 18 can be completely prevented.

Embodiment 2 FIG. 6 shows a partial cross-sectional view of Embodiment 2 of the solar cell according to the present invention. A characteristic point in FIG. 6 is that a V-shaped groove having a triangular cross section is formed from the light receiving surface side where the texture structure 20 is formed toward the n + -type Si layer 14. Thereby, the short-circuit current density is improved.

Also in this embodiment, as in the first embodiment, the depth of the V-groove has an optimum range, and is preferably about 50 to 90% of the thickness of the substrate. This is to satisfy both the improvement of the short-circuit current density and the maintenance of the strength of the substrate, as described above. When the V-groove is formed, the average moving distance of the minority carriers in the crystalline silicon substrate 10 becomes shorter,
Annihilation due to recombination of minority carriers is reduced. Therefore,
The formation of the V-groove is effective for improving the short-circuit current density. However, if the groove width of the V groove is increased, n +
Since it is necessary to increase the width of the p-type Si layer 14, there is a problem that the width of the p + -type Si layer 12 decreases, the area of the positive electrode 16 decreases, and the electric resistance increases. For this reason, the fill factor (ff) decreases as the groove width of the V groove increases.

For example, the short-circuit current density when no V-groove is formed is 35.0 mA / cm ² at the time of non-condensing, while the V-groove has a groove width Lv of 30 μm and two V-grooves. When the width Lf of the flat surface was also 30 μm, the short-circuit current density increased to 36.8 mA / cm ² . On the other hand, the fill factor at the time of 50 times focusing is
The flat surface width Lf was 0.76 at 30 μm or more, whereas 0.71 at 20 μm and 0.7 at 10 μm.
It was 0.68 at 0,0. As described above, the width Lf of the flat surface on the light receiving surface of the solar cell is 30 to
It is considered that the thickness is preferably about 50 μm.

In this embodiment, as shown in FIG. 6, both the positive electrode 16 and the negative electrode 18 are formed in a flat shape on the back surface opposite to the light receiving surface. Therefore, since it is difficult to form electrodes by the lift-off method, aluminum is used for each electrode. In this case, in order to prevent a spike phenomenon from occurring in the n + -type Si layer on the negative electrode side, it is necessary to suppress the firing temperature to 450 ° C. or less. For this reason, titanium,
Adhesion evaluated as tensile strength is reduced to about 30 to 70% as compared with the case where palladium or silver is used. Therefore, the solar cell according to the present embodiment is particularly suitable when used at a relatively uniform temperature within 50 ° C. during use.

Further, in the above description, the groove shape is V-shaped, but it is not necessarily limited to this shape. For example, it may be U-shaped.

Embodiment 3 FIG. 7 shows a partial plan view of the back surface side of the solar cell according to the present invention. As described in the first embodiment, since aluminum is used for the positive electrode 16, the positive bus electrode 26 is also formed by extending this aluminum. Since aluminum cannot be soldered, Au plating is further performed after Ni plating on aluminum to enable soldering.
It is generally plated. However, there is a problem that the aluminum surface is oxidized in a pattern forming step after the aluminum film is formed, and Ni plating cannot secure sufficient adhesion. The solar cells generate power while cooling with cooling water during use, and do not generate power at night, so the cooling water is stopped, but the temperature difference per day reaches a maximum of about 60 to 70 ° C. . As an adhesive force for withstanding such a temperature change, a tensile strength of about 100 to 200 N / cm ² is required, but in the case of the above-described Ni plating, only an adhesive force of about 50 N / cm ² can be obtained. Can not.

Therefore, in the present embodiment, titanium, palladium,
Each layer of silver is stacked in this order to ensure the adhesion to the crystalline silicon substrate 10 and reduce the contact resistance with aluminum as a bus electrode. That is, as shown in FIG. 7, a rectangular square hole 28 is made in the aluminum of the positive electrode bus electrode 26. This square hole 2
8, the positive electrode side bus electrode 26 has a width W = 5 mm and a length l =
When it is about 0.5 to 1 mm, it is a square having a side of about 50 μm. The square hole 28 is drilled until it reaches the crystalline silicon substrate 10, that is, through aluminum.

The titanium-palladium-silver layer 24 described above is formed on the aluminum on which the square holes 28 are formed. As a result, the titanium of the titanium palladium silver layer 24 reaches the crystalline silicon substrate 10 through the square hole 28.
Since silicon and titanium have high adhesion, an adhesion of about 180 to 220 N / cm ² can be obtained as a tensile strength. Further, the formation of the titanium palladium silver layer 24 on the positive electrode side bus electrode 26 is performed simultaneously with the formation of the titanium palladium silver layer 24 as the negative electrode 18. In this embodiment, since aluminum is not used for the negative electrode 18, a so-called spike phenomenon does not occur even if the firing temperature after pattern formation is increased. For this reason, the firing temperature is raised to 600 to 700 ° C., the adhesion between aluminum and titanium is increased, and the contact resistance is reduced to 0.03.
０．２0.20 Ωcm ² .

FIG. 8 shows a procedure for fabricating the solar cell according to the present embodiment by using a section taken along line VIII-VIII of FIG. In FIG. 8A, a texture structure 20 is formed on the light receiving surface of the crystalline silicon substrate 10, and a V-groove-shaped n + type Si layer 14 and a flat p-type Si layer 12 are formed on the back surface side. I have. In this case, n + type Si
Layer 14 is formed by diffusion of phosphorus. Also, p + type S
The i-layer 12 is formed by boron diffusion. However, since aluminum is diffused in a later step, it is not always necessary to form the p + -type Si layer 12 at this time.

In FIG. 8B, the crystalline silicon substrate 1
The aluminum film 30 is vapor-deposited on the entire rear surface of the substrate 0. Next, a pattern is formed using a negative resist by photolithography. At this stage, FIG.
As shown in (c), the aluminum other than the predetermined portion is etched by phosphoric acid, and the positive electrode 16 and the positive electrode bus electrode 26 are formed. A square hole 28 is formed in the positive electrode side bus electrode 26. The square hole is formed until reaching the crystalline silicon substrate 10.

Next, after stripping the resist, FGA annealing is performed at 600 to 700 ° C. to form a p + -type Si layer. The FGA _{annealing, N 2 90%, H 2} 1
This is done with a 0% gas mixture. Further, FIG.
As shown in (1), a negative electrode 18 made of titanium, palladium, and silver is formed in the V groove by the lift-off method.
Further, a titanium palladium silver layer 24 is also formed on the aluminum of the positive electrode side bus electrode 26. This lift-off method is performed as described in the first embodiment. In this case, the thickness of the titanium layer is 2000 angstroms, the thickness of the palladium layer is 1000 angstroms, and the thickness of the silver layer is 2 μm to 3 μm.
It has become.

Thus, the solar cell according to the present embodiment is completed. In the solar battery cell according to the present embodiment, it is also preferable to form a bank-shaped reinforcing portion 32 around the cell to improve the cell strength.

FIG. 9 is a partial plan view of a modified example of the solar cell according to the present embodiment. Also in this case, the back side of the crystalline silicon substrate 10 is shown. In FIG. 9, the aluminum layer forming the positive electrode 16 is extended to a region where the negative electrode 18 is not formed, and a titanium palladium silver layer 24 is formed on this extended portion. In this case, even if the square hole 28 is not formed as shown in FIG. 7, the titanium palladium silver layer 24 is formed in a region wider than the aluminum layer. In the exposed region, the titanium palladium silver layer 24 comes into direct contact with the crystalline silicon substrate 10. As described above, titanium has high adhesiveness to the crystalline silicon substrate 10, and thus a sufficiently high adhesive force can be secured even in the configuration as in the present modification.

[0043]

As described above, according to the present invention,
The average movement distance of minority carriers can be reduced by the V-grooves formed on the crystalline silicon substrate, and diffusion of the minority carriers is blocked by each V-groove, so that carrier recombination is greatly reduced. Therefore, photoelectric conversion efficiency can be improved.

Further, the V-shaped groove is easier to make than the rectangular or U-shaped groove, and the manufacturing cost of the solar cell can be reduced. Further, by forming the V-groove, the area of the electrode formed here increases, so that the resistance of the electrode can be reduced.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of Embodiment 1 of a solar battery cell according to the present invention.

FIG. 2 is a graph showing the relationship between the depth of a V-groove and the short-circuit current density.

FIG. 3 is a graph showing a relationship between a pitch of a V groove, a short circuit current density, and a fill factor.

FIG. 4 is an explanatory view of a conventional lift-off method.

FIG. 5 is an explanatory diagram of a lift-off method for producing the solar battery cell of Embodiment 1 shown in FIG.

FIG. 6 is a partial sectional view of Embodiment 2 of the solar battery cell according to the present invention.

FIG. 7 is a partial plan view of Embodiment 3 of the solar battery cell according to the present invention.

FIG. 8 is an explanatory view showing a method for producing the solar battery cell of Embodiment 3 shown in FIG.

FIG. 9 is a partial plan view showing a modified example of Embodiment 3 of the solar battery cell according to the present invention.

FIG. 10 is a partial cross-sectional view of a conventional solar cell in which a groove is formed on the back side.

[Description of Signs] 10-crystal silicon substrate, 12 p + -type Si layer, 14
n + type Si layer, 16 positive electrode, 18 negative electrode, 20 texture structure, 22 resist, 24 titanium palladium silver layer, 26 positive electrode side bus electrode, 28 square hole, 30 aluminum film, 32 reinforcing part.

──────────────────────────────────────────────────続 き Continuation of front page (72) Inventor Tomomi Nagashima 1 Toyota Town, Toyota City, Aichi Prefecture Inside Toyota Motor Corporation (56) References JP-A-8-213642 (JP, A) JP-A-8- 204214 (JP, A) JP-A-53-10988 (JP, A) JP-A-62-205667 (JP, A) JP-A-1-125988 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H01L 31/04-31/078

Claims (4)

(57) [Claims] 1. A light-receiving surface is formed on the front side of a crystalline silicon substrate, and a p + type silicon (Si) layer and an n + type S
In the case of a solar cell in which i-layers are alternately arranged, and when p-type Si is used for the crystalline silicon substrate, the n +
When the type Si layer uses n-type Si for the crystalline silicon substrate, the p + -type Si layer is formed in a V-shaped groove having a triangular cross section provided on the back surface of the crystalline silicon substrate. Solar cells. 2. The solar cell according to claim 1, wherein
The depth of the groove is 50 to 90% of the thickness of the crystalline silicon substrate
And the pitch of the grooves is 5 times the thickness of the crystalline silicon substrate.
A solar cell, which is 0 to 150%. 3. A solar cell in which a light-receiving surface is formed on a front surface side of a crystalline silicon substrate, and p + -type Si layers and n + -type Si layers are alternately arranged on a back surface side. A photovoltaic cell, wherein a groove is formed in an upper portion of the type Si layer from the light-receiving surface side of the crystalline silicon substrate toward the n + type Si layer. 4. The solar cell according to claim 1, wherein aluminum is patterned as a positive electrode side bus electrode, titanium, palladium,
A solar cell in which silver layers are stacked in this order.