JPH08111537A - Solar battery - Google Patents

Abstract

PURPOSE: To improve efficiency of a solar battery in which a back side field is formed by a microcrystal silicon layer. CONSTITUTION: A solar battery comprises a first conduction type crystal semiconductor substrate 11, a second conduction type semiconductor layer 12 formed on the surface of incidence within the substrate, a first conduction type non- crystalline semiconductor layer 18 formed on the back side of the substrate, and a first conduction type microcrystal semiconductor layer 16 formed on the non-crystalline semiconductor layer and having a higher impurity density than that of the substrate.

Description

Detailed Description of the Invention

[0001]

The present invention relates to a semiconductor solar cell,
In particular, it relates to improvement of photoelectric conversion efficiency of solar cells.

[0002]

2. Description of the Related Art Conventionally, in order to increase the photoelectric conversion efficiency of a semiconductor solar cell, a semiconductor layer of the same conductivity type as that of the semiconductor substrate and having a high impurity concentration is provided on the back surface of the semiconductor substrate opposite to the light incident side. It has been known. An internal electric field is formed between the semiconductor substrate and the back surface semiconductor layer having a high impurity concentration. The minority carriers among the carriers generated in the vicinity of the back surface of the solar cell are pushed back into the semiconductor substrate by the internal electric field, which enhances the photoelectric conversion efficiency of the solar cell (hereinafter, this effect is referred to as "back surface field effect").
Called).

For example, when a semiconductor layer having a high impurity concentration is formed on the back surface of a P-type silicon substrate, there is a method of printing and baking an aluminum paste on the back surface of the P-type silicon substrate. Further, by diffusing boron or aluminum from the back surface of the P-type silicon substrate, the P ⁺ -type back surface semiconductor layer having a high impurity concentration can be formed.

FIG. 6 schematically illustrates a cross section of a solar cell disclosed in Japanese Patent Laid-Open No. 4-192569. Figure 6
In this solar cell, the N-type layer 12 is formed on the light incident side of the P-type single crystal silicon substrate 11. The surface of the N-type layer 12 is made uneven by, for example, etching in order to reduce reflection of incident light. The N-type layer 12 is covered with a silicon oxide film 13, and the silicon oxide film 13 is covered with an antireflection film 14. The electric current penetrates the silicon oxide film 13 and the antireflection film 14 to generate N
It is taken out through the grid electrode 15 connected to the mold layer 12.

On the back surface of the P-type substrate 11 on the side opposite to the light incident side, a back surface electric field layer 16 made of a P ⁺ -type microcrystalline silicon film to which the same P-type impurity is added at a high concentration is formed. . The back surface field layer 16 is covered with a back surface metal electrode 17. That is, in the solar cell of FIG. 6, for the purpose of enhancing the back surface field effect, a heterojunction is formed by providing a P ⁺ -type microcrystalline silicon layer on the back surface of the P-type silicon substrate, thereby improving the effect. To form a typical back surface electric field.

[0006]

[SUMMARY OF THE INVENTION Incidentally, when depositing back surface field layer of the P ⁺ -type microcrystalline silicon on the back surface of the P-type silicon substrate, P ⁺ -type microcrystalline silicon layer under the influence of the crystal structure of the substrate Since it is easy, the portion of the back electrode layer near the substrate is unlikely to have a microcrystalline structure. That is, in order for the back surface field layer to have the original P ⁺ -type microcrystalline structure,
It is necessary to increase the film thickness considerably.

FIG. 7 shows the relationship between the film thickness of the back surface electric field layer of P ⁺ type microcrystalline silicon in the solar cell of the type shown in FIG. 6 and various characteristics of the solar cell. Figure 7
In each of graphs (A), (B), (C), and (D), the horizontal axis represents the film thickness (nm) of the P ⁺ -type microcrystalline silicon layer, but the vertical axis represents a short circuit. The current (mA / cm ² ), open circuit voltage (mV), fill factor and conversion efficiency (%) are shown.

As can be seen from FIG. 7, in order to improve the characteristics of the solar cell by the shape of the back surface field layer, the back surface field layer must have a thickness of 200 nm or more. That is, a part of the back surface electric field layer affected by the crystal structure of the substrate is prevented from forming a heterojunction, and a complete heterojunction cannot be obtained. Therefore, even if the back surface field layer of P ⁺ -type microcrystalline silicon is formed on the back surface of the P-type silicon substrate, a high back surface field effect cannot be obtained if the back surface field layer is thin, and the back surface field effect of the solar cell is reduced. The photoelectric conversion efficiency cannot be improved sufficiently.

In view of such problems of the prior art, it is an object of the present invention to provide a solar cell having high photoelectric conversion efficiency due to a significantly improved back surface field effect even when the back surface field layer is thin. I am trying.

[0010]

A solar cell according to one aspect of the present invention comprises a first-conductivity-type crystalline semiconductor substrate and a second-conductivity-type semiconductor layer formed on the light-incident-side surface of the substrate. A first conductive type amorphous semiconductor layer formed on the back surface of the substrate opposite to the light incident side, and a first conductive type amorphous semiconductor layer having a higher impurity concentration than the substrate. And a conductive type microcrystalline semiconductor layer.

In a solar cell according to another aspect of the present invention, the amorphous layer contains hydrogen.

In a solar cell according to another aspect of the present invention, the amorphous semiconductor layer has a thickness within a range of 2 nm or more and 20 nm or less.

In a solar cell according to still another aspect of the present invention, the amorphous semiconductor layer is in contact with the back surface of the substrate in a plurality of contact holes formed in the passivation film formed on the back surface of the substrate. It is characterized by that.

[0014]

According to the solar cell according to one aspect of the present invention,
Since the amorphous semiconductor layer which is not easily influenced by the crystal structure of the substrate is provided between the crystalline semiconductor substrate and the microcrystalline semiconductor layer, the microcrystalline semiconductor layer can be formed only by a layer having a microcrystalline structure. The amorphous semiconductor layer only serves to control the crystal structure of the microcrystalline silicon layer in the initial stage of deposition, and does not adversely affect the heterojunction between the crystalline semiconductor substrate and the microcrystalline semiconductor layer. Therefore, it becomes possible to form a complete heterojunction between the crystalline semiconductor substrate and the microcrystalline semiconductor layer, the back surface field effect is significantly enhanced, and the photoelectric conversion efficiency of the solar cell is greatly improved.

In the solar cell according to another aspect of the present invention, the amorphous semiconductor layer contains hydrogen, and the hydrogen passivates defects on the back surface of the substrate, so that the substrate and the microcrystals are formed. A good bond can be obtained with the silicon layer.

In the solar cell according to another aspect of the present invention, it is sufficient that the amorphous semiconductor layer has an extremely thin thickness within the range of 2 nm or more and 20 nm or less, so that the manufacturing cost and the manufacturing time are reduced. The characteristics of the solar cell can be improved with almost no increase.

In the solar cell according to still another aspect of the present invention, the amorphous semiconductor layer is in contact with the back surface of the substrate in the plurality of contact holes formed in the passivation layer formed on the back surface of the substrate. Therefore, the electrical characteristics on the back surface of the solar cell are stabilized.

[0018]

FIG. 1 schematically shows a cross-sectional structure of a solar cell according to an embodiment of the present invention. In this solar cell, an N-type layer 12 is formed on the light incident side of a P-type crystalline silicon substrate 11. The surface of the N-type layer 12 is made uneven by, for example, etching in order to reduce reflection of incident light. N-type layer 12 is silicon oxide film 1
3 and the silicon oxide film 13 is covered with an antireflection film 14. The current from the surface penetrates the silicon oxide film 13 and the antireflection film 14 and the N-type layer 1
It is taken out via the grid electrode 15 connected to the No. 2.

On the back surface of the P-type substrate 11 on the side opposite to the light incident side, an amorphous silicon layer 18 to which the same P-type impurities as the substrate are added is formed so as to cover the amorphous silicon layer 18. A back surface electric field layer 16 of P ⁺ -type microcrystalline silicon to which a P-type impurity is added at a high concentration is formed on the surface. The back surface field layer 16 is covered with the back surface electrode 17, and the current from the back surface is taken out from the back surface electrode 17.

FIG. 2 shows the relationship between various characteristics of the solar cell of the type shown in FIG. 1 and the thickness of the P-type amorphous silicon layer 18. That is, as shown in FIG.
In each of the graphs (B), (C) and (D), the horizontal axis is the film thickness (nm) of the P-type amorphous silicon layer.
The vertical axis represents the short-circuit current (mA / cm).
² ) Indicates open circuit voltage (mV), fill factor and conversion efficiency (%).

In FIG. 2, a P-type amorphous silicon layer 18 is formed.
Was formed by plasma CVD in a mixed gas atmosphere of SiH ₄ , H ₂ and B ₂ H ₆ . The composition ratio in the mixed gas is H ₂ / SiH ₄ = 0.3 and B ₂ H ₆
/ SiH ₄ = 0.005, and the input RF (high frequency) power was 7W. The P-type amorphous silicon layer 18 thus formed contains hydrogen, and the hydrogen acts to passivate (deactivate) defects on the back surface of the P-type substrate.

The P ⁺ type microcrystalline silicon layer 16 was formed by the plasma CVD method in the mixed gas atmosphere of SiH ₄ , H ₂ and B ₂ H ₆ as in the case of the P type amorphous silicon layer 18. However, the gas composition ratio during the formation of the P ⁺ -type microcrystalline silicon layer 16 is H ₂ / SiH ₄ = 150 and B.
₂ H ₆ / SiH ₄ = 0.01 and the input RF power was 100 W. In addition, the P ⁺ -type microcrystalline silicon layer 16
Was set to a constant thickness of 100 nm.

From FIG. 2, the ultrathin P-type amorphous silicon layer 1 is formed.
It can be seen that by providing 8 between the P-type substrate 11 and the P ⁺ -type microcrystalline silicon layer 16, the short-circuit current, open circuit voltage and conversion efficiency are improved. This is a P-type substrate 11, P
The result is that the strong back surface electric field formed by the type amorphous silicon layer 18 and the P ⁺ type microcrystalline silicon layer 16 can efficiently push minority carriers near the back surface back into the substrate.

By the way, the P-type amorphous silicon layer 18 is
It has a higher specific resistance than the P-type crystalline silicon substrate 11. Therefore, as can be seen from FIG. 2, if the thickness of the P-type silicon layer 18 becomes too large, various characteristics of the solar cell will rather deteriorate. Therefore, the P-type amorphous silicon layer 18 preferably has a thickness of 20 nm or less. On the other hand, when the thickness of the P-type amorphous silicon layer 18 is 2 nm or less, the amorphous silicon layer 18 becomes heterogeneous and good characteristics of the solar cell cannot be obtained. Therefore, it is also desired that the P-type amorphous silicon layer 18 has a thickness of 2 nm or more.

FIG. 3 shows the relationship between various characteristics of the solar cell of the type shown in FIG. 1 and the thickness of the P ⁺ -type microcrystalline silicon layer 16. That is, in FIG. 3 (A), (B),
In each of the graphs (C) and (D), the horizontal axis represents the film thickness (nm) of the P ⁺ -type microcrystalline silicon layer, and the vertical axes represent short-circuit current (mA / cm ² ), open-circuit voltage (mV), and The fill factor and conversion efficiency (%) are shown. In FIG. 3, the thickness of the P-type amorphous silicon layer 18 is set to a constant value of 10 nm.

FIG. 3 according to the present invention and FIG. 7 according to the prior art.
, P in FIG. ⁺Type microcrystalline silicon
Since the layer 16 is extremely thin,
The improvement in battery characteristics appears, and the absolute amount of the improvement effect is also shown in Fig. 7.
It turns out that it is larger than the case.

FIG. 4 schematically shows a sectional structure of a solar cell according to another embodiment of the present invention. The solar cell of FIG. 4 is similar to that of FIG. 1, but the P-type crystalline silicon substrate 1
The bottom surface of 1 is a passivation film 19 made of a thermal oxide film layer.
Is covered by. This passivation film 19
Are provided to inactivate surface defects on the back surface of the silicon substrate 11. A plurality of openings are opened in the passivation film 19, and the P-type amorphous silicon layer 18 formed so as to cover the bottom surface of the passivation film 19.
Are connected to the substrate 11 through those openings. The P ⁺ type microcrystalline silicon layer 16a covers the P type amorphous semiconductor layer 18a.

That is, in the solar cell of FIG. 4, the P ⁺ -type microcrystalline silicon layer 16a is connected to the substrate 11 through the P-type amorphous silicon layer 18a in the plurality of limited openings of the passivation film 19. Therefore, recombination of carriers in the vicinity of the interface between the substrate 11 and the back surface electric field layer 16a can be further reduced.

FIG. 5 schematically shows a sectional structure of a solar cell according to still another embodiment of the present invention. The solar cell of FIG. 5 is similar to that of FIG. 4, but the P-type amorphous silicon layer 18b and the P ⁺ -type microcrystalline silicon layer 16b are formed only in the openings formed in the passivation film 19. . It will be easily understood that the solar cell shown in FIG. 5 can be improved in various characteristics similarly to the solar cell shown in FIG.

In the above embodiments, the case of using the P-type silicon substrate has been described, but it will be easily understood that the present invention can be applied to the case of using the N-type silicon substrate. In that case, P is formed on the light-incident side surface of the N-type silicon substrate.
Needless to say, the amorphous silicon layer and the microcrystalline silicon layer for forming the back surface electric field are doped with N-type impurities.

Further, instead of the amorphous silicon layer, an amorphous silicon carbon layer or an amorphous carbon layer can be used, and instead of the microcrystalline silicon layer, a microcrystalline silicon carbon layer, a microcrystalline carbon layer, A microcrystalline diamond layer or the like can also be used.

[0032]

As described above, according to the present invention, the back surface field effect can be enhanced by inserting the amorphous semiconductor layer between the crystalline semiconductor substrate and the back surface field layer of the microcrystalline semiconductor. As a result, a solar cell with improved short-circuit current, open circuit voltage, photoelectric conversion efficiency, etc. can be provided.

[Brief description of drawings]

FIG. 1 is a schematic cross-sectional view of a solar cell according to an embodiment of the present invention.

FIG. 2 is a graph showing the relationship between various characteristics and the thickness of the amorphous silicon layer in the solar cell of the type shown in FIG.

FIG. 3 is a graph showing the relationship between various characteristics and the thickness of the microcrystalline silicon layer in the solar cell of the type shown in FIG.

FIG. 4 is a schematic cross-sectional view showing a solar cell according to another embodiment of the present invention.

FIG. 5 is a schematic cross-sectional view showing a solar cell according to still another embodiment of the present invention.

FIG. 6 is a schematic cross-sectional view showing an example of a solar cell according to the prior art.

FIG. 7 is a graph showing the relationship between various characteristics and the thickness of the microcrystalline silicon layer in the solar cell of the type shown in FIG.

[Explanation of symbols]

11 P-type crystalline silicon substrate 12 N-type layer 13 Silicon oxide film 14 Antireflection film 15 Grid electrode 16, 16a, 16b P ⁺ type microcrystalline silicon layer 17 Backside electrode 18, 18a, 18b P-type amorphous silicon layer 19 Passivation film

Front page continuation (72) Inventor Ichiro Yamazaki 22-22 Nagaike-cho, Abeno-ku, Osaka-shi, Osaka

Claims (7)

[Claims] 1. A first-conductivity-type crystalline semiconductor substrate, a second-conductivity-type semiconductor layer formed on the light-incident-side surface of the substrate, and formed on a back surface of the substrate opposite to the light-incident side. First done
A solar cell comprising: a conductive type amorphous semiconductor layer; and a first conductive type microcrystalline semiconductor layer formed on the amorphous semiconductor layer and having a dopant concentration higher than that of the substrate. . 2. The solar cell according to claim 1, wherein the amorphous layer contains hydrogen. 3. The amorphous semiconductor layer has a thickness of 2 nm or more and 20 nm or more.
The solar cell according to claim 1 or 2, which has a thickness within a range of nm or less. 4. The amorphous semiconductor layer is in contact with the back surface of the substrate in a plurality of contact holes formed in a passivation film formed on the back surface of the substrate. The solar cell according to any one of Items 1 to 3. 5. The amorphous semiconductor layer includes at least one selected from amorphous silicon, amorphous silicon carbon, and amorphous carbon.
The solar cell according to any one of items 1 to 4. 6. The microcrystalline semiconductor layer contains at least one selected from microcrystalline silicon, microcrystalline silicon carbon, microcrystalline carbon, and microcrystalline diamond, according to any one of claims 1 to 5. The solar cell described in the section. 7. The solar cell according to claim 1, wherein the crystalline semiconductor substrate contains silicon.