JP2002076396A - Multi-junction thin-film solar cell and method of manufacturing the same - Google Patents

Abstract

(57) Abstract: A multi-junction thin-film solar cell in which a plurality of pin-type cells are stacked has a high conversion efficiency and is easy to manufacture.
Provided are an a-Si solar cell and a method for manufacturing the same. A semiconductor layer having a lower refractive index than another semiconductor layer of a lower cell, for example, an n-type microcrystalline silicon layer having a refractive index of 2.5 to 3 is provided at a boundary between an upper cell and a lower cell. Provide. As a manufacturing method, a substrate temperature of 100 ° C. or less and 100 ° C.
By forming a film with a hydrogen dilution of less than 2 times, the refractive index becomes 3
The following microcrystalline silicon layer is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a multi-junction thin-film solar cell comprising a plurality of pin-type cells mainly composed of amorphous silicon (hereinafter referred to as a-Si) and a method of manufacturing the same.

[0002]

2. Description of the Related Art a-Si solar cells are made of thin films, low-temperature processes,
The development of low-cost solar cells is being promoted as a favorite because of the feature that the area can be easily increased. However, this a-Si
A solar cell has a problem that its conversion efficiency is lower than that of a bulk crystalline solar cell made of single-crystal Si, polycrystalline Si, or the like.
This is because the band gap of the a-Si film is as wide as 1.7 to 1.8 eV, and the wavelength having the spectral sensitivity is 300 to 80 due to the influence.
This is due to the narrowness of 0 nm.

As a method for solving this problem, a tandem cell with a lower cell (hereinafter referred to as a bottom cell) of a narrow gap has been proposed. The main narrow gap materials are amorphous silicon germanium (a-SiGe), thin-film polycrystals and thin-film microcrystals, all of which are a-
Like Si, it can be formed by a plasma CVD method.

Using these narrow gap materials,
A short circuit current (hereinafter, referred to as Jsc) of about ²⁰ to 28 mA / cm ² can be obtained by combining the upper cell (hereinafter, referred to as the top cell) and the bottom cell.
Jsc of 14 mA / cm ² will be taken over. However, since the effect of reflected light can hardly be expected in the top cell, the film thickness needs to be increased to 200 to 300 nm, which causes a problem in the characteristic aspect that the fill factor (hereinafter referred to as FF) is reduced, and a material. There is a cost problem that the cost increases.

As a means for solving the problem of characteristics, a metal oxide having a low refractive index called a mirror layer is sandwiched between n / p junctions between a top cell and a bottom cell, and the difference in refractive index is used intentionally. To reflect some light on the mirror layer,
A technique to increase the short-circuit current of the top cell was proposed by Fisher et al. Of New Chatel University [25th IEEE
PVSC 1053-1056].

Japanese Patent Publication No. 2-37116 discloses that a microcrystalline semiconductor layer is interposed between pin cells made of an amorphous semiconductor in a multi-junction type photovoltaic device in which a plurality of pin cells are stacked. I have. However, the microcrystallized semiconductor layer is for eliminating backward rectification between cells, and has a thickness as thin as 10 nm. No consideration is given to the refractive index. Absent. Therefore, the above publication has a different purpose and configuration from the present invention.

[0007]

Since the metal oxide serving as the mirror layer is formed by sputtering or vapor deposition,
It is difficult to incorporate into a plasma CVD apparatus, and requires two film forming apparatuses. Furthermore, since the film is exposed to the air between the bottom cell film formation and the mirror layer film formation and between the mirror cell film formation and the top cell film formation, there are problems such as incorporation of impurities and generation of pinholes.

[0008] The object of the present invention is to solve these problems,
An object of the present invention is to provide an a-Si solar cell which has high conversion efficiency (hereinafter referred to as Eff) without incorporation of impurities and generation of pinholes, and which is easy to manufacture, and a method for manufacturing the same.

[0009]

In order to solve the above-mentioned problems, the present invention provides a multi-junction thin-film solar cell comprising a plurality of pin-type cells stacked on each other, the two cells forming a boundary between an upper cell and a lower cell. At least one or a part of the layer is a low-refractive-index layer having a lower refractive index than a semiconductor layer above the layer or a part thereof.

The lowermost layer or part of the upper pin cell may have a low refractive index, or the uppermost layer or part of the lower pin cell may have a low refractive index. Alternatively, a low refractive index layer having a lower refractive index than the semiconductor layer of the upper cell may be provided at the boundary between the upper cell and the lower cell. When a semiconductor layer having a low refractive index is provided, the layer plays the same role as the mirror layer, reflects light, and increases the short-circuit current of the upper cell. In addition, since the film can be formed using the same device as the other semiconductor layers, the film is not taken out of the film forming device and exposed to the air as in the case of the mirror layer made of metal oxide.

In particular, it is assumed that the low refractive index layer is an n-type microcrystalline silicon (hereinafter referred to as μc-Si) film. The μc-Si film is a mixed crystal of crystal grains of several tens of nm in size and a-Si. As evidenced by the experiments described later, an n-type μc-Si layer is easier to realize a film having a lower refractive index than a p-type μc-Si layer. Further, even if an a-Si layer can realize a film having a low refractive index, it has a high specific resistance and is not suitable for practical use.

It is assumed that the low refractive index layer has a refractive index in the range of 2.5 to 3. The refractive index of silicon is about 3.5,
The refractive indices of the microcrystalline silicon thin film and the amorphous silicon thin film have almost similar values. Therefore, μ having a refractive index exceeding 3
In the c-Si layer, the effect of reflecting light is reduced. It is practically difficult to form a μc-Si layer having a refractive index of less than 2.5.

The thickness of the i-layer of the upper pin cell is 70 to 20.
It is set to 0 nm. As evidenced by the experiments described below, if the thickness is less than 70 nm, the light absorption is insufficient and the short-circuit current is small. If the thickness exceeds 200 nm, the reflection is reduced and the short-circuit current is reduced. As a method for manufacturing a multi-junction thin-film solar cell as described above, a plasma CVD method with a film formation temperature of 60 to 110 ° C. and a hydrogen dilution degree of 65 to 140 times is formed on the uppermost p-type μc-Si layer of the lower cell. By the method, the lowermost n-type μc-Si layer of the upper cell is formed.

As evidenced by the experiments described later, an n-type μc-Si layer having a refractive index of 3 or less can be obtained by using these film forming conditions. Under other conditions, an n-type μc-Si layer having a refractive index of 3 or less cannot be obtained.

[0015]

[Embodiment 1] a-SiGe is applied to the bottom cell.
An example in which a cell is applied will be described. FIG. 1 is a cross-sectional structural view of the tandem cell created. A-Si / a-SiG with 1cm ² area
e It is a solar cell. A silver (Ag) thin film is provided as a lower electrode 2 on a glass substrate 1. On the lower electrode 2, a bottom n layer 3 of a-Si, a bottom i layer 4 of a-SiGe, μc-
Si bottom p layer 5, μc-Si first top n layer 6, amorphous silicon oxide (hereinafter a-SiO) second top n layer 7, a-Si top i layer 8, a-SiO A top p / i interface layer 9 and an a-SiO top p layer 10 are laminated, and an upper electrode 11 of indium tin oxide (hereinafter referred to as ITO) is provided on the surface of the top p layer 10.

The manufacturing process of the prototype cell will be described below. Asahi Glass Co., Ltd. U-type tin dioxide as the substrate 1 was used (hereinafter referred to as SnO ₂₎ with a glass substrate. A metal electrode 2 having a thickness of 100 to 20 on a glass substrate 1 by a sputtering method.
A silver (Ag) thin film of 0 nm was formed.

Next, a-Si based films 3 to 3 are formed by plasma CVD.
Ten films were formed. First, the substrate temperature is set to 130 to 17
At 0 ° C., monosilane (hereinafter referred to as SiH ₄ ) as the main gas,
Doping gas phosphine (hereinafter referred to as PH _3), hydrogen (hereinafter referred to as H ₂₎ as the diluent gas, the film thickness 10-20
A bottom n layer 3 of a-Si of nm is formed, and then the substrate temperature is set to 2 nm.
When the temperature is set to 100 to 250 ° C., SiH ₄ and germane (hereinafter, referred to as GeH ₄ ) are used as main gases, and H ₂ is used as a diluent gas.
A bottom i-layer 4 made of 0-nm a-SiGe was formed. Here, the band gap of the i-layer 4 was 1.45 eV.

Next, at a substrate temperature of 85 ° C., SiH ₄ is used as a main gas, diborane (hereinafter referred to as B ₂ H ₆ ) as a doping gas,
Hydrogen (hereinafter referred to as H ₂ ) is used as a diluent gas,
A 0 nm bottom p layer 5 of μc-Si was formed. At this time, the hydrogen dilution (H ₂ / SiH ₄ ) was 200 times, and the doping amount was B ₂ H ₆ / SiH ₄ = 0.1 to 1%. Care must be taken when forming μc-Si. This is because an initial transition layer of a-Si having a thickness of about 10 nm is formed depending on film forming conditions. Since the initial transition layer of a-Si is a p-layer of a-Si, the presence of the transition layer adversely affects the absorption coefficient and Voc. Therefore, the film formation conditions must be set so that the initial transition layer of a-Si is not formed. The presence or absence of the a-Si layer can be confirmed by TEM observation. According to the observation results, the presence or absence of the initial transition layer greatly depends on the substrate temperature. At a substrate temperature of 150 ° C. or higher, a transition layer is formed, but at a film forming temperature of 120 ° C. or lower, the initial transition layer does not exist. However, it was found that μc-Si was formed from the interface layer. In this embodiment, since the substrate temperature is 85 ° C., there is no initial transition layer.

After forming the bottom p layer 5 of μc-Si, the same substrate temperature is used.
Degree (85 ° C), SiH_FourThe main gas, PH _ThreeDoping gas
S, H_TwoIs used as a diluting gas, and μc-Si
The first top n layer 6 was formed. Doping amount at this time
Is the PH_Three/ SiH_Four= 0.2-2, and the hydrogen dilution degree is 75-
100 times. What is important here is that the substrate temperature is 100
° C or lower and the hydrogen dilution degree to 100 times or lower.
Thus, a film having a refractive index of 3 or less can be obtained. Sand
The first top n layer plays the role of mirror layer
Become. In this example, a film having a refractive index of 2.65 was obtained.

A substrate temperature of 130 to 170 ° C., a main gas of SiH ₄ and carbon dioxide (CO ₂ ), a doping gas of PH ₃ and a diluting gas of H _2, and a film thickness of 10 to 20 nm
A second top n-layer 7 of a-SiO was formed into a top n-layer having a two-layer structure. The refractive index of the second top n-layer 7 is about 3.5. After that, using SiH ₄ as a main gas and H ₂ as a diluent gas, a top i layer 8 of a-Si having a film thickness of 80 to 300 nm, again doping SiH ₄ and carbon dioxide (CO ₂ ) as a main gas and B ₂ H ₆ Gas, H
_{Using 2} as a diluent gas, a top interface layer 9 of a-SiO ₂ having a thickness of 5 to 20 nm and a top p layer 10 having a thickness of 4 to 15 nm were sequentially formed. The doping amount of the top interface layer 9 and the top p layer is
B ₂ H ₆ / SiH ₄ = 20 to 500 ppm, 0.5 to 3%, respectively
And Finally, ITO having a thickness of 80 to 300 nm was formed as the transparent electrode 11 by a sputtering method.

In addition to the solar cell completed in this manner, a solar cell having a μc-Si first top n-layer having a refractive index of 3.47 was prototyped as a comparative example. First Top n of μc-Si of Comparative Example
The layers were formed under the same conditions as in the example except that the film formation temperature was 150 ° C. That is, in the solar cell of this comparative example, the mirror layer does not exist. FIG.
A-Si / a-SiG of Example 1 and Comparative Example thus produced
FIG. 7 is a characteristic relation diagram showing a relation between a film thickness of a top i-layer 8 and cell characteristics in a tandem cell. The cells of the examples of the present invention are indicated by ●, and the comparative examples are indicated by ○.

From these results, it can be seen that in the a-Si / a-SiGe tandem cell of this embodiment, Jsc increases in the region where the top i layer is thin. In the present embodiment, the film thickness of the optimum matching that maximizes Jsc is about 300 nm, whereas in the present embodiment, it is as thin as about 180 nm.
It can be seen that the same Jsc can be obtained with a top i-layer thickness of about 60 to 70% as compared with the comparative example. With respect to the open circuit voltages Voc and FF, there is little difference between the example and the comparative example.

In the a-Si cell, as the thickness of the i-layer becomes thinner, the internal electric field becomes stronger, thereby improving the FF. In the present embodiment, since the top i-layer for optimal matching can be made thinner, the FF under optimal conditions is improved. As a result, the highest efficiency of this embodiment is 12.1% (at the time of the top i layer of 180 nm), which is about 0.5 point higher than the highest efficiency of the conventional example of 11.6% (at the time of the top i layer of 240 nm). .

These effects are apparently due to the mirror effect in which the amount of reflection of incident light is increased because the first top n layer 6 having a low refractive index is provided at the boundary between the top cell and the bottom cell. It is noted that Japanese Patent Publication No. 2-37116 discloses that a microcrystalline layer is sandwiched between pin cells made of an amorphous semiconductor in a multi-junction type photovoltaic device in which a plurality of pin cells are stacked. However, it was proved by this comparative example that the efficiency was not improved even if a microcrystalline layer not considering the refractive index was interposed between the cells.

In this embodiment, the case where microcrystalline silicon having a refractive index of 2.65 is applied has been described. However, it has been confirmed that a mirror effect can be obtained by applying a film having a refractive index of 3 or less. . Here, an experiment conducted to investigate the relationship between the μc-Si layer deposition conditions and the refractive index will be described.

First, a Corning 7059 substrate was prepared as a substrate, and a 300 μm-thick μc-Si
A film was formed. Next, the transmission spectrum of this sample was evaluated, and the refractive index was derived from the transmittance that became a valley in the wavelength range of 1000 to 2000 nm. Wavelength 1000-20
Since the refractive index of 00 nm is evaluated, the refractive index may be slightly overestimated due to the effect of dispersion.

FIG. 3 shows the relationship between the substrate temperature and the refractive index when forming an n-type μc-Si film formed at a hydrogen dilution ratio (H ₂ / SiH ₄ ) of 100. Here, the n-type impurity doping amount is PH ₃ / Si
H ₄ was fixed at 1%. From this figure, it was found that the refractive index decreased as the substrate temperature decreased, and the refractive index became 3 or less at a substrate temperature of 100 ° C. or less. The refractive index was 2.78 at a substrate temperature of 85 ° C. When the temperature was lowered to 50 ° C., a large amount of powder was generated and no film was formed.

FIG. 4 shows the result of changing the degree of hydrogen dilution while fixing the substrate temperature at 85 ° C. The lower the hydrogen dilution, the lower the refractive index. The refractive index became 2.65 at a hydrogen dilution of 75 times. When it was reduced to 50 times, powder was generated and did not become a film. FIG. 5 shows the relationship between the substrate temperature and the refractive index when forming a p-type μc-Si film formed at a hydrogen dilution of 200 times. Here, the impurity doping amount is B ₂
H ₆ / SiH ₄ was fixed at 0.5%. The p-type film is hardly crystallized as generally known, and therefore, the hydrogen dilution degree is 2
If it was not 00 times or more, it did not crystallize.

From these results, it was found that the refractive index of the p-type μc-Si film hardly changed even when the substrate temperature was lowered. n
Although the behavior differs from that of the mold film, it is not yet clear whether this is due to a difference in dopant or a difference in hydrogen dilution. However, p
It is considered that it is difficult to obtain a p-type μc-Si film having a low refractive index, considering that it is not possible to make the dilution too low with a type film.

Finally, in this embodiment, the low refractive index μc-Si
Although the application of the film has been described, the reason why the a-Si film is not suitable is described. With an a-Si film, it is difficult to reduce the refractive index to 3 or less.
There is a problem that the film cannot be used for a device because the film has a low density and the electric resistance is significantly increased. On the other hand, the μc-Si film has crystal grains of several tens nm in size and a-
It is a mixed crystal of Si. It is considered that the reason why the refractive index becomes low is that the a-Si portion becomes low in density and becomes void-rich, but if the crystal grains are in contact with each other even a little, a current flows through this portion. Low resistance. Therefore, good cell characteristics were obtained only by using μc-Si for the mirror layer.

Japanese Patent Application Laid-Open No. 6-181331 discloses that an amorphous semiconductor solar cell is provided with a low refractive index layer between a pin cell made of an amorphous semiconductor and an electrode on the light incident side. However, the purpose of the low refractive index layer is to reflect the reflected light, and in this embodiment, the titanium oxide layer (refractive index =
1.9) does not reflect incident light while partially transmitting it between the stacked cells as in the present invention, and further does not intend a semiconductor layer having a low refractive index. Therefore, this publication also has a different purpose and configuration from the present invention.

[0032]

As described above, according to the present invention, p
In a multi-junction thin-film solar cell in which in-type cells are stacked,
By providing a semiconductor layer having a low refractive index at the boundary between the upper cell and the lower cell, the incorporation of impurities and the generation of pinholes can be suppressed, and a multi-junction thin-film solar cell with high conversion efficiency can be obtained. In particular, by using microcrystalline silicon as a low-refractive-index film, it is possible to perform cell film formation collectively with one apparatus, and to reduce equipment costs and manufacture low-cost solar cells with high yield. it can. Detailed manufacturing methods are also described.

Therefore, the present invention greatly contributes to the practical use and spread of multi-junction thin-film solar cells.

[Brief description of the drawings]

FIG. 1 is a sectional structural view of an a-Si / a-SiGe tandem cell according to the present invention.

FIG. 2 is a graph showing the dependence of the characteristics of the tandem cells of the embodiment of the present invention and the comparative example on the thickness of the top i-layer.

FIG. 3 is a characteristic diagram showing the relationship between the substrate temperature and the refractive index when forming an n-type microcrystalline silicon film formed at a dilution of 100 times.

FIG. 4 is a characteristic diagram showing the relationship between the degree of dilution and the refractive index of an n-type microcrystalline silicon film formed at a substrate temperature of 100 ° C.

FIG. 5 is a characteristic diagram showing the relationship between the substrate temperature and the refractive index when forming a p-type microcrystalline silicon film formed at a dilution of 200 times.

[Explanation of symbols]

 Reference Signs List 1 substrate 2 metal electrode 3 bottom cell n layer 4 bottom cell i layer 5 bottom cell p layer 6 first top cell n layer 7 second top cell n layer 8 top cell i layer 9 top cell p / i interface layer 10 top cell p layer 11 Transparent electrode

Claims (8)

[Claims] In a multi-junction thin-film solar cell in which a plurality of pin cells are stacked, at least one or a part of two layers forming a boundary between an upper cell and a lower cell is the layer or the layer. A multi-junction thin-film solar cell, which is a low-refractive-index layer having a lower refractive index than a semiconductor layer above a part thereof. 2. The multi-junction thin-film solar cell according to claim 1, wherein the lowermost layer or a part of the upper pin cell has a low refractive index. 3. The multi-junction thin-film solar cell according to claim 1, wherein the uppermost layer of the lower pin cell or a part thereof has a low refractive index. 4. A multi-junction thin-film solar cell comprising a plurality of pin cells stacked on each other, a low refractive index having a lower refractive index than a semiconductor layer of the upper cell at a boundary between the upper cell and the lower cell. A multi-junction thin-film solar cell comprising a layer. 5. The thin-film solar cell according to claim 1, wherein the low refractive index layer is an n-type microcrystalline silicon layer. 6. The multi-junction thin-film solar cell according to claim 5, wherein the low refractive index layer has a refractive index in the range of 2.5 to 3. 7. The thickness of the i-layer of the upper pin cell is 70 to 20.
The multi-junction thin-film solar cell according to claim 5, wherein the thickness is 0 nm. 8. A semiconductor layer in which a plurality of pin type cells are stacked, and one or a part of two layers forming a boundary between an upper cell and a lower cell is a semiconductor layer above the layer or a part thereof. In the method for manufacturing a multi-junction thin-film solar cell which is a low refractive index layer having a lower refractive index than that of
A low-refractive-index n-type microcrystalline silicon layer is formed on the microcrystalline silicon layer by a plasma CVD method at a film forming temperature of 60 to 110 ° C. and a hydrogen dilution degree of 60 to 140 times. A method for manufacturing a junction type thin film solar cell.