JP3248227B2 - Thin film solar cell and method of manufacturing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

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

[0002]

2. Description of the Related Art A solar cell mainly composed of a-Si formed by glow discharge decomposition of a source gas or a photo-CVD method has features of being thin and capable of being easily enlarged, and is expected to be a low-cost solar cell. I have. As a structure of this type of solar cell, a pin-type a-Si solar cell having a pin junction is generally used. FIG. 2 shows a structure of such a solar cell, in which a transparent electrode 2, a p-type a-Si layer 31, a p /
It is manufactured by sequentially laminating an i interface layer 4, an i-type a-Si layer 5, an n-type a-Si layer 6, and a metal electrode 7. In this solar cell, power is generated by light incident through the glass substrate 1.

Here, photocarriers contributing to power generation are mainly generated in the i-layer, and the p and n layers are dead layers. Therefore, in a solar cell in which light is incident from the p-layer, it is important to increase the light transmittance of the p-layer corresponding to the window layer so that as much light as possible reaches the i-layer in order to increase the output. . To that end, it is effective to increase the optical gap Eg of the p-layer to reduce the optical absorption loss. For this purpose, a carbon atom may be added to the p-type a-Si layer as known in, for example, JP-A-56-64476, or a nitrogen atom may be added as known in JP-A-57-181176. Or an oxygen atom as known in JP-A-56-142680, or JP-A-58-19.
Attempts have been made to add oxygen atoms and carbon atoms as known in JP-A-6064 or JP-A-61-242085.

[0004]

A large amount of C is formed during the formation of a p-layer.
The optical gap can be increased by alloying by adding N or N. However, at the same time, a large amount of dangling bonds are induced in the film, so that the electric conductivity is reduced. Usually, in an a-Si solar cell, the electric conductivity at 25 ° C. of a film used for a p-layer is limited to 10 ⁻⁸ S / cm or more in order to suppress a decrease in the fill factor. In order to satisfy this condition, the optical gap has to be set to 2 eV or less in the above-mentioned p-type a-SiC: H (B) and p-type a-SiN: H (B). Therefore, the light absorption loss in the p-layer is relatively large, and is 1 to ² mA / cm ² in terms of short-circuit current density.
Loss. Also, according to the description of JP-A-61-242085, undoped a-Si: O: C:
In the H film, the electric conductivity (σ _d ) is 10 when the optical gap is 2.0 eV or more when AMI (simulated sunlight) is irradiated at 100 mW / cm ^2.
It is about ^-12 to 10 ^-13 s / cm, and it is not clear how much the optical layer has a p-layer when doped.

An object of the present invention is based on the above-mentioned situation, and is based on the above-mentioned situation, and achieves higher efficiency by using an a-Si based film having an electric conductivity of 0.5 to 1 × 10 ⁻⁶ S / cm and a larger optical gap as a window layer. An object of the present invention is to provide a thin-film solar cell and a method for manufacturing the same.

[0006]

According to the present invention, there is provided a thin-film solar cell having a pin junction structure containing amorphous silicon as a main material.
The p-layer or the n-layer on the light incident side of the layer has the general formula aS
i is represented by _{(1-X) O X,} 0.10 <X < consists amorphous silicon oxide which is 0.40, and the to the optical gap is not 2.0 eV 2.2 eV, the light conductivity at 25 ° C. der ratio of dark conductivity of 5 or less as is, amorphous Shirikon'o
Kisaido source of oxygen is carbon dioxide der shall. Here, the layer on the light incident side of the i-layer is the p-layer, and it is effective that an amorphous silicon carbide layer having a lower addition amount of III impurity than the p-layer is interposed between the p-layer and the i-layer. . Also,
According to the present invention, p-i containing amorphous silicon as a main material
In the thin-film solar cell having the n-junction structure, the light of the i-layer
The p-layer or the n-layer on the incident side has the general formula a-Si
It is represented by _{(1-X) O X,} 0.10 <X <0.40 and is amorphous
Made of porous silicon oxide and its optical gap
Is 2.0 eV to 2.2 eV, photoconductive at 25 ° C.
Thin-film solar cell with a ratio of conductivity to dark conductivity of 5 or less.
The layer on the light incident side of the i-layer is a p-layer, and an amorphous silicon oxide layer having a lower addition amount of III is lower than the p-layer between the p-layer and the i-layer. Further, according to the present invention, in a method for manufacturing a thin-film solar cell having a pin junction structure mainly composed of amorphous silicon, the p-layer or the n-layer on the light incident side of the i-layer may be monosilane. Amorphous silicon oxide generated on a substrate by adding a gas containing impurities for doping to a mixed gas in which carbon dioxide is diluted with hydrogen of 10 to 50 times that of monosilane and decomposing at a substrate temperature of 150 to 200 ° C. Shall be formed by Here, it is effective to decompose the mixed gas by glow discharge.

[0007]

[Function] SiH_Four, CO_Two, H_TwoGlow discharge of mixed gas
The film formed by decomposition should not capture more than 0.5% of carbon.
A-Si_(1-x)O_x0.10 <x <0.
A-SiO which is 40 is obtained. Generally, SiH_Four, O_TwoEtc.
The film formed is SiH in the gas phase._FourAnd O_TwoReacts violently
Si_TwoIs generated, and this is taken into the membrane and
This results in a film with many defects. In contrast, the above method
Then SiH_FourAnd CO _TwoDoes not react in the air,_TwoBut
A good film with few defects is not generated.
Furthermore, the optical gap is set in the range of 2.0 to 2.3 eV,
Conductivity σ_phAnd dark conductivity σ_dRatio σ_ph/ Σ_dNot more than 5
Electrical conductivity in the optical gap of 2.0 eV or more
Ten^-8P-i- with p or n layer of S / cm or more as window layer
An n-junction thin-film solar cell is obtained. And such a membrane
A-SiO with its characteristics can be obtained at substrate temperature and hydrogen dilution during film formation.
It can be formed by adjusting the degree of inclination.

[0008]

FIG. 1 shows a cross-sectional structure of an a-Si solar cell according to one embodiment of the present invention. The same reference numerals as in FIG. 2 denote the same parts. This solar cell is manufactured as follows. First, SnO ₂ was used as a transparent electrode 2 on a glass substrate 1.
Is formed to a thickness of 5000 to 10000 mm. This substrate was mounted on a plasma CVD apparatus, and SiH ₄ (monosilane),
A p-type a-SiO film 3 is formed by a glow discharge decomposition method using CO ₂ as a main gas, H ₂ as a diluent gas, and B ₂ H ₆ as a doping gas.
Is formed to a thickness of 100 to 200 mm. Here, the addition amount as _{B 2 H 6 / SiH 4 =} 0.2 ~1% are of B ₂ H _6, the substrate temperature during film formation 140 to 250 DEG ° C., hydrogen dilution (H ₂ / SiH ₄₎
Is in the range of 10 to 50. Subsequently, a SiH _4, CH ₄ and main gas, the a-SiC and H ₂ as diluent gas p / i interface layer 4 to a thickness of 50 to 200 Å. This p / i interface layer 4
Was added without boron, and the film forming conditions were selected so that the optical gap was an intermediate value (1.8 to 2.0 eV) between the p layer 3 and the i layer. Further, after the substrate temperature was raised to 200 to 250 ° C., SiH ₄ was used as a main gas and H ₂ was used as a diluent gas to obtain i-type
A Si layer 5 is formed to a thickness of 2000 to 5000 mm, and SiH ₄
Is used as a main gas, H ₂ as a diluent gas, and PH ₃ as a doping gas to form an n-type a-Si layer 6 having a thickness of 150 to 300 °.
Finally, the metal electrode 7 is formed by vapor deposition of Ag or Al or by sputtering and patterning.

[0009] SiH _4, the CO ₂ main gas, a film formed of H ₂ as diluent gas, the carbon concentration in the film less 0.5% a-
It becomes an SiO film. This was confirmed by analyzing the composition of the formed film by X-ray photoelectron spectroscopy (XPS).
FIG. 3 shows the gas flow ratio (CO ₂ / SiH ₄ ) and the composition ratio of Si, O, and C determined by XPS. Carbon is below the detection limit (0.5%) in all films.

Next, the conditions for forming the a-SiO film are as follows:
That is, the influence on the relationship between the electric conductivity and the optical gap was examined. First, the following experiment was performed to investigate the effect of the hydrogen dilution degree. FIG. 4 shows the results. Gas used: SiH ₄ , CO ₂ , H ₂ , B ₂ H ₆ gas flow ratio: H ₂ / SiH ₄ = 10, 15, 20, 30, 50 B ₂ H ₆ / SiH ₄ = 0.006 CO ₂ / SiH ₄ = 0.5 to 8 Substrate temperature... 200 ° C. Pressure... 0.5 Torr From the results shown in FIG. High conversion efficiency can be obtained by using a-Si solar cells because electric conductivity is high as mentioned above.
It is a film of 10 ^-8 S / cm or more. Therefore, the electric conductivity is 1 ×
An optical gap of 10 ⁻⁶ S / cm was obtained. Hydrogen dilution 1
Looking at the films 0, 15, 20, 30, and 50, 2.0
They are 4 eV, 2.11 eV, 2.12 eV, 2.09 eV, and 2.03 eV. In other words, an optical gap of 2.0 eV or more can be obtained in any case, but particularly desirable is a hydrogen dilution degree of 20,
It has been found that very good characteristics can be obtained if the range is 15 to 30.

The following experiment was conducted to examine the effect of the substrate temperature. FIG. 5 shows the results. Gas used: SiH ₄ , CO ₂ , H ₂ , B ₂ H ₆ Gas flow ratio: H ₂ / SiH ₄ = 20 B ₂ H ₆ / SiH ₄ = 0.006 CO ₂ / SiH ₄ = 0.5 to 8 Substrate temperature: 140, 170, 200, 250 ° C. Pressure: 0.5 Torr From the results shown in FIG. 5, it can be seen that the characteristics of the a-SiO film greatly depend on the substrate temperature. Electric conductivity is 1 × 10 ^-6 S / cm
The optical gap becomes substrate temperature 140 ° C, 170 ° C, 200 ° C.
2.08 eV and 2.13 e
V, 2.12 eV, and 2.05 eV. That is, although a hydrogen gap of 2.0 eV or more can be obtained in each case, a particularly desirable substrate temperature is 170 ° C., and it is found that good film characteristics can be obtained in the range of 150 to 200 ° C. It should be noted that the electrical conductivity of 10 ^-8 S / cm or more is shown in FIGS.
It was found that it was obtained at 2.2 eV or less.

As described above, in terms of the hydrogen dilution degree and the substrate temperature, the optimum conditions are a hydrogen dilution degree of 20 and a substrate temperature of 170 ° C. At this time, the optical gap at which the electric conductivity is 1 × 10 ⁻⁸ S / cm is obtained. Is 2.13 eV. This optical gap is higher than the conventional a-SiC film and a-SiN film by 0.1 eV or more, and it can be seen that extremely excellent characteristics can be obtained.
Further, if the hydrogen dilution degree is 15 to 30 and the substrate temperature is in the range of 150 to 200 ° C., good characteristics such as an electric conductivity of 1 × 10 ⁻⁶ S / cm and an optical gap of about 2.1 eV can be obtained. It is understood that it is possible.

Table 1 shows the photoconductivity and dark conductivity at 25 ° C. of the p-type a-SiO film formed under the above-mentioned optimum film forming conditions, hydrogen dilution degree of 20 and substrate temperature of 170 ° C.

[0014]

[Table 1]

From Table 1, it was found that σ _ph / σ _d must be 5 or less in order to obtain an electric conductivity of 10 ⁻⁸ S / cm or more. Next, a p-type a-SiC film and a p-type a-
The characteristics of a single cell having the same structure as that shown in FIG. 1 manufactured by applying the SiO film and having a structure in which the p / i interface layer 4 was omitted were compared. Here, the p-layer, i-layer, and n-layer of the single cell had a thickness of 100 °, 4000 °, and 200 °, respectively. The conditions for forming the p-type a-SiO film applied to the cell are as follows.

Gas used: SiH ₄ , CO ₂ , H ₂ , B ₂ H ₆ gas flow ratio: H ₂ / SiH ₄ = 20 B ₂ H ₆ / SiH ₄ = 0.006 CO ₂ / SiH ₄ = 2.5 Substrate temperature: 170 ° C. Pressure: 0.5 Torr The characteristics of the obtained p-type a-SiO film were as follows.

Optical gap: 2.16 eV Photoconductivity: 1.1 × 10 ⁻⁶ S / cm Electrical conductivity: 0.5 × 10 ⁻⁶ S / cm Table 2 shows the results measured under irradiation with light of 100 mW / cm ² .

[0018]

[Table 2]

From this table, it can be seen that when the a-SiO film is applied to the p-layer, a larger J _SC is obtained than when the a-SiC film is applied, and the JSC is larger in each case with and without the p / i interface layer. It can be seen that high efficiency can be obtained. Moreover, p / of a-SiC
The effect of the i-interface layer insertion is as follows when the p-layer is made of a-SiC and when the
It turned out that it is almost the same as when it consists of -SiO. By the way, it has been considered that the role of the p / i interface layer is to alleviate lattice mismatch caused by different materials (for example, a-SiC and a-Si). Therefore, the p / i interface layer used so far is always of the same material as the p-layer or the i-layer, and has a composition intermediate between the p-layer and the i-layer. Therefore, the a-Si p-layer and the a-Si
No attempt has been made to insert an a / SiC p / i interface layer between the i-layers of the above, and it has been said that the conventional theory only generates a new lattice mismatch. However, in reality, the conversion efficiency is improved by inserting the p / i interface layer, which is a surprising fact that cannot be explained by the conventional theory.
We explain this fact in the following model. That is, the defects generated at the p / i interface are not caused by lattice mismatch, but are caused by boron in the p layer being doped in the i layer by autodoping. Therefore, at the p / i interface,
If an interface layer having a large optical gap without boron is inserted, autodoping of boron into the i-layer is suppressed, and interface defects can be reduced.

FIG. 6 shows the spectral sensitivity characteristics of two types of cells each having an a / SiC p / i interface layer inserted therein. From this figure, it can be seen that the short-wavelength light sensitivity of the cell using the p-type a-SiO film shown by the solid line 61 is higher than that of the cell using the p-type a-SiC film shown by the dotted line 62. From these results,
It can be seen that when the p-type a-SiO film is applied to the cell, the light absorption loss can be reduced as compared with the case where the p-type a-SiC film is used, so that high conversion efficiency with improved short-circuit current can be obtained.

Of course, a-SiO can also be used for the p / i interface layer 4. A-Si inserted into such a cell
The conditions for forming the p / i interface layer of O are as follows. Gas used: SiH ₄ , CO ₂ , H ₂ gas flow ratio: H ₂ / SiH ₄ = 20 CO ₂ / SiH ₄ = 0.25 Substrate temperature: 170 ° C. Pressure: 0 The characteristics of the film obtained at 5 Torr are as follows.

Optical gap: 2.00 eV Photoconductivity: 1.7 × 10 ^-6 S / cm Electrical conductivity: 1.2 × 10 ^-11 S / cm FIG. Shows the relationship between the characteristics of the cell having the p / i interface layer 4 and the thickness of the interface layer. As can be seen from the figure, the insertion voltage increases due to the insertion of the p / i interface layer, and the conversion efficiency improves. When the thickness of the p / i interface layer 4 is 90 °, V _OC = 0.899 V, J _SC = 18.81 mA / cm ² , and FF = 0.
The highest conversion efficiency of 12.5% was obtained with the 740. And
The thickness of the p / i interface layer is desirably 60 to 120 mm, but is preferably 30 to 18 mm.
It can also be seen that the effect is within the range of 0 °.

Although only the application of the a-SiO film to the p-layer has been described above, the application of the a-SiO film of the present invention to the n-layer is also effective. As a result of forming an n-layer under the following film forming conditions, as shown in FIG. 8, good optical properties were obtained with an optical gap of 2.1 eV at which the electrical conductivity was 1 × 10 ⁻⁶ S / cm. Gas used: SiH ₄ , CO ₂ , H ₂ , PH ₃ gas flow ratio: H ₂ / SiH ₄ = 20 PH ₃ / SiH ₄ = 0.01 CO ₂ / SiH ₄ = 0.5 to 8 Substrate temperature・ ・ 170 ℃ Pressure ・ ・ ・ ・ ・ 0.5Torr

[0024]

According to the present invention, the p layer is made of SiH ₄ , C
An optical gap of 2.0 to 2.2 eV, σ _ph / σ _d > 5 or more is formed by decomposing a mixed gas of O ₂ and H ₂ and controlling the substrate temperature and the hydrogen dilution degree during film formation to appropriate values. By doing so, the electrical conductivity is 10 ^-8 S / cm or more and the optical gap is 2.
A film having a voltage of 1 eV or more can be formed. This makes it possible to reduce the light absorption loss and improve the short-circuit current density as compared with the case where a conventional p layer having an optical gap of 2.0 eV or less is used. Further, in addition to inserting an interface layer of a-SiC or a-SiO formed without adding boron at the p / i interface, a solar cell having a higher conversion efficiency than the conventional p-layer is used. You can get a battery.

[Brief description of the drawings]

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

FIG. 2 is a cross-sectional view of a single cell of a conventional a-Si solar cell.

FIG. 3 is a relationship diagram between a gas flow ratio and a composition ratio of an a-SiO film.

FIG. 4 is a diagram showing the relationship between p-type a-SiO film characteristics and hydrogen dilution.

FIG. 5 is a diagram showing the relationship between p-type a-SiO film characteristics and substrate temperature.

FIG. 6 is a spectral sensitivity characteristic diagram of a cell using an a-SiO film as a p-layer and a cell using an a-SiC film.

FIG. 7 is a diagram showing the relationship between the characteristics of a solar cell in which ap / i interface layer of a-SiO is inserted and the interface layer thick film.

FIG. 8 is a diagram showing the relationship between the electrical conductivity of the n-type a-SiO film and the optical gap.

[Explanation of symbols]

 Reference Signs List 1 glass substrate 2 transparent electrode 3 p-type a-SiO layer 4 p / i interface layer 5 i-type a-Si layer 6 n-type a-Si layer 7 metal electrode

Claims (5)

(57) [Claims] 1. A pin comprising amorphous silicon as a main material.
In a thin-film solar cell having a junction structure, the p-layer or the n-layer on the light incident side of the i-layer has a general formula a-Si _(1-X) O _x
And 0.10 <X <0.40, and has an optical gap of 2.0.
It is no eV 2.2 eV, Ri der ratio of photoconductivity and dark conductivity is 5 or less at 25 ° C., the amorphous silicon oxide
Thin film solar cell source of oxygen and wherein the carbon dioxide der Rukoto. 2. A layer on the light incident side of the i-layer is a p-layer,
2. The thin-film solar cell according to claim 1, wherein an amorphous silicon carbide layer in which the amount of Group III impurities added is lower than that of the p layer is interposed between the layer and the i layer. 3. A pin using amorphous silicon as a main material.
Light incident on the i-layer in a thin-film solar cell having a junction structure
The p-layer or the n-layer on the side has the general formula a-Si _(1-X) O _x
Amorphous silicon satisfying 0.10 <X <0.40
And an optical gap of 2.0
eV to 2.2eV, photoconductivity and dark conductivity at 25 ° C
A thin-film solar cell having a power ratio of 5 or less, wherein a layer on the light incident side of the i-layer is a p-layer, and II is provided between the p-layer and the i-layer.
A thin-film solar cell comprising an amorphous silicon oxide layer in which the amount of I added is lower than that of a p-layer. 4. A pin using amorphous silicon as a main material.
In a method for manufacturing a thin film solar cell having a junction structure,
The p-layer or the n-layer on the light incident side of the layer is added to a mixed gas obtained by diluting monosilane and carbon dioxide with hydrogen 10 to 50 times that of monosilane and containing a gas containing doping impurities, and the substrate temperature is 150 to 200. A method for manufacturing a thin-film solar cell, wherein the thin-film solar cell is formed by amorphous silicon oxide generated on a substrate by decomposition at a temperature of ° C. 5. The method according to claim 4, wherein the decomposition of the mixed gas is performed by glow discharge decomposition.