JPH08139346A - Amorphous solar cell - Google Patents

Abstract

PURPOSE: To obtain an amorphous solar cell with high open-circuit voltage and high reliability. CONSTITUTION: An amorphous solar cell has a profile that the band gap of an i-layer, a photoelectric conversion layer, gradually decreases from the p-layer side until the minimum value is reached, and then gradually increases towards the n-layer side. One or more low light absorption layers 6 having a band gap above the minimum value is formed in the i-layer in proximity to the p-layer.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an amorphous solar cell, and more particularly to a highly efficient amorphous solar cell.

[0002]

2. Description of the Related Art Various attempts have been made in the above-mentioned amorphous solar cell in order to improve its efficiency.
For example, amorphous silicon (hereinafter, a-
It is called Si: H. When a solar cell is manufactured using only), the band gap is about 1.75 eV,
Even if the thickness of the i layer (intrinsic semiconductor layer) is increased, the wavelength is 800 n
Light above m is hardly available. Therefore, attempts have been made to effectively use light by using amorphous silicon germanium (hereinafter referred to as a-SiGe: H), which is a narrow band gap material. Since this a-SiGe: H has a large light absorption, the short-circuit current can be increased, but the level in the gap is a-S.
Since it has more than i: H, it has a drawback that the fill factor is lowered, and it is difficult to improve the efficiency.

In order to overcome this drawback, a method of providing a profile in the band gap of the i-layer has been proposed, and this method enables high efficiency to be obtained in a laminated solar cell using a-SiGe: H. became.

In the above method of providing a profile in the band gap of the i layer, as shown in FIG. 5, the band gap in the i layer is continuously changed, and the p layer (p-type semiconductor layer) and the n layer (n layer) are formed. (Type semiconductor layer), the band gap is wide on the side in contact with the p-type semiconductor layer, and the minimum band gap is provided near the p-layer on the light incident side. Such a band gap structure can be formed by changing the composition ratio of materials such as a-SiGe: H.

[0005]

In the solar cell having the above-mentioned bandgap profile structure, i
The closer the minimum bandgap of the layer is to the p-layer on the light incident side, the smaller the photodegradation and the higher the reliability of the device. This is because the collection of holes is improved as the light absorption distribution increases near the p-layer. However, when the minimum bandgap portion is formed in the vicinity of the p-layer in this way, there is a problem that the bandgap of the i-layer in the vicinity of the p-layer is reduced and the open circuit voltage is lowered. Also,
In this method, the bandgap of the i layer is reduced to increase the light absorption. However, when the band gap of the i layer is about 1.4 eV or less, the fill factor decreases,
The efficiency does not improve even if the amount of light absorption increases. Furthermore, in order to improve the open circuit voltage, amorphous silicon carbon with a wide band gap of about 2.1 eV (hereinafter referred to as aS
A method is known in which a layer referred to as iC: H) is provided at the p / i interface. However, in this method, a-SiC:
Since it is not possible to obtain a film having good film quality in the H layer,
There has been a problem that the light deterioration becomes large, which causes a decrease in the hole traveling after the light deterioration.

The present invention has been made in order to solve the problems of the prior art, and an object thereof is to provide an amorphous solar cell having high efficiency, high open circuit voltage and high reliability. .

[0007]

In the amorphous solar cell of the present invention, the band gap of the i-type semiconductor layer made of the amorphous semiconductor material formed between the p-type semiconductor layer and the n-type semiconductor layer is It has a minimum value between the end on the p-type semiconductor layer side and the end on the n-type semiconductor layer side, and is close to the end on the p-type semiconductor layer side between the minimum value and the end on the p-type semiconductor layer side. An amorphous solar cell having a profile that becomes larger as it gets closer to the end on the n-type semiconductor layer side between the minimum value and the end on the n-type semiconductor layer side. One or more low light absorption layers having a band gap equal to or larger than the minimum value are formed in the i-type semiconductor layer closer to the i-type semiconductor layer, thereby achieving the above object.

The amorphous solar cell of the present invention may have a structure in which two or more i-type semiconductor layers having the low light absorption layer are laminated.

[0009]

In the present invention, the low light absorption layer having a bandgap equal to or larger than the minimum bandgap of the i layer is present in the i layer near the p layer. The open circuit voltage of an amorphous solar cell is
Since it largely depends on the bandgap of the i layer near the p layer, the presence of such a low light absorption layer can improve the open circuit voltage.

The band gap of the i layer has a minimum value that gradually decreases from the p layer side and gradually increases from the minimum value portion to the n layer side, so that the light absorption efficiency can be increased. it can.

As the band gap minimum value portion is closer to the p-layer, the absorption distribution of light in the vicinity of the p-layer is larger and the collection of holes is improved.
Since the band gap of the i layer near the layer is reduced, the open circuit voltage is reduced. In the present invention, since the low light absorption layer is formed in the vicinity of the p layer, the open circuit voltage can be improved similarly to the case where the band gap in the vicinity of the p layer is increased. Therefore, p
By forming a bandgap minimum value portion in the vicinity of the layer to reduce photodegradation, efficiency after photodegradation can be improved.

In the present invention, the photoelectric conversion layer (i
The present invention is not limited to a single-layer type amorphous solar cell in which a layer is a single layer, but is also applied to a stacked-type amorphous solar cell in which a photoelectric conversion layer (i layer) has a laminated structure such as a tandem structure. .

Further, as the low light absorption layer, a
When a-SiGe having a film quality close to that of -Si or a-Si is used, an increase in photodegradation unlike in the case of using a-SiC does not occur.

[0014]

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

Example 1 FIG. 1 shows a schematic sectional view of an amorphous solar cell of Example 1, and FIG. 2 shows a schematic view of a bandgap structure of an i layer 4 of this amorphous solar cell. In this amorphous solar cell, a transparent conductive film 2 is formed on a transparent insulating substrate 1. In this embodiment, the substrate 1 has a thickness of 1 m.
Although a glass substrate having a thickness of about m is used, any transparent insulating substrate can be used, and a polymer film or the like may be used. The transparent conductive film 2 preferably has an uneven shape, and the material thereof is ZnO, SnO ₂ or ITO (In
aluminum tin oxide) or the like can be used.

On the transparent conductive film 2, a p-layer 3 made of a-SiC, an i-layer 4 made of a-SiGe, and a-Si.
The n layer 5 made of is sequentially laminated.

As shown in FIG. 4, the i layer 4 linearly decreases in band gap from the p layer 3 side to the minimum value portion of the band gap near the p layer 3, and from the minimum value portion to the n layer 5 side. It has a profile that the band gap increases linearly. The i-layer 4 has a film thickness of 250 nm or less, preferably 150 nm or less, and the thinner the film thickness, the smaller the photodegradation. The minimum part of the band gap is within 80 nm, preferably within 20 nm from the p layer 3, and the closer to the p layer 3, the smaller the photodegradation. Further, it is desirable that the minimum band gap value is around 1.4 eV. The smaller the band gap minimum value is, the more the light absorption amount is increased. The band gap on the n layer 5 side and the band gap on the p layer 3 side do not necessarily have to be the same. Further, the band gap does not have to change linearly, and may increase or decrease in a curve or in steps.

A low light absorption layer 6 having a band gap larger than the minimum band gap of the i layer 4 is formed in the i layer near the p layer 3. The film thickness of the low light absorption layer 6 is 2 to 30 nm, preferably 10 nm, and the formation position is 5 to 40 nm from the p layer 3 side, preferably 10 to 20 nm near the minimum band gap portion. To do. The band gap is preferably a band gap close to that of a-Si: H having a good film quality, that is, 1.6 to 1.8 eV. The low light absorption layer 6 has a good film quality, and the formation of this layer does not adversely affect the photodegradation.

A back electrode 7 is formed on the n layer 5. The back electrode 7 may be made of any metal having a relatively high reflectance such as Ag or Al. Further, in this embodiment, only the metal electrode is formed for simplification, but a transparent conductive film is formed on the n layer 5 in order to effectively use the reflected light on the back surface, and the metal electrode is formed thereon. May be formed.

This amorphous solar cell can be manufactured as follows. First, the transparent conductive film 2 made of, for example, ZnO, SnO ₂ or ITO having a thickness of about 1 μm is formed on the glass substrate 1 having a thickness of about 1 mm.

Next, plasma CVD (Chemical Vapo) as shown in FIG. 3 is formed on the transparent conductive film 2.
The p layer 3, the i layer 4 on which the low light absorption layer 6 is formed, and the n layer 5 are sequentially laminated using a ur deposition device. In FIG. 4, 12 is a charging chamber, 13 is a p-layer film formation chamber, 14 is an i-layer film formation chamber, and 15 is n.
A layer deposition chamber, 16 is a take-out chamber, 17 is a substrate on which a film is to be formed, 18 is a cathode, 19 is a SiH ₄ gas storage chamber, 20 is a H ₂ gas storage chamber, 21 is a GeH ₄ gas storage chamber,
22 is a PH ₃ gas storage room, 23 is a B ₂ H ₅ gas storage room, 2
4 is a CH ₄ gas storage chamber, 25 is an RF power source, and 26 is a gate valve. The growth conditions are RF power of 10 W and pressure of 0.35.
Torr, the gas used and its flow rate are shown in Table 1 below.
As shown in.

[0022]

[Table 1]

First, a p-layer 3 made of a-SiC having a thickness of 15 nm is formed on a substrate, and an i-layer 4 made of a-SiGe having a low light absorption layer 6 formed thereon has a thickness of 150 nm.
To form. At this time, the GeH ₄ gas flow rate is controlled so that the bandgap profile shown in FIG. 2 is obtained, and the low resistance layer 6 having a thickness of 10 nm is formed in the i layer 4 near the p layer 3. An n-layer 5 made of a-Si having a thickness of 30 nm is formed thereon.
To form.

Thereafter, the back electrode 7 made of Al or Ag is formed to have a thickness of about 100 nm to 1 μm.

The characteristics of the thus obtained amorphous solar cell of this example are shown in Table 2 below. As a comparative example, FIG.
At the same time, the characteristics of the conventional amorphous solar cell having the band gap profile as shown in FIG.

[0026]

[Table 2]

Here, Isc is a short circuit current Voc is an open circuit voltage F. F. The fill factor η is the conversion efficiency. According to Table 2 above, the open-circuit voltage Voc of the amorphous solar cell of this example is improved by 0.05 V as compared with the conventional amorphous solar cell. Therefore, the bandgap minimum value portion of the i layer 4 is formed in the vicinity of the p layer 3 to improve the optical deterioration, and
The low light absorption layer 6 can improve the open circuit voltage. This is because the low light absorption layer 6 is formed in the i layer 4 in the vicinity of the p layer 3, so that the same effect as increasing the band gap of the i layer 4 in the vicinity of the p layer 3 can be obtained. Is.

The amorphous solar cell of this example was irradiated with light of AM 1.5, 100 mW / cm ² for 500 times.
It was carried out for a time and compared with a conventional solar cell, but it was confirmed that the deterioration rate was equivalent to the initial value within an error range. It is considered that this is because the low light absorption layer 6 has a good film quality, so that increasing the number of layers having such a thickness has no adverse effect.

Example 2 In this example, an i-layer having a bandgap structure as shown in FIG. 4 was formed, and an amorphous solar cell similar to that of Example 1 in other structures was prepared.

As shown in FIG. 4, in the i layer, the band gap decreases linearly from the p layer side to the minimum band gap portion in the vicinity of the p layer, and the band linearly decreases from the minimum value portion to the n layer side. It has a profile that the gap becomes large. The thickness of the i layer is 200 nm or less, preferably 150 nm or less, and the thinner the thickness, the smaller the photodegradation. The minimum value of the band gap is 80 nm or less, preferably 20 nm or less from the p layer, and the closer to the p layer, the smaller the photodegradation. Further, it is desirable that the minimum band gap value is around 1.4 eV. The smaller the band gap minimum value, the greater the light absorption amount.
If it is less than 4 eV, the fill factor is lowered and the efficiency cannot be improved. The band gap on the n layer 5 side and the band gap on the p layer side do not necessarily have to be the same. Also,
The band gap does not have to change linearly and may increase or decrease in a curve or in steps.

In the i layer near the p layer, a plurality of low light absorption layers 6 having a band gap larger than the band gap minimum value of the i layer are formed at intervals. The film thickness of the low light absorption layer 6 is 2 to 30 nm, preferably 10 nm, and the formation position is 5 to 40 nm from the p-layer side, and preferably several points are formed at intervals of several nm near the minimum value portion of the band gap. . The band gap is a- with good film quality.
Bandgap close to Si: H, that is, 1.6 to 1.8e
It is preferably V. The low light absorption layer 6 has a good film quality, and the formation of this layer does not adversely affect the photodegradation.

Such an i layer is formed by Ge as in the first embodiment.
It can be formed by controlling the H ₄ gas flow rate.

The characteristics of the amorphous solar cell of this example are shown in Table 2 above.

According to Table 2 above, the open-circuit voltage Voc of the amorphous solar cell of this example is improved by 0.05 V as compared with the conventional amorphous solar cell. Therefore, it is possible to improve the photodegradation by forming the band gap minimum value portion of the i layer near the p layer and improve the open circuit voltage by the low light absorption layer. This is because the low light absorption layer 6 is formed in the i layer near the p layer, and therefore, the same effect as when the band gap of the i layer near the p layer is increased can be obtained.

Further, the amorphous solar cell of this example was irradiated with light of AM 1.5, 100 mW / cm ² for 500 times.
It was carried out for a time and compared with a conventional solar cell, but it was confirmed that the deterioration rate was equivalent to the initial value within an error range. It is considered that this is because the low light absorption layer 6 has a good film quality, so that increasing the number of layers having such a thickness has no adverse effect.

In the above embodiment, a transparent conductive film, p
The layer, the i layer, the n layer, and the back surface electrode are formed in this order, but the present invention is not limited to this, and when a non-translucent insulating substrate is used as the substrate, the metal electrode is used instead of the transparent conductive film. May be formed, a transparent electrode may be formed instead of the back surface electrode, and the n layer, the i layer, and the p layer may be laminated in this order from the substrate side.

In Examples 1 and 2 described above, a single-layer type amorphous solar cell having a single photoelectric conversion layer (i layer) was formed. It may be a crystalline solar cell.

[0038]

As is apparent from the above description, according to the present invention, a low light absorption layer having a bandgap equal to or larger than the minimum bandgap of the i layer is formed in the i layer near the p layer. Therefore, the open circuit voltage can be improved. Further, since the bandgap minimum value portion can be brought close to the p-layer to increase the light absorption distribution in the vicinity of the p-layer, photodegradation can be reduced and the reliability of the device can be improved. Even in that case, the open circuit voltage can be improved and the efficiency after photodegradation can be improved in the same manner as when the band gap of the i layer near the p layer is increased. Furthermore, when a-Si having a good film quality or a-SiGe having a film quality close to a-Si is used as the low light absorption layer, an increase in photodegradation unlike in the case of using a-SiC does not occur.

[Brief description of drawings]

FIG. 1 is a cross-sectional view illustrating the structure of an amorphous solar cell of Example 1.

2 is a schematic diagram illustrating a bandgap structure of the amorphous solar cell of Example 1. FIG.

FIG. 3 is a schematic view showing the structure of a CVD apparatus used for manufacturing the amorphous solar cells of Examples 1 and 2.

FIG. 4 is a schematic diagram illustrating a bandgap structure of the amorphous solar cell of Example 1.

FIG. 5 is a schematic diagram illustrating a bandgap structure of a conventional amorphous solar cell.

[Explanation of symbols]

1 transparent insulating substrate 2 transparent conductive film 3 p layer made of a-SiC 4 i layer made of a-SiGe 5 n layer made of a-Si 6 low light absorption layer 7 back electrode 12 charging chamber 13 p layer deposition Chamber 14 i-layer deposition chamber 15 n-layer deposition chamber 16 ejection chamber 17 substrate to be deposited 18 cathode 19 SiH ₄ gas storage chamber 20 H ₂ gas storage chamber 21 GeH ₄ gas storage chamber 22 PH ₃ gas storage chamber 23 B ₂ H ₅ gas storage room 24 CH ₄ gas storage room 25 RF power supply 26 Gate valve

Claims (2)

[Claims] 1. A band gap of an i-type semiconductor layer made of an amorphous semiconductor material formed between a p-type semiconductor layer and an n-type semiconductor layer has a band gap between an end on the p-type semiconductor layer side and an n-type semiconductor layer side. Has a minimum value between the edge and the end on the p-type semiconductor layer side, and becomes larger as it approaches the edge on the p-type semiconductor layer side. An amorphous solar cell having a profile between the edge and the edge that is closer to the edge on the n-type semiconductor layer side, the amorphous solar cell having a profile larger than the minimum value in the i-type semiconductor layer near the p-type semiconductor layer. An amorphous solar cell in which at least one low light absorption layer having a band gap is formed. 2. The amorphous solar cell according to claim 1, wherein two or more i-type semiconductor layers on which the low light absorption layer is formed are laminated.