JPH0794768A - Photovoltaic device - Google Patents

Abstract

PURPOSE:To prevent the characteristics from being lowered owing to impurities of oxygen and nitrogen mixed in a photoelectric conversion layer. CONSTITUTION:A photovoltaic device includes a pin structure photovoltaic conversion layer 3, and an i-type amorphous semiconductor layer 3i contains therein oxygen and/or nitrogen by 10<18> to 10<21>cm<-3>, wherein 0.1 to 100ppm boron is doped within a range of from 500Angstrom to 3000Angstrom from the interface between the i-type amorphous semiconductor layer 3i to a p-type amorphous semiconductor layer 3p in the direction of film thickness.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a photovoltaic device having an amorphous semiconductor as a photoelectric conversion layer.

[0002]

2. Description of the Related Art Generally, in a photovoltaic device made of an amorphous semiconductor typified by amorphous silicon, its characteristics greatly vary depending on the amount of impurities such as oxygen and nitrogen contained in the photoelectric conversion layer.

Usually, as impurities mixed in the formation of a film of an amorphous semiconductor, although impurities contained in the reaction gas also exist,
Most of the impurities that have a great influence in the mass production stage are caused by vacuum leak in a vacuum device, so-called leak, and oxygen and nitrogen are the main ones.

As a method of minimizing this characteristic variation, for example, a device is devised such as using a high-performance vacuum device for increasing the initial vacuum degree of the film forming device, or its photoelectric conversion layer is formed. An element structure in which the intrinsic amorphous semiconductor layer (hereinafter, referred to as an i-type amorphous semiconductor layer) is doped with boron, which is a conductivity type determining impurity, so that the concentration thereof is uniform, to suppress the fluctuation. The device has been devised (see Japanese Examined Patent Publication No. 4-60353 and Japanese Examined Patent Publication No. 61-97874).

[0005]

However, in the method using the vacuum device, in addition to requiring a large investment in equipment, the exhaust capability deteriorates due to the long-term use of the vacuum device itself. As a result, the characteristic of the photovoltaic device is deteriorated, and it is not a fundamental solution.

Further, in the method of doping the i-type amorphous semiconductor layer with boron, although the effect of impurities such as oxygen and nitrogen can be suppressed by this boron, the i-type amorphous substance is affected by the effect of boron itself. Since it also deteriorates the film quality of the entire quality semiconductor layer, it is generally extremely difficult to control boron doping in terms of the balance between the film quality deterioration and the suppression of the influence of impurities.

In particular, in a photovoltaic device having a photoelectric conversion layer having a pin junction as a basic structure, when impurities such as oxygen or nitrogen are mixed into the i-type amorphous semiconductor layer, these impurities are added to the i-type amorphous semiconductor layer. A weak donor level is formed in the i-type amorphous semiconductor layer, and the internal electric field due to the pin junction, particularly the electric field strength in the vicinity of the p / i interface, is locally increased. Make the electric field distribution in the layer non-uniform. This non-uniformity of the electric field distribution becomes a hindrance to the external extraction of photocarriers, which leads to a decrease in photoelectric conversion efficiency, which is a comprehensive characteristic evaluation of the photovoltaic device.

Therefore, the present invention provides a photovoltaic device capable of sufficiently collecting photocarriers obtained by photoelectric conversion while suppressing the influence of boron and impurities caused by oxygen and nitrogen.

[0009]

The photovoltaic device of the present invention comprises:
comprising a photoelectric conversion layer consisting of a pin structure, the i-type amorphous semiconductor layer is an oxygen or and nitrogen 10 ¹⁸ to within the layer 10
²¹ cm ⁻³ , and from the interface between the i-type amorphous semiconductor layer and the p-type amorphous semiconductor layer, boron is added in the range of 500 Å to 3000 Å in the thickness direction of 0.1 to 1
The i-type amorphous semiconductor layer is composed of a plurality of layers having different optical band gaps, and the plurality of layers are arranged so that the optical band gap becomes larger toward the light incident side. There is something I did.

[0010]

According to the present invention, a photovoltaic device provided with an i-type amorphous semiconductor layer containing oxygen or nitrogen in a certain concentration range or impurities composed of oxygen and nitrogen is provided. By doping boron to the i-type amorphous semiconductor layer from the interface between the p-type amorphous semiconductor layer and the i-type amorphous semiconductor layer in a predetermined thickness range by doping with boron in an amount of 0.1 to 100 ppm, It is possible to suppress the local increase in the electric field strength in the vicinity of the p / i interface.

This makes it possible to make the internal electric field in the pin junction uniform throughout the photoelectric conversion layer.

Further, since the action as a donor due to the impurities of oxygen and nitrogen at the p / i interface is compensated by boron as an acceptor, the efficiency as a photovoltaic device is improved and at the same time, It is considered that photodegradation can be suppressed, and the influence of dangling bond defects in the amorphous semiconductor caused by light irradiation can be reduced.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is an element structure diagram for explaining a first embodiment of a photovoltaic device of the present invention. In the figure, (1) is a transparent substrate made of quartz, glass or the like, and (2) is made of tin oxide or indium oxide formed on the surface of the transparent substrate (1), or a mixture thereof. Transparent conductive film, which becomes (3)
Is a p-type amorphous semiconductor layer formed on the transparent conductive film (2)
(3p) and i-type amorphous semiconductor layer (3i) and n-type amorphous semiconductor layer (3
n), the i-type amorphous semiconductor layer (3i) further comprises two layers described later. And (4) is a metal electrode made of aluminum, chromium or the like.

In this photovoltaic device, a transparent substrate
Light incident on the photoelectric conversion layer (3) via (1) is absorbed in the photoelectric conversion layer to become holes and electrons, and the transparent conductive film
It is taken out from the (2) and the metal electrode (4) respectively.

In this embodiment, amorphous silicon formed by the plasma CVD method is used as the amorphous semiconductor layer (3), and specific film forming conditions are shown in Table 1.

[0016]

[Table 1]

First, in forming the photoelectric conversion layer (3), a p-type amorphous semiconductor layer (3p) is formed on the transparent conductive film (2) within a film thickness range of 50Å to 300Å. . Although amorphous silicon carbide is used in the present embodiment, the present invention is not limited to this, and simple p-type amorphous silicon or the like may be used.

Next, as an i-type amorphous semiconductor layer, silane is used.
(SiH ₄ ) and carbon dioxide (CO ₂ ) mixed with diborane (B ₂ H ₆ ) as a reaction gas under the conditions of i layer (I) in the table, the film thickness is in the range of 500 to 3000 Å A first i-type amorphous semiconductor layer is formed, and then a second i-type amorphous semiconductor layer is formed by a reaction gas containing no diborane (B ₂ H ₆ ) mixed therein.
The i-type amorphous semiconductor layer (i layer (II)) is formed by the remaining film thickness required for the entire i-type amorphous semiconductor layer. Its typical film thickness is 1000 to 5000Å.

Therefore, in the i-type amorphous semiconductor layer (3i),
Oxygen is mixed from the i layer (I) to the i layer (II), and boron is doped only in the i layer (I), that is, the i type amorphous semiconductor layer near the p type amorphous semiconductor layer. Will be done.

Further, in this embodiment, in order to clearly separate the influence of nitrogen, the leak from the apparatus main body is suppressed as much as possible.

In the description of the present invention, oxygen and nitrogen, which are impurities in the i-type amorphous semiconductor layer, are intentionally mixed by using a gas such as carbon dioxide (CO ₂ ) and characteristic evaluation is performed. However, this is for clearly grasping the optimum impurity concentration range and the like, and the present invention relates to the case where oxygen or nitrogen mixed with these impurities by a leak from a vacuum device or a film forming device is used. Also has exactly the same effect.

FIG. 2 is a band diagram (a) schematically showing the band profile of the photoelectric conversion layer of this photovoltaic device. In FIG. 2, only oxygen impurities are mixed and boron band is shown. (B) of the conventional photovoltaic device, which is different only in that the i-type amorphous semiconductor layer not doped with is used, is also shown. That is, according to the present invention, as shown in the same figure, the inclination of the band is substantially constant in the i-type amorphous semiconductor layer of the photovoltaic device of the embodiment,
We believe that the internal electric field strength is uniform. On the other hand, in the conventional example, the band inclination in the i-type amorphous semiconductor layer becomes steep near the p-type amorphous semiconductor layer, and the internal electric field strength becomes nonuniform in the i-type amorphous semiconductor layer. I think it has become.

Next, the optimum concentration range of oxygen and nitrogen, and further boron, which sufficiently exerts the effect of the present invention, will be described. FIG. 3 is a characteristic diagram showing the relationship between the oxygen concentration contained in the i-type amorphous semiconductor layer and the photoelectric conversion efficiency in the present invention. Example photovoltaic device (a) doped with boron (about 10 ppm) up to 1000 Å, and conventional photovoltaic device in which only oxygen was mixed in the i-type amorphous semiconductor layer and boron was doped in the entire region of the layer. The characteristics with the power device (b) are shown respectively. However, the film thickness of the entire i-type amorphous semiconductor layer is approximately 5000Å constant.

According to the figure, as compared with the conventional photovoltaic device (b), in the photovoltaic device (a) of the present invention, when the oxygen concentration is within the range of 10 ²¹ cm ^-3 or less, its photoelectric It can be seen that the conversion efficiency shows an extremely high value. It should be noted that the characteristic improvement by suppressing impurities such as oxygen as much as possible is clear,
Further, to realize this is a large capital investment as described above, so it is practically preferable to set the concentration of oxygen as an impurity to 10 ¹⁸ cm ⁻³ or more.

Next, referring to FIG. 4, the oxygen concentration in the i-type amorphous semiconductor layer (film thickness: about 5000 Å) is kept constant at about 10 ²⁰ cm ^-3 ,
Boron doping in the i-type amorphous semiconductor layer (about 10 pp
It is a characteristic view which shows the photoelectric conversion efficiency of the photovoltaic device of this invention when variously changing the film thickness of the part m).

According to the figure, the thickness of the semiconductor layer is 50.
It can be seen that a high photoelectric conversion efficiency is exhibited in the range of 0Å to 3000Å. This is because when the film thickness is less than 500 Å, the donor-like state due to oxygen cannot be sufficiently compensated by boron, and when the film thickness exceeds 3000 Å, i-type due to boron itself is caused. It is considered that the film quality of the amorphous semiconductor layer is significantly deteriorated.

Further, the doping concentration of boron in the i-type amorphous semiconductor layer will be described. FIG. 5 shows a case where the oxygen concentration in the i-type amorphous semiconductor layer is kept constant at about 10 ²⁰ cm ^-3 and the thickness of the i-type amorphous semiconductor layer doped with boron is kept constant at about 1000Å. Shows the change in photoelectric conversion efficiency with respect to the boron concentration. According to this, a high photoelectric conversion efficiency can be obtained by doping the boron concentration in the range of 0.1 to 100 ppm, and it is most suitable in the range of 1.0 to 50 ppm.

Although oxygen has been described as an impurity in the above embodiments, the effect of the present invention is exactly the same when nitrogen is used as the impurity or when both oxygen and nitrogen are mixed as impurities. It has been confirmed to be effective. However, when considering both oxygen and nitrogen as impurities, the optimum amount of these impurities in the i-type amorphous semiconductor layer is in the range of 10 ^{18 to} 10 ²¹ cm ⁻³ as the sum of these impurities. It is necessary to be inside.

Next, a second embodiment of the present invention is shown in FIG. This example is different from the first example in that the i-type amorphous semiconductor layer is composed of a plurality of layers having different optical band gaps, and the same parts as those in the first example are described. Are given the same reference numerals. (3i in the figure
f) is a layer having a large optical bandgap disposed on the light incident side of the i-type amorphous semiconductor layer, and (3ib) is a layer having an optical bandgap smaller than the layer. In this example, the difference in the optical band gap was controlled by the methane gas mixed in the reaction gas. In particular, increasing the optical bandgap on the light incident side is
This is because more light can enter the photoelectric conversion layer.

In the photovoltaic device of the present invention having such a structure, the i-type amorphous semiconductor layer composed of a plurality of layers is formed over the entire region.
Oxygen or nitrogen impurities will be mixed in.
Table 2 shows the conditions for forming the photoelectric conversion layer in the photovoltaic device of the present example, and FIG. 7 shows only the band diagram (a) of the photoelectric conversion layer of the photovoltaic device of the present example and not mixing boron. FIG. 7B is a diagram showing that (b) of a conventional photovoltaic device having different numbers.

[0031]

[Table 2]

As shown in the figure, in the photovoltaic device of this embodiment, as in the case of the first embodiment, the inclination of the band in the i-type amorphous semiconductor layer is two layers having different optical band gaps. However, the internal electric field strength in the i-type amorphous semiconductor layer is considered to be uniform.

In the case of the photovoltaic device of the second embodiment, the boron-doped region from the p / i interface is in the i-type amorphous semiconductor layer having a large optical band gap. The element design may be limited to (3if), and it may be included only in the range of 500 Å to 3000 Å from the above interface. Further, even if the boron doping region is formed over a wide region up to (3ib) in the i-type amorphous semiconductor layer having a small optical bandgap as in the second embodiment, the condition of the above range is satisfied. It has been confirmed that the effects of the present invention can be obtained as long as they are satisfied.

[0034]

[Table 3]

Table 3 shows the initial photoelectric conversion efficiency of this photovoltaic device and the photoelectric conversion efficiency after light irradiation for 310 hours under the irradiation condition of AM-1,100 mW / cm ^2. The table also shows those characteristics in the case of the conventional photovoltaic device.

However, in the embodiment, the i-type amorphous semiconductor layer 40 is used.
00Å, 10 oxygen^{twenty one}cm^-3I-type non-
From the p / i interface in the crystalline semiconductor layer to 2000 Å
The conventional example is an i-type amorphous half.
5 × 10 oxygen in the conductor layer¹⁸cm ^-3Use the one that is contained
It was done.

According to this, in the present invention, the initial 1
Although the characteristic declines from 1.0% to 8.0% after light irradiation, the initial value of the conventional photovoltaic device is 10.
It is as low as 5% in the first place, and after irradiation with light, it is significantly deteriorated to 7.0%. From this, it can be seen that the present invention has an effect of suppressing photodegradation and improves reliability.

Although the thin film formed by the plasma CVD method is used as the amorphous semiconductor in the embodiments, the present invention is not limited to this, and the non-crystalline semiconductor formed by the photo CVD method or the sputtering method is used. A crystalline semiconductor may be used.

[0039]

According to the present invention, a photovoltaic device provided with an i-type amorphous semiconductor layer containing oxygen or nitrogen in a constant concentration range or impurities containing oxygen and nitrogen is provided.
From the interface between the p-type amorphous semiconductor layer and the p-type amorphous semiconductor layer,
Boron is added to the film thickness direction within a predetermined film thickness range of 0.1.
By doping with 100 to 100 ppm, the internal electric field in the pin junction can be made uniform throughout the entire photoelectric conversion layer, and the photoelectric conversion efficiency can be improved.

Further, since the action as a donor due to the impurities of oxygen and nitrogen at the p / i interface is to be compensated by boron which is an acceptor, in the amorphous semiconductor layer generated by light irradiation. The characteristics can be improved by acting to reduce the influence of defects and suppressing the optical deterioration of the photovoltaic device.

[Brief description of drawings]

FIG. 1 is a cross-sectional view of an element structure showing a first embodiment of a photovoltaic device of the present invention.

FIG. 2 is a band diagram showing a band profile in a photoelectric conversion layer of the photovoltaic device according to the first embodiment.

FIG. 3 is a characteristic diagram showing the relationship between the oxygen concentration in the i-type amorphous semiconductor layer of the photovoltaic device of the example and the photoelectric conversion efficiency.

FIG. 4 is a characteristic diagram showing a relationship between a film thickness of a boron-doped portion and photoelectric conversion efficiency of an example photovoltaic device.

FIG. 5 is a characteristic diagram showing a relationship of photoelectric conversion efficiency with respect to boron concentration in the photovoltaic device of the embodiment.

FIG. 6 is a sectional view of an element structure showing a photovoltaic device according to a second embodiment of the present invention.

FIG. 7 is a band diagram showing a band profile in a photoelectric conversion layer of a photovoltaic device according to a second embodiment.

[Explanation of symbols]

(3p) ... p-type amorphous semiconductor layer (3i) ... i-type amorphous semiconductor layer (3n) ... n-type amorphous semiconductor layer

Claims (4)

[Claims] 1. A photovoltaic device comprising a photoelectric conversion layer comprising a p-type amorphous semiconductor layer, an i-type amorphous semiconductor layer and an n-type amorphous semiconductor layer, wherein the i-type amorphous The semiconductor layer contains 10 ^{18 to} 1 oxygen in the layer.
0 ²¹ cm ^-3 , and from the interface between the i-type amorphous semiconductor layer and the p-type amorphous semiconductor layer toward the film thickness direction of the i-type amorphous semiconductor layer, 500 Å to 3000 Å A photovoltaic device, characterized in that boron is doped in the range of 0.1 to 100 ppm. 2. A photovoltaic device comprising a photoelectric conversion layer comprising a p-type amorphous semiconductor layer, an i-type amorphous semiconductor layer and an n-type amorphous semiconductor layer, wherein the i-type amorphous The semiconductor layer contains 10 ^{18 to} 1 nitrogen in the layer.
0 ²¹ cm ^-3 , and from the interface between the i-type amorphous semiconductor layer and the p-type amorphous semiconductor layer toward the film thickness direction of the i-type amorphous semiconductor layer, 500 Å to 3000 Å A photovoltaic device, characterized in that boron is doped in the range of 0.1 to 100 ppm. 3. A photovoltaic device comprising a photoelectric conversion layer comprising a p-type amorphous semiconductor layer, an i-type amorphous semiconductor layer and an n-type amorphous semiconductor layer, wherein the i-type amorphous The semiconductor layer contains nitrogen and oxygen within the layer.
^{18 to} 10 ²¹ cm ⁻³ , and 500Å from the interface between the i-type amorphous semiconductor layer and the p-type amorphous semiconductor layer in the thickness direction of the i-type amorphous semiconductor layer. Through 30
A photovoltaic device characterized by being doped with boron in an amount of 0.1 to 100 ppm in a range of 00Å. 4. The i-type amorphous semiconductor layer comprises a plurality of layers having different optical bandgabs, and these layers are arranged so that the optical bandgap becomes larger toward the light incident side. The photovoltaic device according to any one of claims 1 to 3.