JP2962915B2 - Method for manufacturing photovoltaic element - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a photovoltaic device, and more particularly to a photovoltaic device using a polycrystalline semiconductor as a base material.

[0002]

2. Description of the Related Art A photovoltaic device made of a semiconductor material has a semiconductor junction for performing a photoelectric conversion function. This portion is composed of electrons generated by absorption of light incident on the photovoltaic device. It has the function of separating holes and extracting these electrons and holes to the outside.

As such a semiconductor material, there are a polycrystalline material and an amorphous material in addition to a single crystal material which has been widely used. Among these materials, a photovoltaic element formed of a polycrystalline material has attracted attention in recent years because of its relatively high photoelectric conversion efficiency and easy characteristics of large area.

As a method for forming such a semiconductor junction, a thermal diffusion method is generally used. This thermal diffusion method is, for example, p
In a photovoltaic element having a polycrystalline semiconductor substrate as a substrate, the substrate is left in an atmosphere containing n-type impurities,
By performing a heat treatment in this state, the n-type impurity is diffused into the substrate, and an n-type semiconductor having a desired concentration is formed in the diffused portion. Due to this diffusion, a contact surface between the n-type semiconductor and the p-type semiconductor as a base material is formed in the substrate, and this portion becomes a semiconductor junction.

A method for manufacturing a photovoltaic element by such a thermal diffusion method is described, for example, in Japanese Patent Application Laid-Open No. 62-108579.

FIG. 4 is an element structure diagram for explaining a process of forming a pn junction by the thermal diffusion method. FIG.
Is a p-type polycrystalline semiconductor (41), and a number of crystal grains (41b) surrounded by many grain boundaries (41a) (usually several mm)
Are constituted by gathering.

FIG. 1B is a cross-sectional view taken along the line AA ′ of FIG. 1A. Such a grain boundary (41 a) is formed in the polycrystalline semiconductor (41).
.. Exist along the film thickness direction.

Next, FIG. 1C shows that the p-type polycrystalline semiconductor (41) is left in an atmosphere containing n-type conductivity-determining impurities, for example, phosphorus (for example, a POCl ₃ gas atmosphere). This semiconductor (41) is heat-treated at 850 ° C. to diffuse the n-type impurity from the surface (41c) side of the p-type polycrystalline semiconductor (41). Thereby, the p-type polycrystalline semiconductor (4
The surface side of 1) becomes an n-type polycrystalline semiconductor (42),
The contact surface between the p-type polycrystalline semiconductor (41) and the n-type polycrystalline semiconductor (42) becomes a junction interface (43) as a semiconductor junction.

[0009]

In a photovoltaic device having such a semiconductor junction, the junction interface (43) is flat and the distance from the light incident surface to the interface (43) is smaller. It is preferable for the light wavelength sensitivity characteristic that the distance is equal over the entire interface.

However, as shown in FIG. 1C, when the polycrystalline semiconductor is used as a base material, the bonding interface (43) cannot be flat, and the bonding interface (41) at the grain boundary (41a) is usually used. 43)
Are formed deeper from the surface (41c) as compared to those formed at the crystal grains (41b).

This is because the crystal grains (41b) ... and the grain boundaries (41a)
This is because there is a large difference in the diffusion coefficients for various elements with the part, and this is particularly due to the abnormally large diffusion coefficient in the part of the grain boundary (41a). The reason for this from the viewpoint of physical properties is that the bonding energy between the semiconductor elements is small in the portion of the grain boundary (41a), and as a result, the semiconductor element is very easily bonded to a foreign element that enters from the outside.

Therefore, even in the above-mentioned thermal diffusion method, an effect based on the difference in the diffusion coefficient appears, and the conductivity-type-determining impurities diffuse particularly deeply along the grain boundary (41a). The bonding interface (43) in the portion (1) is not uniform in the position plane with the bonding interface (43) in the crystal grains (41b) formed by ordinary diffusion.

Here, the relationship between the position of the semiconductor junction and the characteristics of the photovoltaic element is such that when the semiconductor junction is generally arranged in a portion near the light incident surface, the sensitivity characteristic of short-wavelength light is excellent, On the other hand, when it is arranged in a distant portion, the sensitivity characteristic of short wavelength light tends to be small, and the sensitivity characteristic of long wavelength light tends to be large.

For this reason, if there is an irregular portion at the position of the junction interface (43) in the semiconductor junction, the light wavelength sensitivity characteristics as a photovoltaic element become unstable.

Therefore, the specific light wavelength sensitivity characteristic of a conventional photovoltaic device using a polycrystalline semiconductor is that when the photovoltaic device is made of a polycrystalline semiconductor containing a large number of grain boundaries, it is deep as viewed from the light incident side surface. Since the amount of light absorbed by the semiconductor junction increases, sensitivity characteristics mainly in a long wavelength region increase. On the other hand, in the case of a polycrystalline semiconductor having only a relatively small number of grain boundaries, in other words, in the case of a polycrystalline semiconductor having large crystal grains, the sensitivity to short-wavelength light is rather high.

This means that, in the case of a polycrystalline semiconductor in which the size of a crystal grain greatly changes due to a slight change in formation conditions, the sensitivity characteristic to light varies variously depending on the degree of the size of the crystal grain. Become. Further, even if a slight change occurs in the set temperature for the above-mentioned thermal diffusion method, the degree of diffusion of the conductivity-type determining impurity in the grain boundary greatly changes, and the device characteristics can be stabilized. It will not be.

[0017]

A feature of the method for manufacturing a photovoltaic element of the present invention is to form an amorphous semiconductor film containing hydrogen on the surface of a polycrystalline semiconductor, The polycrystalline semiconductor on which the amorphous semiconductor film is formed is subjected to a heat treatment in an atmosphere containing a conductivity type determining impurity, thereby diffusing the conductivity type determining impurity from the surface side of the amorphous semiconductor film, It is to make an amorphous semiconductor film polycrystalline.

[0018]

According to the manufacturing method of the present invention, an amorphous semiconductor film containing hydrogen is deposited on the surface of a polycrystalline semiconductor including grain boundaries. As a result, the hydrogen diffuses sufficiently in the grain boundary by utilizing the characteristic of the grain boundary having a large diffusion coefficient.

Then, the grain boundaries subjected to the diffusion of hydrogen are
The characteristic of having a large diffusion coefficient up to that point is lost, and a state of having a diffusion coefficient substantially equal to that of a crystal grain is obtained.

Accordingly, even if the polycrystalline semiconductor after the formation of the amorphous semiconductor film diffuses impurities for determining the conductivity type in a later step, the polycrystalline semiconductor diffuses into the grain boundaries and forms the crystal grains. To the same extent.

As a result, according to the present invention, even in the case of a polycrystalline semiconductor having a large number of grain boundaries, the junction interface is flat and the distance from the light incident surface to the junction interface is made equal over the entire interface. it can.

Further, according to the method of manufacturing a photovoltaic element of the present invention, an amorphous semiconductor is formed on the surface of a polycrystalline semiconductor, and impurities for determining the conductivity type are diffused from the surface of the amorphous semiconductor. Therefore, when the thickness of the conductive semiconductor layer formed by the diffusion of the conductive type determining impurity is equal to or less than the thickness of the amorphous semiconductor film, the conductive type formed by the diffusion is determined. Since the base material of the semiconductor layer is the amorphous semiconductor, a more uniform layer can be formed as compared with a case where the base material is a polycrystalline semiconductor as a base material.

That is, although an amorphous semiconductor is inferior to a polycrystalline semiconductor in terms of film quality, it does not have a grain boundary exhibiting unique physical properties like a polycrystalline semiconductor in a film, and therefore, it is necessary that the amorphous semiconductor is uniform as a whole. I can say.

Therefore, if the impurities determining the conductivity type are diffused to such an extent that they remain in the amorphous semiconductor, abnormal diffusion such as that at the grain boundaries described above will not occur, and the junction formed by the diffusion will not occur. The interface can also be flat.

[0025]

1 is a sectional view of an element structure for each step for explaining a method of manufacturing a photovoltaic element according to the present invention.

FIG. 1A shows a p-type polycrystalline semiconductor (1) serving as a base material of a photovoltaic element, and a large number of crystal grains (1b) surrounded by a grain boundary (1a). Contains.

Next, in a first step shown in FIG. 1B, an amorphous semiconductor film (2) made of an amorphous silicon film containing hydrogen is conventionally formed on the p-type polycrystalline semiconductor (1). Plasma C
It is formed by the VD method. Table 1 shows typical conditions for forming the amorphous semiconductor film (2).

[0028]

[Table 1]

[0029] The conditions employed in the examples, flow 100sccm silane (SiH ₄₎ as a reactive gas, a substrate temperature of 400 ° C., under a reaction time of vacuum 0.1 Torr, between the counter electrodes a high-frequency power 50 mW / cm ² To generate plasma and decompose the reactive gas. The thickness of this amorphous silicon film was about 1000 °.

Particularly, according to the formation of the plasma CVD method or the like, since a silane gas containing hydrogen is used as a reactive gas, hydrogen is inevitably contained in the amorphous silicon film formed by the decomposition reaction. It will be taken in. Other than this silane gas, the same applies to the case where a higher silane gas such as disilane or a mixed gas with hydrogen or argon is used.

In the formation of an amorphous silicon film by sputtering, solid silicon is used as a target, and hydrogen is added to an inert gas such as argon as a sputtering gas to form the film. Similarly, an amorphous silicon film containing hydrogen can be formed in the film.

According to this step, hydrogen contained in the amorphous semiconductor film (2) diffuses into the grain boundaries (1a) of the polycrystalline semiconductor (1), and as a result, the grain boundaries (1a) are It loses the characteristic of having a large diffusion coefficient up to the crystal grain (1b)…
In this case, the degree of diffusion of the impurity is substantially the same as the degree of diffusion of the impurity, and the degree of diffusion of the impurity for determining the conductivity type in the subsequent process becomes uniform throughout the polycrystalline semiconductor.

In the subsequent second step shown in FIG.
A heat treatment is performed while flowing a mixture of POCl ₃ gas (100 mg / min mass control), oxygen gas (300 sccm) and nitrogen gas (10 slm) to determine the conductivity type on the surface side of the polycrystalline semiconductor (1). Phosphorus, which is an impurity, is diffused to form a semiconductor junction by a pn junction.
(3) in the figure indicates the bonding interface. Incidentally, the temperature of this heat treatment was 850 ° C., and the treatment time was 20 minutes.

In this step, the surface side of the polycrystalline semiconductor (1) becomes an n-type semiconductor (1d), and at the same time, the amorphous semiconductor film (2) formed earlier is subjected to heat treatment.
(2a).

In this step, the diffusion of hydrogen into the grain boundary (1a), which was conventionally caused by diffusing hydrogen in the grain boundary (1a) in the previous step, and in this embodiment, the abnormal diffusion of phosphorus into the grain boundary (1a) is suppressed. Does not occur.

Therefore, the junction interface (3) formed in the polycrystalline semiconductor has a substantially uniform depth from the surface (4). Light wavelength sensitivity characteristics as a power element can be stabilized.

Incidentally, there may be a problem whether the hydrogen introduced in the previous step is released due to the influence of the heat treatment in this step, but it has been confirmed from experimental results that this is not a problem. The reason for this is considered to be that the amorphous semiconductor (2) itself used for diffusing hydrogen has an effect of suppressing hydrogen release from the polycrystalline semiconductor (1) as if to cap it. .

Next, in a third step shown in FIG. 3D, a p ⁺ layer (5) for forming a BSF (Back Surface Field) structure on the back surface of the polycrystalline semiconductor (1), and a light incident surface side and a back surface. And a light reflection preventing film (7) made of silicon nitride or the like are formed by a conventionally known forming method.

FIG. 2 is a characteristic diagram showing the relationship between the heat treatment temperature in the above-described second step and the electrical characteristics of the photovoltaic element formed by the heat treatment. The heat treatment conditions other than the heat treatment temperature are the same as in the above embodiment. The vertical axis in FIG.
Open-circuit voltage (V _OC ), short-circuit current (I _SC ), fill factor (FF), and photoelectric conversion efficiency (%) are shown as characteristic items of the photovoltaic element.

According to the figure, as the heat treatment temperature rises from 890 ° C. to 930 ° C., the open-circuit voltage slightly decreases, but this increases the film thickness of the n ⁺ layer formed by this increase in temperature. As a result, the short-circuit current is reduced, which is affected by this.

At this higher temperature, the fill factor is improved, so that the photoelectric conversion efficiency, which is a comprehensive evaluation, is improved.

On the other hand, when the temperature is lowered from 850 ° C. to 810 ° C., the thickness of the n ⁺ layer is reduced, but the surface recombination of carriers at the interface between the half-surface n ⁺ layer and the underlying semiconductor increases, resulting in a short-circuit current. It shows a downward trend again.

In the evaluation based on the photoelectric conversion efficiency, the heat treatment temperature is in the range of 810 to 930 ° C., preferably 83
The temperature is preferably in the range of 0 to 890 ° C.

FIG. 3 shows a distribution diagram of the photoelectric conversion efficiency when 100 samples are formed as the photovoltaic device of the embodiment (product of the present invention). The figure also shows the characteristics of the same number of conventional photovoltaic elements (conventional products).
This conventional photovoltaic element is different from the conventional photovoltaic element only in that an amorphous semiconductor film containing hydrogen, which is a feature of the present invention, is not formed, and other forming conditions are the same.

According to the figure, most of the photovoltaic elements of the present invention show an efficiency of 13%, and the variation is 12.5%.
% Within the range of 13.5%. On the other hand, in the conventional photovoltaic device, the number of devices exhibiting an efficiency of 12.5%, which is lower than 13% of the photovoltaic device of the present invention, is the largest, and the range of variation in the efficiency is large. Also 11.5%
From 13.5%.

This is because, in the photovoltaic element of the present invention, since the grain boundaries are treated with hydrogen, there is almost no influence by the grain boundaries, and variations in characteristics are reduced.

In the photovoltaic device of the embodiment, the surface (4) of the polycrystalline semiconductor on which the amorphous semiconductor film (2) is formed is the light incident surface, but the present invention is not limited to this. Instead, the back surface side of the polycrystalline semiconductor facing the surface on which the amorphous semiconductor film is formed may be used as the light incident surface.

In the formation of the n-type semiconductor (1d) by the diffusion of the conductivity type determining impurity in the second step of the embodiment, the n-type semiconductor (1d) is formed even inside the polycrystalline semiconductor (1). However, the method of the present invention is not limited to this, and the diffusion may be such that the diffusion stays in the amorphous semiconductor (2).

Even in such a case, the n-type semiconductor (1d) formed by diffusion is formed using a homogeneous amorphous semiconductor (2) as a base material. Can form a flat bonding interface.

[0050] In addition, in the embodiment, to form an amorphous semiconductor film only on the light incident side of the photovoltaic element, processing was carried out to intergranular, the back surface side, that is p ⁺ layer in addition to An amorphous semiconductor film may be formed on the formed side, and the grain boundary may be similarly processed on the back surface.

In the embodiment, an amorphous silicon film is used as the hydrogen-containing amorphous semiconductor film. However, the present invention is also applicable to an amorphous silicon nitride film containing hydrogen and an amorphous silicon carbide film. Etc. may be used.

[0052]

According to the method of manufacturing a photovoltaic element of the present invention, it is possible to suppress the abnormal diffusion of impurities determining the conductivity type into many grain boundaries in a polycrystalline semiconductor due to hydrogen contained in an amorphous semiconductor film. Become.

Since the diffusion of the impurities determining the conductivity type at the grain boundaries and the crystal grains can be made substantially equal, the junction interface of the semiconductor junction formed by such diffusion is made flat. Can be. This makes it possible to easily make the light incident surface and the bonding interface thereof parallel, and the distance between the surfaces can be equal over the entire interface.

Therefore, in the photovoltaic device of the present invention,
Light wavelength sensitivity characteristics are stable regardless of the number of grain boundaries contained in the polycrystalline semiconductor.

[Brief description of the drawings]

FIG. 1 is a sectional view of an element structure in each step of a method for manufacturing a photovoltaic element of the present invention.

FIG. 2 is a characteristic diagram showing a relationship between a heat treatment temperature and electrical characteristics of the photovoltaic device.

FIG. 3 is a characteristic distribution diagram of the photovoltaic element.

FIG. 4 is a view showing a heat treatment step when a conventional photovoltaic element is formed.

[Explanation of symbols]

(1) ... one conductivity type polycrystalline semiconductor (1a) ... grain boundary (2) ... hydrogen-containing amorphous semiconductor film (3) ... junction interface

Continuation of the front page (56) References JP-A-4-26169 (JP, A) JP-A-2-94625 (JP, A) JP-A-55-95375 (JP, A) , A) (58) Field surveyed (Int.Cl. ⁶ , DB name) H01L 31/04-31/078

Claims (1)

(57) [Claims] An amorphous semiconductor film containing hydrogen is deposited on a surface of a polycrystalline semiconductor, and then the polycrystalline semiconductor on which the amorphous semiconductor film is formed contains impurities for determining conductivity type. A photovoltaic element characterized by diffusing the conductivity-type determining impurities from the surface side of the amorphous semiconductor film and polycrystallizing the amorphous semiconductor film by performing a heat treatment in a heated atmosphere. Manufacturing method.