JPH10247737A - Functional thin film and photovoltaic device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To manufacture a functional thin film combining the merits of an amorphous material in continuous phase with those of an crystalline material in discontinuous phase for improving the generating characteristics of photovoltaic device by mixing the continuous phase in amorphous state with the discontinuous phase in crystalline state. SOLUTION: The figure is a section of an SiGe base thin film, crystalline grains 2 as discontinuous phase in crystalline state are dispersed in a matrix comprising a-Si:H as a continuous phase in amorphous state. Such a thin film can be formed by selecting the thin film forming requirements selectively crystallizing Ge e.g. in vapor phase. Besides, the SiGe base thin film diminishes the defects in the grain boundary of crystalline Ge 2 due to the existance of amorphous silicon on the surface of the crystalline Ge grains 2. Furthermore, the long wave absorption can be increased by increasing the volume ratio of the crystalline grains 2 to the whole thin film thereby enabling the conductivity in the photoirradiation time to be increased.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a functional thin film and a photovoltaic device using the thin film.

[0002]

2. Description of the Related Art In a photovoltaic element, a semiconductor pin junction or p-type semiconductor is generally used.
An n-junction is formed to convert light incident into the semiconductor into electric energy. As a semiconductor film of such a photovoltaic element, an amorphous semiconductor film such as a-Si, a-SiC, a-SiGe, or a crystalline semiconductor film such as a microcrystalline silicon film or a polycrystalline silicon film is used. Have been.

Semiconductor films conventionally used in photovoltaic elements are roughly classified into amorphous semiconductor films and crystalline semiconductor films, and each has the following advantages and disadvantages. That is, since the amorphous semiconductor film has a large light absorption coefficient, a photovoltaic element can be formed with a relatively small thickness.
Another advantage is that a thin film can be formed at a low temperature. However, there is a drawback that the increase in defect density due to light degradation is large, and furthermore, there is a drawback that photoconductivity is low.

On the other hand, a crystalline semiconductor film has the advantages of a small defect density, a small increase in the defect density due to light degradation, and a high photoconductivity. There is a disadvantage that the thickness of the film to be a layer must be increased.

SUMMARY OF THE INVENTION An object of the present invention is to provide a functional thin film having a combination of the advantages of an amorphous semiconductor film and the advantages of a crystalline semiconductor film, and a photovoltaic device having improved power generation characteristics by using the thin film. It is to provide a power element.

[0006]

The present inventors have conducted intensive studies to develop a functional thin film having the advantages of the amorphous semiconductor film and the crystalline semiconductor film as described above. It has been found that by mixing a continuous phase in a state and a discontinuous phase in a crystalline state, a functional thin film having both advantages can be obtained, and the present invention has been completed.

That is, the functional thin film of the present invention is characterized in that a continuous phase in an amorphous state and a discontinuous phase in a crystalline state are mixed. According to the present invention, a discontinuous phase in a crystalline state is dispersed in a continuous phase in an amorphous state, whereby a functional thin film having advantages of the amorphous state and the crystalline state can be obtained. it can.

In the present invention, the advantages of different materials can be combined by forming the continuous phase in the amorphous state and the discontinuous phase in the crystalline state from materials having different compositions. The content of the main component element in the discontinuous phase in the crystalline state is preferably 10 atomic% or less in the continuous phase in the amorphous state, and the content of the main component element in the continuous phase in the amorphous state is preferably It is preferably 10 atomic% or less in the crystalline discontinuous phase.

As the amorphous component constituting the continuous phase in the amorphous state, for example, a-SiC, a-SiN, aC,
a-Si, a-Ge, a-SiGe, a-SiSn, a
—SiO, a-GaAs, a-SiO ₂ , a-Se, a
—As ₂ Se ₃ , a-BaO, a-P ₂ O _{3 and the} like. These amorphous components may be passivated by containing hydrogen.

Also, a crystal constituting a discontinuous phase in a crystalline state
As the quality component, for example, C, Si, Ge, Sn, B,
P, Al, Ga, In, S, GaAs, GaP, In
P, CIS, CdS, Cu_TwoS, SiO_Two, BN, Si
_ThreeN_Four, Mg_TwoF_Two, Al_TwoO _Three, ZnO, ITO, S
nO_Two, Ca, Mg, Co, Cr, Mo, Ag, W, C
d, Au, Cu, Ni, Zn, Fe, Mn, etc.
It is.

The combination of the amorphous continuous phase and the crystalline discontinuous phase can be arbitrarily performed, and the functional thin film of the present invention can be formed by an arbitrary combination. By such a combination, functional thin films having various characteristics can be formed, and semiconductor materials such as photovoltaic elements, sensors and transistors, mechanical materials such as hard coatings, conductive materials such as cables, electrode materials, and contact materials. It can be a thin film used in applications such as application materials, heat-generating materials, insulating materials, and dielectric materials.

In the present invention, the volume ratio of the crystalline discontinuous phase to the entire thin film is preferably 20% to 80%. Outside of such a range, the effect as a functional thin film in which the amorphous state and the crystalline state are mixed is not sufficiently exhibited. (Photoelectric conversion efficiency) tends to decrease.

In the present invention, when the crystalline discontinuous phase is dispersed in the form of crystalline particles, the size of the crystalline particles is preferably 40 ° or more. When the size of the crystalline particles is less than 40 °, for example, when used as a power generation layer of a photovoltaic element, high energy conversion efficiency may not be obtained.

The functional thin film of the present invention can have both the advantages of an amorphous material and the advantages of a crystalline material. For example, it is known that the color tone of an amorphous material such as a-Si: H can be changed by changing its film thickness. It is known that crystalline materials generally have high hardness. Therefore, by using an amorphous material whose color tone can be controlled by changing the film thickness for the continuous phase and using a crystalline material having high hardness for the discontinuous phase, the functional thin film material of the present invention can be used, for example, as a colored coating material. Can be used as In addition, as described above, in the conductive material, the heat generating material, the insulator material, the dielectric material, and the like, the characteristics of the amorphous material and the characteristics of the crystalline material are combined to form a functional thin film having various functions. be able to.

The main component of the amorphous continuous phase is a-Si:
H, and the main component of the discontinuous phase in the crystalline state is Ge
The iGe-based thin film exhibits excellent characteristics particularly as a thin film constituting a photovoltaic element. In recent years, to make effective use of light,
There is a demand for a material that can use light having a longer wavelength than before, and a-Si having a narrow band gap is used as an amorphous material.
The development of a photovoltaic element using a Ge thin film has been advanced. However, generally, when elements for adjusting a band gap such as Ge, Sn, or carbon are added to a-Si, the defect density due to these elements increases,
There is a problem that the film quality is deteriorated. S according to the present invention
An iGe-based thin film is a thin film having both the advantages of amorphous Si and the advantages of crystalline Ge. That is, crystalline Ge has a narrower band gap than a-Si, and therefore can absorb light having a longer wavelength than a-Si. Further, since crystalline Ge has a low defect density, the addition of Ge does not cause an increase in the defect density unlike conventional a-SiGe. Further, since the light absorption coefficient of an amorphous material is generally at least two orders of magnitude greater than that of a crystalline material, the absolute value of the light absorption coefficient of an amorphous and crystalline Is determined by
The SiGe-based thin film according to the present invention has a high light absorption coefficient as a whole thin film, and does not need to be thick like the conventional amorphous thin film. Further, by adding an n-type dopant such as P to the SiGe-based thin film, it can be used as an n-layer of a photovoltaic element.

In the above-mentioned SiGe-based thin film, it is preferable that 90 atomic% or more of Ge in the thin film exists in a discontinuous phase in a crystalline state. If Ge in the crystalline discontinuous phase is less than 90 atomic% of Ge in the thin film, it may be difficult to obtain a thin film having a large light absorption coefficient and high photoconductivity.

Further, as the functional thin film according to the present invention, an SiC-based material in which the main component of the amorphous continuous phase is a-Si: H and the main component of the crystalline discontinuous phase is carbon (C) is used. A thin film can be mentioned. By combining a-Si with crystalline carbon having a wide band gap, excellent light absorption in a short wavelength region is obtained without increasing the defect density due to the addition of carbon as in conventional a-SiC. A functional thin film can be obtained. Further, by adding a p-type dopant such as B to the thin film, it can be used as a p-layer on the light incident side of the photovoltaic element.

When the functional thin film of the present invention is used for a photovoltaic device, light absorption and accompanying generation of photocarriers such as electrons and holes are caused by an amorphous continuous phase having a large light absorption coefficient. As a result, the generated carriers move through a discontinuous phase in a crystalline state with high photoconductivity. Therefore, the carrier traveling characteristics are improved as compared with the photovoltaic element using the amorphous semiconductor thin film, and the photovoltaic element is manufactured with a smaller thickness than the photovoltaic element using the crystalline semiconductor thin film. be able to. Therefore, a photovoltaic element having both the advantages of a photovoltaic element using an amorphous semiconductor thin film and the advantages of a photovoltaic element using a crystalline semiconductor thin film can be obtained.

In the photovoltaic device using the functional thin film of the present invention, when the photocarrier moves to the discontinuous phase in the crystalline state, the amorphous material in the continuous phase and the crystalline material in the discontinuous phase Loses energy corresponding to the band gap difference of Therefore, when the functional thin film of the present invention is used for a photovoltaic device, it is preferable that the band gap difference between the amorphous continuous phase and the crystalline discontinuous phase be as small as possible.

[0020]

FIG. 1 is a cross-sectional view schematically showing a SiGe-based thin film of one embodiment according to the present invention. As shown in FIG. 1, crystalline Ge particles 2 that are a discontinuous phase in a crystalline state are dispersed in a matrix 1 made of a-Si: H that is a continuous phase in an amorphous state. Such a thin film can be formed, for example, by selecting a thin film forming condition under which crystallization of Ge from the gas phase occurs selectively. For example, in the case of a plasma CVD method, amorphous SiGe
As compared with the thin film formation conditions described above, the substrate can be formed by raising the substrate temperature and highly diluting the source gas with a larger amount of hydrogen than usual. Table 1 shows typical forming conditions in the plasma CVD method.

[0021]

[Table 1]

In Table 1, the flow rate of GeH ₄ is
It is a GeH ₄ conversion flow rate. That is, since the GeH ₄ gas is usually diluted to about 1% with hydrogen, the flow rate of the raw material gas diluted to 1% with hydrogen is about 100 times this flow rate. Further, within the flow rate range of the raw material gas, SiH ₄
The flow rate of each source gas is set so that the gas is diluted 50 times or more with the H ₂ gas.

Table 2 shows the light absorption coefficient, optical gap, dark conductivity, and photoconductivity of one example of the SiGe-based thin film according to the present invention. The conditions for forming this thin film are SiH
₄ flow rate 20 sccm, GeH ₄ flow rate 4 sccm, H ₂ flow rate 1200 sccm, substrate temperature 350 ° C., RF power 8
0 mW / cm ² and pressure 20 Pa. Further, α ₄₀₀ and α ₇₀₀ in Table 2 represent light wavelengths of 400 nm and 7 nm, respectively.
The light absorption coefficient at 00 nm is shown.

[0024]

[Table 2]

As shown in Table 2, as compared with the a-Si: H film, the a-Si: H film has characteristics of high photoconductivity and a high light absorption coefficient for long-wavelength light. Si: H
It shows a characteristic that light absorption of short wavelength light is higher than that of the film.
The high photoconductivity and the high light absorption coefficient of long wavelength light are thought to be due to the inclusion of crystalline Ge. On the other hand, the high light absorption coefficient of short wavelength light is considered to be due to the fact that silicon exists in an amorphous state. Further, in the SiGe-based thin film according to the present invention schematically shown in FIG. 1, since amorphous silicon is present on the surface of the crystalline Ge particles, the crystalline Ge grain boundary is made of amorphous silicon. It is considered that passivation can be performed, and defects at the crystal grain boundaries are reduced.

Under the above conditions for forming a thin film, the source gas is S
iH_FourAnd GeH_FourThe gas is exemplified, but the source gas is
Other hydrogenated gases,
Si _TwoCl₆, SiF_FourAnd GeF_FourSuch as halogen
Gas and the like can also be used as a source gas.

In the present invention, the characteristics of the thin film can be controlled by changing the volume ratio of the crystalline particles to the whole thin film. For example, in the case of the SiGe-based thin film shown in FIG. 1, by increasing the volume ratio of the crystalline Ge particles, the absorption of long-wavelength light can be increased, and the conductivity during light irradiation can be increased. The above-mentioned plasma CVD
When forming by the method, the volume ratio of the crystalline Ge particles can be increased by increasing the substrate temperature relatively, increasing the flow rate of the Ge source gas relatively, and increasing the dilution ratio with hydrogen.

In the present invention, the characteristics of the thin film can be controlled by changing the size of the crystalline particles. By increasing the size of the crystalline particles, defects associated with crystal grain boundaries can generally be reduced. Plasma CVD of SiGe based thin film as shown in FIG.
When forming by the method, the RF power is relatively low,
By relatively increasing the substrate temperature and increasing the flow rate of the Ge source gas, the size of the crystalline particles can be increased. Although the influence of the hydrogen dilution rate is small, the size of the crystalline particles tends to increase as the hydrogen dilution rate increases.

FIG. 2 shows the SiGe according to the present invention shown in FIG.
It is sectional drawing which shows an example of the photovoltaic element which used the system thin film for the electric power generation layer. Referring to FIG. 2, A functioning as a back electrode
An n-type microcrystalline silicon layer 12 having a thickness of 150 ° is formed on a metal substrate 11 made of g. On this microcrystalline silicon layer 12, a SiGe-based thin film 13 according to the present invention having a thickness of 4000 ° is formed. In this SiGe-based thin film, as schematically shown in FIG.
e particles are dispersed in the a-Si: H matrix.
A non-doped a-SiC: H film 14 having a thickness of 100 ° is formed on the SiGe-based thin film 13, and a p-type a-SiC: H film 15 having a thickness of 100 ° is formed thereon. . Further thereon, a transparent electrode 16 made of ITO having a thickness of 700 ° is formed, and a collector electrode 17 made of metal is formed thereon. In addition, as shown in FIG. 2, the light 10 enters from the transparent electrode 16 side.

Microcrystalline silicon layer 12, SiGe thin film 1
3. Non-doped a-SiC: H film 14 and p-type a-Si
Each of the C: H films 15 is formed by a glow discharge plasma CVD method. SiGe based thin film 13 according to the present invention
Is formed under the same forming conditions as those of the thin film whose characteristics are shown in Table 2. The volume ratio of the crystalline Ge particles in the thin film was 70%, and the size was 400 °. The volume ratio of the crystalline Ge particles was measured by Raman spectroscopy, and the size of the crystalline Ge particles was measured by X-ray diffraction. In addition, the photoelectric conversion efficiency before light irradiation for light degradation was 10.6%.

A photovoltaic device was manufactured by changing the volume ratio of crystalline Ge particles in the power generation layer 13 of the photovoltaic device shown in FIG. 2, and the relationship between the volume ratio of crystalline Ge particles and the photoelectric conversion efficiency. It was investigated. The volume ratio of the crystalline Ge particles was changed mainly by changing the substrate temperature. As described above, by increasing the substrate temperature relatively, crystalline Ge
The volume ratio of the particles can be increased. Table 3 shows the photoelectric conversion efficiency after light irradiation (AM 1.5, 5 sun, 25 ° C., 160 minutes).

[0032]

[Table 3]

As is clear from Table 3, a relatively high photoelectric conversion efficiency is obtained when the volume ratio of the crystalline Ge particles is in the range of 20% to 80%. Next, a photovoltaic element was manufactured by changing the size of the crystalline Ge particles in the power generation layer 13 of the photovoltaic element shown in FIG. 2, and the relationship between the size of the crystalline Ge particles and the photoelectric conversion efficiency was examined. did. The size of the crystalline Ge particles was changed mainly by changing the substrate temperature and the RF power. As described above, when the substrate temperature is increased, the size of the crystalline Ge particles tends to increase, and when the RF power is decreased, the size of the crystalline Ge particles tends to increase. Table 4 shows the relationship between the size of the crystalline Ge particles and the conversion efficiency after light irradiation. In addition,
In this example, crystalline Ge particles exceeding 400 ° could not be formed.

[0034]

[Table 4]

As is apparent from Table 4, the conversion efficiency is improved as the size of the crystalline Ge particles is increased. In particular, the size of the crystalline Ge particles is 40 °.
It can be seen that good conversion efficiency can be obtained above.

FIG. 3 is a cross-sectional view showing an example of a photovoltaic element using the SiGe-based thin film according to the present invention for the power generation layer and the n-layer. Referring to FIG. 3, Ag functioning as a back electrode
An n-type doped SiGe-based thin film 22 having a thickness of 150 ° is formed on a metal substrate 21 made of. On this thin film 22, an SiGe-based thin film 23 having a thickness of 4000 ° is formed. These SiGe-based thin films 22 and 23 are thin films according to the present invention, and are thin films in which crystalline Ge particles are dispersed in an a-Si: H matrix as shown in FIG. The thin film 23 is formed under the same forming conditions as in the above embodiment, and the thin film 22 is formed by using PH ₃ as an impurity gas.

On the SiGe-based thin film 23, a film thickness of 100
ノ ン non-doped a-SiC: H film 24 and film thickness 100
A p-type a-SiC: H film 25 is formed thereon, and a transparent electrode 26 made of ITO having a thickness of 700 is further formed thereon.
Are formed. A collecting electrode 27 made of metal is formed on the transparent electrode 26. Light 10 is transmitted through transparent electrode 2
It enters from the 6 side. The thin films 24 and 25 are formed in the same manner as the thin films 14 and 15 shown in FIG.

The volume ratio of the crystalline Ge particles in the n-layer 22 was 80%, and the size was 500 °. The volume ratio of crystalline Ge particles in the power generation layer 23 is 70%.
% And its size was 400 °. The energy conversion efficiency before light degradation was 10.7%. The energy conversion efficiency after light irradiation was 10.3%, indicating higher energy conversion efficiency than the photovoltaic device of the example shown in FIG. This is because, by using an a-Si: H film containing crystalline Ge particles according to the present invention as the n-layer, the doping efficiency is improved, the Fermi level is increased, the internal electric field is increased, and the fill factor (F .F.) Was improved and the conversion efficiency was improved.

In each of the above embodiments, the S
Although the composition of the iGe-based thin film is substantially uniform in the thickness direction of the power generation layer, the composition may be changed in the thickness direction to form a graded structure. For example, a graded structure may be obtained by changing the volume ratio of crystalline Ge particles. In this case, a graded structure in which the volume ratio of the crystalline Ge particles increases toward the center of the power generation layer is provided, so that the interface between the p-layer or the n-layer and the i-layer is inclined to facilitate the movement of carriers. can do.

In the above embodiment, an example is shown in which the thin film containing crystalline particles according to the present invention is used for the power generation layer and the doped layer. However, the functional thin film of the present invention may be used for a buffer layer and the like.

Further, the structure of the photovoltaic element using the functional thin film of the present invention is not limited to the above embodiment, but can be used for other photovoltaic element structures. Further, the functional thin film of the present invention is not limited to the use of the photovoltaic element, for example, can be used as an infrared detection film of an infrared detector, furthermore, It can be widely used for semiconductor devices and the like.

[0042]

According to the present invention, by mixing a continuous phase in an amorphous state and a discontinuous phase in a crystalline state, the characteristics of the amorphous material in the continuous phase and the characteristics of the crystalline material in the discontinuous phase are combined. A functional thin film can be provided.

In the present invention, the characteristics of the material of the continuous phase and the characteristics of the material of the discontinuous phase are obtained by forming the continuous phase in the amorphous state and the discontinuous phase in the crystalline state from materials having different compositions from each other. And a functional thin film having both of the above.

For example, as shown in the embodiment, a
A thin film in which crystalline Ge particles are dispersed in a matrix of -Si: H has characteristics of high photoconductivity and low defect density, and exhibits a high light absorption coefficient. Therefore, by using such a thin film for a power generation layer of a photovoltaic element, initial characteristics such as short-circuit current can be improved, light deterioration can be reduced, and high energy conversion efficiency can be obtained.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view schematically showing a SiGe-based thin film of one embodiment according to the present invention.

FIG. 2 is a cross-sectional view showing an example of a photovoltaic element using a thin film of one embodiment for a power generation layer according to the present invention.

FIG. 3 is a cross-sectional view showing an example of a photovoltaic element using a thin film of one embodiment for an n-layer and a power generation layer according to the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... a-Si: H matrix 2 ... crystalline Ge particle 11, 21 ... metal substrate 12 ... n-type microcrystalline silicon layer 22 ... n-type SiGe-based thin film (a-S containing crystalline Ge particle)
13,23 ... SiGe-based thin film (a-S including crystalline Ge particles)
i, H film) 14, 24 ... non-doped a-SiC: H film 15, 25 ... p-type a-SiC: H film 16, 26 ... transparent electrode 17, 27 ... collector electrode

Claims (9)

[Claims] 1. A functional thin film wherein a continuous phase in an amorphous state and a discontinuous phase in a crystalline state are mixed. 2. The functional thin film according to claim 1, wherein the continuous phase in the amorphous state and the discontinuous phase in the crystalline state are made of materials having different compositions from each other. 3. The method according to claim 1, wherein the content of the main component element in the discontinuous phase in the crystalline state is 10 atomic% in the continuous phase in the amorphous state.
The content of the main component element in the continuous phase in the amorphous state is 10 atomic% in the discontinuous phase in the crystalline state.
The functional thin film according to claim 1, wherein: 4. The method according to claim 1, wherein the continuous phase in the amorphous state is substantially a-
The functional thin film according to any one of claims 1 to 3, wherein the functional thin film is composed of Si: H, and the discontinuous phase in the crystalline state is substantially composed of Ge. 5. The functional thin film according to claim 4, wherein 90 atomic% or more of Ge in the thin film exists in the discontinuous phase in the crystalline state. 6. The functional thin film according to claim 1, wherein a volume ratio of the discontinuous phase in the crystalline state to the whole thin film is 20% to 80%. 7. The functional thin film according to claim 1, wherein the discontinuous phase in a crystalline state is crystalline particles, and the size of the crystalline particles is 40 ° or more. 8. A photovoltaic device comprising the functional thin film according to claim 1 as a part of a constituent layer. 9. A photovoltaic device comprising the functional thin film according to claim 1 as a power generation layer.