JPH04338679A - Manufacture of photovoltaic element - Google Patents

Abstract

PURPOSE:To provide a method for manufacturing a photovoltaic element having high optical conversion efficiency and a small amount of using raw gas. CONSTITUTION:A photovoltaic element having a structure in which at least one set of p-, i-, n-type semiconductor films are deposited on a substrate having a conductive surface, the i-type film is amorphous silicon-germanium, content of germanium in the i-type film is varied in a thickness direction thereby to once monotonically reduce a band gap energy in the thickness direction and to again monotonically increase it, is manufactured. In a step of depositing the i-type film, a supplying flow rate of raw gas containing the germanium is first linearly increased from '0' to a predetermined flow rate with time, and the flow rate is linearly reduced to '0' with the time from when it reaches the predetermined flow rate.

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 element, which is a photoelectric conversion element that converts sunlight into electrical energy.

0002

[Prior Art] Photovoltaic elements are widely used as low-power power sources for consumer electronics such as calculators and wristwatches, and may be put to practical use in the future as an alternative power source to so-called fossil fuels such as oil and coal. It is attracting attention as a promising technology. A photovoltaic device is a technology that utilizes photovoltaic power at the pn junction of a semiconductor.A semiconductor such as silicon absorbs sunlight and generates photocarriers of electrons and holes, which are then transferred to the inside of the pn junction. It is caused to drift by an electric field and taken out to the outside. A semiconductor process is often used as a method for manufacturing such a photovoltaic element. Specifically, p
A single crystal or polycrystal of silicon whose valence electrons are controlled to be type or n-type is produced, and the single crystal or polycrystal is sliced into a silicon wafer having a thickness of about 300 μm. Furthermore, layers of different conductivity types are formed by using a suitable means such as diffusion with a valence electron control agent so as to have a conductivity type opposite to that of the wafer, thereby forming a pn junction.

By the way, monocrystalline silicon or polycrystalline silicon is mainly used in photovoltaic devices currently in practical use because of its high reliability and conversion efficiency.
As described above, crystalline photovoltaic elements using single crystal silicon or polycrystalline silicon generally have the disadvantage of high production costs because a semiconductor process is used in the manufacturing method.

Furthermore, since crystalline silicon has a relatively small light absorption coefficient, it needs to have a thickness of at least 50 μm when used as a photovoltaic device, and its band gap energy is about 1.1 eV, making it difficult to use as a photovoltaic device. 1 suitable for
．． It is narrower than 5 eV and has drawbacks such as inability to effectively utilize long wavelength components of incident light. Polycrystalline silicon also has grain boundary problems.

Furthermore, it is difficult to make crystalline silicon large in area, and in order to extract a large amount of power, it is necessary to connect unit elements in series or in parallel, which requires complicated wiring. For several reasons, including the need for expensive packaging to protect the photovoltaic device from mechanical damage.
There is a problem in that the production cost per unit of power generation is relatively high compared to other existing power generation methods.

[0006] Under these circumstances, the important technical issues in the practical application of photovoltaic devices for power use are the realization of low production costs and large-area devices, and various studies are being carried out. . As a result, low cost materials, e.g.
Materials with high conversion efficiency for elements include tetrahedral amorphous semiconductors such as amorphous silicon, amorphous silicon/germanium, and amorphous silicon carbide, II-IV group semiconductors such as CdS and Cu2S, and GaAs and GaAlAs. Compound semiconductors have been discovered. Among them, thin-film photovoltaic devices using amorphous semiconductors, especially amorphous silicon, are easy to increase in area, have a larger absorption coefficient than crystalline silicon, and can be thinner. It is viewed as promising because of its advantages, such as fewer restrictions on the substrate material and shape on which it is deposited. However, photovoltaic elements using amorphous silicon described above also have drawbacks such as low photoelectric conversion efficiency and low reliability, and various attempts have been made to solve these problems.

One of the reasons for the low photoelectric conversion efficiency of amorphous silicon is as follows. That is, the band gap energy of amorphous silicon is about 1.7 eV, and it does not absorb light in a long wavelength range of 700 nm or more. This is because the light in this area is not used effectively. In response, materials with small band gap energy that are sensitive even in this long wavelength region are being considered. Amorphous silicon germanium is one example. The band gap energy can be adjusted from about 1.3eV to about 1.7e depending on the ratio of silicon-containing source gas and germanium-containing source gas used during thin film deposition.
It can be controlled to any value up to V.

Further, US Pat. No. 2,949,498 discloses the use of a so-called stack structure in which a plurality of unit photovoltaic element structures are stacked as a method for improving photoelectric conversion efficiency. Although a pn junction crystal semiconductor is used in the above patent, incident light can be effectively utilized over a wide wavelength range by sharing the absorption wavelength range of incident light with unit photovoltaic element structures having different band gaps and energies. This idea is common to both crystalline and amorphous semiconductors. By using this method, the open circuit voltage Voc of the photovoltaic element characteristic
increases, and photoelectric conversion efficiency improves.

[0009] In the method using the above-mentioned stacked structure, the unit photovoltaic element structure is designed so that its band gap energy decreases in order from the light incident side. In this respect, amorphous silicon germanium is suitable as a material because it has a smaller band gap energy than amorphous silicon and whose band gap energy can be easily controlled.

On the other hand, by stacking a plurality of amorphous silicon unit photovoltaic device structures having the same band gap energy without an insulating layer between them, the Voc of the entire photovoltaic device is increased. A method has been proposed.

By the way, amorphous silicon/germanium is generally inferior to amorphous silicon in terms of film quality. Furthermore, when compared as a unit photovoltaic element, amorphous silicon has higher photoelectric conversion efficiency. This is said to be due to the fact that there are more localized levels in the depletion layer of amorphous silicon and germanium than in amorphous silicon.
Attempts have been made to reduce the above localized levels. For example, in Japanese Patent Publication No. 63-48197, activated fluorine atoms compensate for dangling bonds in amorphous silicon/germanium and reduce the localized levels. It is disclosed to reduce the number of.

On the other hand, studies have been made to improve the characteristics of amorphous silicon germanium photovoltaic elements by methods other than improving film quality as described above. For example, p-type semiconductor layer and/or
Alternatively, a method using a so-called buffer layer that provides a band width gradient at the junction interface between an n-type semiconductor layer and an i-type semiconductor layer is disclosed in U.S. Pat. No. 4,254,429, U.S. Pat.
No. 377,723. The role of the buffer layer is to increase the internal electric field and increase Voc.

Furthermore, as another method, there is a method of improving the characteristics of a photovoltaic device by changing the composition ratio of silicon and germanium contained in the film thickness direction to provide a compositional distribution in the i-layer, that is, a so-called graded layer. There is.

For example, according to US Pat. No. 4,816,082, the band gap energy of the i-layer in the portion in contact with the valence-controlled semiconductor layer on the first surface on the light incident side is increased; A method is disclosed in which the band gap energy is gradually decreased toward the center and further increased toward the second valence-controlled semiconductor layer. According to this method, carriers generated by light are efficiently separated by the action of an internal electric field, and the film properties are improved.

[0015]

[Problems to be Solved by the Invention] However, amorphous photovoltaic devices such as those made of amorphous silicon have lower photoelectric conversion efficiency than crystalline silicon photovoltaic devices, and the power generation cost per 1 W is low. is higher than existing thermal, hydropower, and nuclear power generation. Therefore, the reality is that amorphous photovoltaic devices have not yet become as widespread as crystalline photovoltaic devices and existing power generation means.

The present invention solves the above-mentioned problems and provides a method for manufacturing an amorphous photovoltaic element that has high light conversion efficiency and requires relatively little raw material gas.

[0017]

SUMMARY OF THE INVENTION The present invention provides at least one set of p-, i-, and n-type semiconductor films deposited on a substrate having a conductive surface, the i-type semiconductor film being amorphous silicon germanium; A method for manufacturing a photovoltaic device having a structure in which the germanium content in an i-type semiconductor film changes in the film thickness direction so that the band gap energy monotonically decreases once in the film thickness direction and then monotonically increases again. Therefore, in the step of depositing an i-type semiconductor film, the supply flow rate of the source gas containing germanium is first increased linearly with time from 0 to a predetermined flow rate, and from the time when the predetermined flow rate is reached, This is a method of manufacturing a photovoltaic device in which the supply flow rate is linearly reduced to 0 with respect to time.

[0018] Setting the point at which the predetermined flow rate is reached at a point at least 0.75 and no more than 0.8 with respect to the total deposition time from the start to the end of deposition of the i-type semiconductor film further increases the conversion efficiency. Therefore, it is preferable.

The band gap energy of the film deposited at the time when the predetermined flow rate is reached is set to 1.3 eV or more.
It is also preferable to set the voltage to 4 eV or less, since this further increases the conversion efficiency.

As a result of our studies, the present inventors first obtained the following knowledge. In other words, the change in the band gap energy in the film thickness direction in the i-type semiconductor thin film can be calculated using
The photoactivation was controlled by increasing linearly with time from SCCM to the maximum flow rate of the source gas, and then decreasing linearly with time from the maximum flow rate of the source gas to 0SCCM at the end of the deposition of the i-layer semiconductor thin film. A power device and a photovoltaic device in which band gap energy is reduced by introducing a constant flow rate of a raw material gas containing germanium in a total amount equal to that of the above from the start of deposition to the end of deposition of an i-type semiconductor thin film. In comparison, the former has higher photoelectric conversion efficiency. The present invention has been completed through further studies based on this knowledge.

The present invention can also be applied to a photovoltaic device in which a plurality of unit photovoltaic device structures are laminated, and in that case, the present invention can be applied to the i-type semiconductor thin film of at least one unit photovoltaic device structure. The effect can be obtained by applying.

FIGS. 6 to 9 show ps suitable for applying the present invention.
This is a schematic representation of an example of an in-type amorphous solar cell. FIG. 6 shows a solar cell with a structure in which light enters from the top of the figure, and in the figure, 1 is a substrate, 2 is a lower electrode, 3 is an n-type semiconductor layer, 4 is an i-type semiconductor layer, 5 is a p-type semiconductor layer, 6 represents an upper electrode, and 7 represents a current collecting electrode.

FIG. 7 shows a solar cell having a structure in which the substrate 1 is transparent and light enters from the bottom of the figure. FIG. 8 shows a solar cell having a structure in which two layers of the pin structure of the solar cell of FIG. 15 is p
type semiconductor layer. FIG. 9 shows a solar cell having a structure in which the pin structure of the solar cell shown in FIG. 6 is stacked in three layers.
represents a unit element of a solar cell, 23 is an n-type semiconductor layer, 2
4 represents an i-type semiconductor layer, and 25 represents a p-type semiconductor layer. Note that in any photovoltaic element, the n-type semiconductor layer and the p-type semiconductor layer can be used by changing the stacking order of each phase depending on the purpose. The configurations of these photovoltaic elements will be explained below.

The substrate p, i and n type semiconductor layers 3, 4, 5 have a thickness of at most 1μ.
Since it is a thin film of about 100 m thick, it can be deposited on a suitable substrate. Such a substrate 1 may be single crystal or non-single crystal, and may be electrically conductive or electrically insulating. Furthermore, they may be transparent or non-transparent, but it is preferable that they have little deformation or distortion and have desired strength. Specifically, Fe, N
i, Cr, Al, Mo, Au, Nb, Ta, V, Ti,
Metals such as Pt, Pb, or alloys thereof, such as thin plates such as brass and stainless steel, and composites thereof, as well as polyester, polyethylene, polycarbonate, cellulose acetate, polypropylene, polyvinyl chloride, polyvinylidene chloride, polystyrene, polyamide, polyimide, Films or sheets of heat-resistant synthetic resins such as epoxy, composites of these with glass fibers, carbon fibers, boron fibers, metal fibers, etc., and metal thin films of different materials on the surfaces of these metal thin plates, resin sheets, etc. /or SiO2, Si3
Examples include those whose surface is coated with an insulating thin film of N4, Al2O3, AlN, etc. by sputtering, vapor deposition, plating, etc., as well as glass and ceramics.

If the substrate is electrically conductive such as a metal, the substrate may be used as an electrode for direct current extraction, or if it is electrically insulating such as a synthetic resin, the surface on which the deposited film is formed may be used as an electrode. Al, Ag, Pt, Au, Ni, Ti,
Mo, W, Fe, V, Cr, Cu, stainless steel, brass, nichrome, SnO2, In2O3, ZnO, ITO
It is desirable to form electrodes for current extraction by subjecting so-called simple metals or alloys, such as metals, and transparent conductive oxides (TCO) to surface treatment in advance by plating, vapor deposition, sputtering, or other methods.

Of course, even if the substrate is electrically conductive such as metal, it is possible to improve the reflectance of long wavelength light on the surface of the substrate, or to change the composition of constituent elements between the substrate material and the deposited film. For the purpose of preventing mutual diffusion, a layer of different metals or the like may be provided on the side of the substrate on which the deposited film is formed. Also,
In the case of a solar cell having a layered structure in which the substrate is relatively transparent and light is incident from the side of the substrate, a conductive thin film such as the transparent conductive oxide or metal thin film is deposited in advance. This is desirable.

[0027] The surface of the substrate may be a so-called smooth surface or a minutely uneven surface.

[0028] In the case of forming a minutely uneven surface, the uneven shape is spherical, conical, pyramidal, etc., and the maximum height (Rmax) is preferably 500 to 5000 angstroms. Light reflection at the surface becomes diffused reflection, resulting in an increase in the optical path length of the reflected light at the surface. The shape of the substrate can be a flat plate with a smooth surface or an uneven surface, a long belt shape, a cylindrical shape, etc. depending on the purpose, and the thickness can be determined as appropriate to form a photovoltaic element as desired. However, if flexibility is required as a photovoltaic element, or if light is incident from the side of the substrate, it should be made as thin as possible within the range that allows the substrate to fully function. Can be done. However, from the viewpoint of manufacturing and handling of the substrate, mechanical strength, etc., the thickness is usually set to 10 μm or more.

In the photovoltaic device of the present invention, appropriate electrodes are selected and used depending on the configuration of the device. These electrodes include a lower electrode, an upper electrode (transparent electrode),
Examples include current collecting electrodes. (However, the upper electrode here refers to the one provided on the light incident side, and the lower electrode refers to the one provided opposite the upper electrode with the semiconductor layer in between.) The electrodes will be explained in detail below.

Lower electrode As for the lower electrode 2 used in the present invention, the surface to which light for photovoltaic generation is irradiated differs depending on whether the material of the substrate 1 described above is translucent or not (for example, the surface of the substrate When 1 is made of a non-transparent material such as a metal, light for generating photovoltaic force is irradiated from the upper electrode 6 side as shown in FIG. 6.) The location where it is installed differs.

Specifically, in the case of a layer structure as shown in FIGS. 6, 8 and 9, it is provided between the substrate 1 and the n-type semiconductor layer 2. However, if the substrate 1 is conductive, it can also serve as the lower electrode. However, even if the substrate 1 is conductive, if the sheet resistance value is high, the lower electrode may be used as a low-resistance electrode for current extraction or for the purpose of increasing the reflectance on the substrate surface and making effective use of incident light. 2 may be installed.

In the case of FIG. 7, a transparent substrate 1 is used, and since light is incident from the side of the substrate 1, the lower electrode 2 is used for the purpose of extracting current and reflecting light at the electrode.
is provided facing the substrate 1 with a semiconductor layer in between.

In addition, when an electrically insulating material is used as the substrate 1, the substrate 1 and n
A lower electrode 2 is provided between the type semiconductor layer 3 and the lower electrode 2 .

[0034] As the electrode material, Ag, Au, Pt, N
Examples include metals such as i, Cr, Cu, Al, Ti, Zn, Mo, and W, or alloys thereof, and thin films of these metals are formed by vacuum evaporation, electron beam evaporation, sputtering, or the like. Further, care must be taken so that the formed metal thin film does not become a resistance component with respect to the output of the photovoltaic element, and it is desirable that the sheet resistance value is preferably 50Ω or less, more preferably 10Ω or less.

Although not shown in the figure, a diffusion prevention layer such as conductive zinc oxide may be provided between the lower electrode 2 and the n-type semiconductor layer 3. The effect of the diffusion prevention layer is not only to prevent the metal element constituting the lower electrode 2 from diffusing into the n-type semiconductor layer, but also to provide a slight resistance value to the lower part provided with the semiconductor layer in between. Electrode 2 and transparent electrode 6
This can have effects such as preventing short circuits caused by defects such as pinholes between the photovoltaic element and the photovoltaic element, and confining incident light within the photovoltaic element by generating multiple interference due to the thin film.

Upper electrode (transparent electrode) The upper electrode (transparent electrode) 6 used in the present invention has a light transmittance of 85% in order to efficiently absorb light from the sun, a white fluorescent lamp, etc. into the semiconductor layer. The sheet resistance value is preferably 100Ω or less so as not to become a resistance component electrically with respect to the output of the photovoltaic element. SnO2 is a material with such properties.
, In2O3, ZnO, CdO, CdSnO4, ITO
Metal oxides such as (In2O3+SnO2), Au,
Examples include metal thin films made of extremely thin, translucent metals such as Al and Cu. The upper electrode 6 is laminated on the five p-type semiconductor layers in FIGS. 6, 8 and 9, and in FIG.
Since these are laminated on the substrate 1, it is necessary to select materials that have good adhesion to each other. As a method for manufacturing the upper electrode, a resistance heating evaporation method, an electron beam heating evaporation method, a sputtering method, a spray method, etc. can be used, and the method is appropriately selected depending on the necessity.

Current Collector Electrode The current collector electrode 7 used in the present invention is provided on the upper electrode 6 for the purpose of reducing the surface resistance value of the upper electrode 6. Electrode materials include Ag, Cr, Ni, Al, Au,
Examples include thin films of metals such as Ti, Pt, Cu, Mo, and W, or alloys thereof. These thin films can be used in a stacked manner. Further, the shape and area of the semiconductor layer are appropriately designed so that a sufficient amount of light incident on the semiconductor layer is ensured.

For example, it is desirable that the shape spreads uniformly over the light-receiving surface of the photovoltaic element, and that its area is preferably 15% or less, more preferably 10% or less of the light-receiving area.

Further, the sheet resistance value is preferably 50Ω or less, more preferably 10Ω or less.

The semiconductor layers 3, 4, and 5 are manufactured by a normal thin film manufacturing process, such as a vapor deposition method, a sputtering method, or a sputtering method.
It can be produced by using known methods such as high frequency plasma CVD, microwave plasma CVD, ECR, thermal CVD, and LPCVD as desired. As a method employed industrially, a plasma CVD method is preferably used, in which a raw material gas is decomposed by plasma and deposited on a substrate. Further, as the reaction apparatus, a batch type apparatus, a continuous film forming apparatus, etc. can be used as desired. When producing a semiconductor with controlled valence electrons, it is carried out by simultaneously decomposing PH3, B2H6 gases, etc. containing phosphorus, boron, etc. as constituent atoms.

I-type semiconductor layer The semiconductor material constituting the i-type semiconductor layer suitably used in this photovoltaic device is a-SiGe:H, a-SiGe:H, a-SiGe:H, a-SiGe:H,
-SiGe:F, a-SiGe:H:F, and other so-called group IV alloy semiconductor materials of the periodic table can be mentioned. In addition, in the stacked cell structure in which unit element configurations are stacked, semiconductor materials constituting the i-type semiconductor layer other than amorphous silicon/germanium include a-Si:H, a-Si:F
, a-Si:H:F, a-SiC:H, a-SiC:F
, a-SiC:H:F, poly-Si:H, poly
-Si:F, poly-Si:H:F, and other so-called group IV and IV group alloy semiconductor materials of the periodic table, as well as so-called compound semiconductor materials of groups III-V and II-VI of the periodic table. Can be mentioned.

As the raw material gas used in the CVD method, a chain or cyclic silane compound is used as a compound containing the silicon element, and specifically, for example, SiH4, SiF
4, (SiF2)5, (SiF2)6, (SiF2)4
, Si2F6, Si3F8, SiHF3, SiH2F2
, Si2H2F4, Si2H3F3, SiCl4, (S
iCl2)5, SiBr4, (SiBr2)5, SiC
l6, SiHCl3, SiHBr2, SiH2Cl2,
Examples include those in a gaseous state or easily gasifiable, such as SiCl3F3.

Compounds containing germanium elements include chain germane or germanium halides, cyclic germane or germanium halides, chain or cyclic germanium compounds, and organic germanium compounds having an alkyl group, specifically GeH4 , Ge
2H6, Ge3H3, n-Ge4H10, t-Ge4H
10,GeH6,Ge5H10,GeH3Cl,GeH
2F2,Ge(CH3)4,Ge(C2H5)4,Ge
(C6H5)4,Ge(CH3)2F2,GeF2,G
Examples include eF4.

P-type semiconductor layer and n-type semiconductor layer As the semiconductor material constituting the p-type or n-type semiconductor layer that is preferably used, the semiconductor material constituting the above-mentioned i-type semiconductor layer is doped with a valence electron control agent. obtained by As a manufacturing method, a method similar to the method for manufacturing the i-type semiconductor layer described above can be suitably used. Further, as a raw material, when obtaining a deposited film of Group IV of the periodic table, a compound containing an element of Group III of the periodic table is used as a valence electron control agent for obtaining a p-type semiconductor. Group III elements include B
, Al, Ga, and In. Specifically, compounds containing Group III elements include BF3, B2H6, B
4H10, B5H9, B5H11, B6H10, B(C
H3)3, B(C2H5)3, B6H12, AlX3,
Al(CH3)2Cl, Al(CH3)3, Al(OC
H3)2Cl, Al(CH3)Cl2, Al(C2H5
)3, A(OC2H5)3, Al(CH3)3Cl3,
Al(i-C4H9)5, Al(C3H7)3, Al(
OC4H9)3,GaX3,Ga(OCH3)3,Ga
(OC2H5)3,Ga(OC3H7)3,Ga(CH
3) 3, Ga2H6, GaH(C2H5)2, Ga(O
C2H5), (C2H5)2, In(CH3)3, In
(C3H7)3, In(C4H9)3, and the like.

As a valence electron control agent for obtaining an n-type semiconductor, a compound containing an element of Group V of the periodic table is used. Group V elements include P, N, As, and Sb. Specifically, compounds containing Group V elements include:
NH3HN3, N2H5N3, N2H4, NH4N3,
PX3, P(OCH3)3, P(OC2H5)3, P(
C3H7)3, P(OC4H9)3, P(CH3)3,
P(C2H5)3, P(C3H7)3, P(C4H9)
3,P(OCH3)3,P(OC2H5)3,P(OC
3H7)3,P(OC4H9)3,P(SCN)3,P
2H4, PH3, AsH3, AsX3, As(OCH3
)3, As(OC2H5)3, As(OC3H7)3,
As(OC4H9)3, AS(CH3)3, As(CH
3) 3,As(C2H5)3,As(C6H5)3,S
bX3, Sb(OCH3)3, Sb(OC2H5)3,
Sb(OC3H7)3, Sb(OC4H9)3, Sb(
CH3)3, Sb(C3H7)3, Sb(C4H9)3
Examples include.

Of course, these raw material gases may be used alone, or two or more types may be used in combination.

When the above-mentioned raw material is in a gaseous state at room temperature and pressure, a mass flow controller (hereinafter referred to as MF) is used.
The amount introduced into the film forming space is controlled by
If it is in a liquid state, it is gasified using a bubbler that can control the temperature as necessary using a rare gas such as Ar or He or hydrogen gas as a carrier gas, and if it is in a solid state, it is A rare gas such as or hydrogen gas is gasified using a heating sublimation furnace as a carrier gas, and the amount introduced is controlled mainly by the carrier gas flow rate and furnace temperature.

[0048]

EXAMPLES After preliminary tests were conducted to investigate the properties of amorphous silicon germanium thin films, tests of Examples and Comparative Examples were conducted. First, the film formation procedure will be explained. An amorphous silicon/gemanium film was produced in the following manner using a known RF discharge plasma CVD film forming apparatus shown in FIG.

In FIG. 10, 100 is a reaction chamber;
101 is a substrate, 102 is an anode electrode, 103 is a cathode electrode, 104 is a heater for heating the substrate, 105 is a grounding terminal, 106 is a matching box, 107 is an RF power source, 108 is an exhaust pipe, 109 is an exhaust pump, 110 is an exhaust pump. A membrane gas introduction pipe, 120 and 122 are valves, and 121 is a mass flow controller.

First, a 5 cm square substrate 101 is attached to the cathode in the reaction chamber 100, and the exhaust pump 10 is
The reaction chamber 100 was sufficiently evacuated using an ion gauge (not shown), and the vacuum level in the reaction chamber 100 was adjusted to 10 −6 Torr. Next, the substrate 101 is heated by the substrate heating heater 104.
was heated to 300°C. After the substrate temperature becomes constant, the valves 120 and 122 are opened, and the mass flow controller 121 is controlled to supply Si from a SiH4 gas cylinder (not shown).
H4 gas is introduced into the reaction chamber 1 through the gas introduction pipe 110.
It was introduced into 00. Similarly, the mass flow controller was controlled to introduce H2 gas and GeH4 gas. After adjusting the pressure controller (not shown) to maintain the internal pressure of the reaction chamber 100 at 1.5 Torr, power is applied from the RF power source 107 and the matching box 106
Plasma discharge was carried out for 60 minutes while minimizing the reflected waves by adjusting , and an amorphous silicon germanium film was deposited. The gas supply was stopped and the sample was taken out from the reaction chamber 100.

The above is the general deposition process for amorphous silicon germanium thin film deposition used in this experiment. A similar method was used when using a metal substrate.

Preliminary Test In this experiment, the relationship between the amount of GeH4 introduced during i-layer deposition and the film deposition rate, and the relationship between the amount of GeH4 introduced and the band gap energy of the deposited film were investigated.

First, a sample was prepared in which only a SiGe:H film corresponding to the i-layer film was deposited on a glass substrate by introducing constant amounts of Si2H6 gas, H2 gas, and GeH4 gas from the start to the end of i-layer film deposition. was created. However, in order to examine the relationship between the amount of GeH4 and the band gap energy of the i-layer, a plurality of samples were prepared with different amounts of GeH4 introduced.

The glass substrate used was a commercially available 7059 glass plate manufactured by Corning Corp. with a thickness of about 1 mm. The amount of raw material gas introduced was Si2H6 10SCCM,H for all samples.
2 100SCCM. And GeH4 is 1,
There were four types: 5, 10, and 20 SCCM. Applied RF
The power was 10 W, the temperature of the glass substrate was 350° C., and the pressure inside the chamber was 2 Torr. The film deposition time was 1 hour for all samples.

For each of the prepared samples, the film deposition rate, band gap energy, and Ge composition amount in the film were measured as follows. The deposition rate of the film is alpha
Step (Product name, TENCOR INSTRUME
The film thickness of the deposited film on the glass substrate was measured using a NTSC (manufactured by NTS), and the value was divided by the film deposition time.
The gap energy was determined using the absorption coefficient in the ultraviolet and visible light regions and the measured value of the film thickness of the deposited film on the glass substrate.

The relationship between the amount of GeH4 introduced and the film deposition rate and the relationship between the amount of GeH4 introduced and band gap energy are shown in FIGS. 4 and 5, respectively, for the four types of samples prepared as described above.

As a result, from FIG. 4, the film deposition rate was
It increases linearly as the amount of introduction increases, and Fig. 5
It was found that the band gap energy decreases convexly downward as the amount of GeH4 introduced increases.

Example 1 and Comparative Example 1 In Example 1, GeH as shown in FIG.
4 introduction profile, and in Comparative Example 1, a photovoltaic device was manufactured by continuously introducing GeH4 at a constant flow rate, and the energy conversion efficiency of the device was examined. In FIG. 1, T is the total deposition time, and t indicates the point in time when the amount of GeH4 introduced becomes maximum. Here, amorphous S is formed on a metal substrate in the order of n, i, and p layers.
An i:H thin film was deposited by a glow discharge method, and a transparent electrode and a grid-shaped metal electrode were vacuum-deposited thereon to form a photovoltaic device.

Since this example is particularly concerned with the i-layer, first, the metal substrate, the n- and p-layers, the transparent electrode (upper electrode),
The grid-like electrode (collecting electrode) will be briefly explained, and then the i-layer will be explained in detail.

[0060] As the substrate, a stainless steel SUS304 with a thickness of about 1 mm and polished on both sides was used.

[0061] The n-layer thin film is Si2H6, H2, PH3/
H21% dilution gas 10SCCM and 300SCCM respectively
, 12SCCM, introduced into the chamber, RF power 20
W was applied between the electrodes and deposited at a pressure of 1.5 Torr. The thickness of the n layer was 200 angstroms.

[0062] Furthermore, the p-layer thin film is made of SiH4, H2, BF
3/H22% dilution gas each 5.0SCCM, 300S
CCM, 5.0SCCM, introduced into the chamber, RF
A power of 150 W was applied between the electrodes, and a pressure of 1.5 Torr was applied.
It was deposited in The film thickness was 100 angstroms.

Indium tin oxide was used as the transparent electrode. Further, as a grid-shaped electrode, Cr, Ag, and Cr were vacuum-deposited in order using a grid-shaped mask.

Next, the i-layer will be explained. GeH4 introduced during i-layer film deposition was linearly increased from 0 SCCM at the start of film deposition to the maximum GeH4 amount, and then linearly decreased to 0 SCCM at the end of film deposition.

[0065] The amount of raw material gas introduced is Si2H610CC
M, H2100SCCM, maximum GeH4 introduction amount is 10S
CCM. The applied RF power was 10W. The film thickness was set to 2000 angstroms by adjusting the deposition time. Further, the position in the film thickness direction at which the amount of GeH4 introduced is maximum was set at 2/3 of the film deposition time.

As Comparative Example 1, the following i-layer manufacturing conditions were set. That is, the amount of GeH4 introduced during the i-layer deposition was kept constant at 10 SCCM from the start to the end of the film deposition.

[0067] The amount of raw material gas introduced is Si2H610SC.
CM, H2100SCCM. The applied RF power was 10W. The film thickness can be adjusted by adjusting the deposition time.
000 angstroms. The reason why the film thickness is set to 2000 angstroms is that the film thickness of the i-layer affects the conversion efficiency, and the conversion efficiency tends to decrease as the film thickness of the i-layer increases. By making the film thicknesses the same, changes in conversion efficiency due to differences in film thickness can be avoided.

A photovoltaic device was produced under each of the above conditions, the device temperature was maintained at 25° C., and the current/voltage curve was measured by irradiating simulated sunlight of 1.5 AM and 100 mW to determine the conversion efficiency. .

As a result, the conversion efficiency was approximately 40% higher with the i-layer prepared according to the GeH4 introduction profile of the present invention than with the case where a fixed amount of GeH4 was introduced, demonstrating the effect of the present invention.

Example 2 An experiment was conducted to find the optimal value of the position in the film thickness direction at which the band gap energy of the i-layer is minimized.

The thickness of the i-layer affects the conversion efficiency, and as the thickness of the i-layer increases, the conversion efficiency tends to decrease.
In this experiment, the deposition time of the i-layer film was kept constant so that the film thickness did not change even if the position in the film thickness direction where the band gap energy of the i-layer was minimized (the position of the maximum GeH4 supply flow rate) was changed. The conditions were set. This is based on the following knowledge.

A preliminary test revealed that when the amount of Si2H6 gas introduced is constant, the deposition rate of the i-layer film increases in proportion to the amount of GeH4 gas introduced (FIG. 4).

From this result, when the amount of GeH4 gas introduced is increased linearly with the film deposition time from 0SCCM at the start of film deposition to the maximum amount of GeH4 introduced, and then linearly decreased to 0SCCM at the end of film deposition, i The thickness of the layer is the area of a triangle whose base is the deposition time of the film and whose height is the maximum amount of GeH4 introduced. Therefore, if the maximum GeH4 introduction amount and the i-layer film deposition time are kept constant, even if the maximum GeH4
The film thickness of the i-layer can be kept constant no matter where in the film thickness direction the time from the start of the i-layer film deposition with the amount of . Therefore, after removing the influence of the thickness of the i-layer on the conversion efficiency, it is possible to determine the optimal value of the position in the thickness direction where the band gap energy of the i-layer is minimized.

The experiment was conducted under the following conditions in consideration of the above findings.

[0075] The amount of raw material gas introduced is Si2H610SC.
CM, H2100SCCM, maximum GeH4 introduction amount 10
It was called SCCM. The applied RF power was 10W. i
The deposition time of the layers was kept constant, and a plurality of photovoltaic elements having i-layer films having different times t from the start of i-layer film deposition at which the maximum amount of GeH4 introduced was produced. The same device as in Example 1 was used.

The conversion efficiency of each of the photovoltaic devices produced as described above was measured under the same conditions as in Example 1. The horizontal axis in Figure 2 shows the value obtained by dividing t by the total deposition time T of the i layer (t/
T), data is plotted with conversion efficiency taken on the vertical axis. However, relative values of conversion efficiency were plotted with the value of conversion efficiency of an element having t/T of 0.78 as 1. As a result, as t/T increases from 0.73 to 0.78, the conversion efficiency increases once and reaches the maximum at about 0.77, and then as the value on the horizontal axis increases from 0.77 to 1.00. It was found that it decreases.

That is, the optimum value of the position in the film thickness direction at which the band gap energy in the i-layer film is minimum is 0.75 to 0 from the start of deposition, assuming that the total deposition time of the i-layer film is 1.
．． I found out that it was when I was 80 years old.

Example 3 In this example, photovoltaic elements were produced in the same manner as in Example 1 except for the i-type semiconductor layer, and their conversion efficiencies were measured and compared. The materials and manufacturing conditions for the metal substrate, n- and p-layers, transparent electrodes, and grid-like substrate were the same as in Example 1, and the manufacturing conditions for the i-layer were as follows.

In this experimental example, an experiment was conducted to find the optimum value of the band gap energy, that is, the optimum value of the maximum amount of GeH4 to be introduced.

In this example as well, the manufacturing conditions were set according to the following procedure in order to avoid changes in conversion efficiency due to changes in the thickness of the i-layer.

First, the thickness of the i-layer is set to 2000 angstroms, and the amount of raw material gas introduced is Si2H610SC.
CM, H2100SCCM, and the maximum amount of GeH4 introduced were 1.5, 10, and 20SCCM, respectively. Each maximum G
The i-layer deposition film speed corresponding to the amount of eH4 introduced was determined from FIG. 4. Based on this value, the film deposition time of each i-layer was determined so as to achieve the set film thickness.

As described above, by setting the film deposition time of the i-layer to a length suitable for each maximum amount of GeH4 introduced, the film thickness of the i-layer can be kept constant and is not affected by the film thickness of the i-layer. The optimum value of the maximum amount of GeH4 to be introduced can be determined.

In addition, in this experimental example, the band gap
Considering the results of Example 2, the position in the film thickness direction where the energy is minimum was determined for each film deposition time so that the film deposition time of the i layer was 1 and the position was 0.78 from the start of film deposition. . The applied RF power was 10W.

The conversion efficiency of each of the photovoltaic devices produced as described above was measured. Further, for each band gap energy and Ge composition ratio in the i-layer film, values corresponding to the maximum amount of GeH4 introduced were determined.

FIG. 3 shows measured data plotted with band gap energy plotted on the horizontal axis and conversion efficiency plotted on the vertical axis. However, the conversion efficiency is such that the band gap energy of the deposited portion is 1.4 when the maximum GeH4 is supplied.
The conversion efficiency of the element, which was 2 eV, was expressed as a relative value, taking it as 1. From this figure, the band gap when the maximum GeH4 is supplied is
The conversion efficiency should be approximately constant in the energy range of about 1.3 eV to 1.4 eV. Furthermore, in the region from about 1.4 eV to about 1.7 eV, there is a tendency for the conversion efficiency to decrease as the band gap energy increases when the maximum GeH4 is supplied.

From the above results, it was found that the optimum value of the band gap energy is 1.3 eV to 1.4 eV.

[0087]

As explained above, according to the present invention, a photovoltaic device can be obtained which has high light conversion efficiency and requires relatively little raw material gas.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram showing a method of introducing germanium-containing gas in the method of the present invention.

FIG. 2 is a diagram showing the relationship between the relative time representing the point of maximum introduction of germanium-containing gas in the method of the invention and the conversion efficiency of the produced photovoltaic device.

FIG. 3 is a diagram showing the relationship between the bandgap energy of the portion deposited at the time of maximum introduction of germanium-containing gas and the conversion efficiency of the manufactured photovoltaic device in the method of the present invention.

FIG. 4 is a diagram showing the relationship between the amount of germanium-containing gas introduced and the film deposition rate in a preliminary test.

FIG. 5 is a diagram showing the relationship between the amount of germanium-containing gas introduced and the band gap energy of the deposited film in a preliminary test.

FIG. 6 is a schematic diagram showing an example of a photovoltaic device manufactured by the method of the present invention.

FIG. 7 is a schematic diagram showing an example of a photovoltaic device manufactured by the method of the present invention.

FIG. 8 is a schematic diagram showing an example of a photovoltaic device manufactured by the method of the present invention.

FIG. 9 is a schematic diagram showing an example of a photovoltaic device manufactured by the method of the present invention.

FIG. 10 is a schematic diagram showing an outline of a deposited film forming apparatus used in Examples.

[Explanation of symbols]

1,101 Substrate 2 Lower electrode 3, 13, 23 N-type semiconductor layer 4, 14, 24 I-type semiconductor layer 5, 15, 25 P-type semiconductor layer 6 Upper electrode 7 Current collecting electrode 10, 11, 12 Unit element 100 Reaction Chamber 102 Anode electrode 103 Cathode electrode 104 Substrate heating heater 105 Grounding terminal 106 Matching box 107 RF power supply 108 Exhaust pipe 109 Exhaust pump 110 Filming gas introduction pipe 120 Valve 121 Mass flow controller 122
valve

Claims (3)

[Claims] 1. At least one set of p-, i-, and n-type semiconductor films is deposited on a substrate having a conductive surface, the i-type semiconductor film being amorphous silicon germanium, and germanium in the i-type semiconductor film being amorphous. A method for manufacturing a photovoltaic device having a structure in which the band gap energy monotonically decreases once in the film thickness direction and monotonically increases again as the content of i-type semiconductor film changes in the film thickness direction. In the step of depositing germanium, the supply flow rate of the raw material gas containing germanium is first increased linearly over time from 0 to a predetermined flow rate, and from the point when the predetermined flow rate is reached, the supply flow rate is increased over time. A method for manufacturing a photovoltaic device in which the amount of energy decreases linearly to zero. 2. The method according to claim 1, wherein the point at which the predetermined flow rate is reached is a point at least 0.75 and no more than 0.8 with respect to the total deposition time from the start to the end of deposition of the i-type semiconductor film. . 3. The bandgap energy of the film deposited when the predetermined flow rate is reached is 1.3 eV or more.
The method according to claim 1, wherein the voltage is 4 eV or less.