JPH04192373A - Photovoltaic element - Google Patents

Abstract

PURPOSE:To manufacture a photoelectric conversion film excellent in quality while minimizing the conductivity to continuous photoirradiation as well as the increase in the defectives in the films by a method wherein the stoichiometric ratio of silicon base gas to germanium base gas used for the film formation step is controlled. CONSTITUTION:An i type semiconductor layer 104 comprising an amorphous semiconductor containing silicon and germanium is structured of a buffer layer 111, a germanium containing layer 112 and a buffer layer 113 successively laminated from the interface side to an n-type semiconductor layer 103. The germanium containing layer 112 comprising silicon and germanium whose content is to be increased in the direction from the n-type semiconductor layer 103 to a p-type semiconductor layer 105. That is, the composition ratio of 0 in the interface to the n-type semiconductor layer 103 side is to be increased in the ratio of about 0.02-0.10%/Angstrom in the film thickness direction toward the p-type semiconductor layer 105 side. In such a constitution, the conversion efficiency of the title photovoltaic element can be improved, further enabling the film thickness of the i type semiconductor layer 104 to be made thinner.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a photovoltaic element that generates an electromotive force when irradiated with light, and in particular, a photovoltaic element that uses an amorphous silicon germanium alloy and has a high photoelectric conversion efficiency. This invention relates to a highly reliable photovoltaic device.

[Conventional technology] Photovoltaic elements, which convert light energy such as sunlight into electrical energy, are widely used as small power sources for consumer products such as calculators and wristwatches. It is attracting attention as a technology that can be put into practical use as an alternative to fossil fuel electricity. A photovoltaic element utilizes the photovoltaic force of a semiconductor pn junction.
A semiconductor such as silicon (Si) absorbs sunlight to generate a pair of optical carriers consisting of electrons and holes, and the optical carriers are caused to drift by the internal electric field of the pn junction,
It is extracted externally as electrical energy. Such photovoltaic elements are manufactured using a semiconductor process.

Specifically, a single crystal of silicon whose valence electrons are controlled to be p-type or n-type is produced using a crystal growth method such as the Czochralski method (CZ method), and the single crystal is sliced into approximately 3
A silicon wafer with a thickness of 0 μm is made. Furthermore, layers of different conductivity types are formed to form a pn junction by appropriate means such as diffusing a valence electron control agent that makes the conductivity type opposite to that of the wafer.

By the way, from the viewpoint of reliability and conversion efficiency, single-crystal silicon is mainly used in photovoltaic devices currently in practical use, but as mentioned above, photovoltaic devices are manufactured using semiconductor processes. The production cost is high due to the use of In addition, single crystal silicon has a small optical absorption coefficient due to indirect transition, and a single crystal silicon photovoltaic device must be at least 50 μm thick in order to absorb incident sunlight, and the band gap is approximately 11eV,
1. Suitable for absorbing sunlight as a photovoltaic element.
Since the voltage is narrower than 5 eV, there are problems such as not being able to effectively utilize short wavelength components of sunlight.

Furthermore, even if polycrystalline silicon were used to reduce production costs, the problem of indirect transition would remain and the thickness of the photovoltaic element could not be reduced. Furthermore, polycrystalline silicon has grain boundaries and other problems.

Furthermore, because it is crystalline, it is not possible to manufacture large-area wafers, making it difficult to increase the area, and when extracting large amounts of power, wiring must be done to connect unit elements in series or parallel. The production cost per unit of power is lower than that of existing power generators due to the high cost of production and the need for expensive implementations to protect photovoltaic devices from mechanical damage caused by various weather conditions when used outdoors. The problem is that it is more expensive than other methods. Due to these circumstances, various studies are being carried out as lowering costs and increasing area are important technical issues in promoting the practical application of photovoltaic devices for power use.
The search for materials such as low-cost materials and materials with high conversion efficiency has been carried out.

Materials for such photovoltaic elements include amorphous silicon, amorphous silicon germanium (S i Ge),
Tetrahedral amorphous semiconductors such as amorphous silicon carbide (SiC) and II-V'l such as CdS and Cu2S
family and GaAs.

Examples include m-v group compound semiconductors such as GaAlAs. In particular, compared to single-crystal photovoltaic devices, thin-film photovoltaic devices made of amorphous semiconductors can be easily formed into large-area films, require thin films, and can be deposited on any substrate material. It has the following advantages and is considered promising.

However, in the photovoltaic device using the amorphous semiconductor described above, problems remain in terms of improvement in photoelectric conversion efficiency and reliability as a power device.

As a method for improving the photoelectric conversion efficiency of a photovoltaic element using an amorphous semiconductor, for example, narrowing the band gap to increase the sensitivity to long wavelength light has been carried out. In other words, amorphous silicon has a band gap of approximately 1
．． Since the voltage is 7 eV, light with a long wavelength of 700 nm or more cannot be absorbed and cannot be used effectively. Therefore, the use of a material with a narrow band gap that is sensitive to long wavelength light is being considered. An example of such a material is amorphous silicon germanium, whose band gap can be easily changed arbitrarily from about 1.3 eV to about 1.7 eV by changing the ratio of silicon source gas and germanium source gas during film formation.

In addition, as another method for improving the conversion efficiency of photovoltaic elements, the use of so-called tandem cells, in which a plurality of photovoltaic elements with a unit element structure are stacked, is disclosed in US Pat.
No. 949498. This tandem cell uses a pn junction crystalline semiconductor,
The idea is common to both crystalline and amorphous materials, and by stacking elements with different band gaps and efficiently absorbing each part of the sunlight spectrum, the open-circuit voltage (Voc) is increased. This is to improve power generation efficiency, that is, to improve conversion efficiency. In this case, the bandgap of the so-called bottom layer, which is a layer located below the top layer, is narrower than the bandgap of the so-called top layer, which is a fake located on the light incident side of the stacked elements. Designed to.

On the other hand, in Japanese Patent Publication No. 48197/1983, Shibukawa et al.
They have reported a so-called cascade-type battery in which multiple layers are stacked to increase the open-circuit voltage (Voc) of the entire device.

Amorphous silicon germanium has a narrow band gap and excellent sensitivity to long wavelength light, so it is used as a suitable material for the bottom layer of the tandem cell described above.

By the way, amorphous silicon germanium, which improves long-wavelength sensitivity and can be used as the bottom layer of tandem cells, is inferior to amorphous silicon in terms of film quality and is not suitable for photovoltaic elements. Even when fabricated, there is a drawback that the conversion efficiency is low. This is because there are more localized levels within the bandgap than in amorphous silicon. For this reason, research and development efforts are underway to improve film quality by reducing the localized levels within the bandgap, for example by using activated fluorine atoms to compensate for tumpling ponds in amorphous silicon germanium. Amorphous silicon germanium is known to have a reduced localized level density.

On the other hand, improvements in the characteristics of amorphous silicon germanium photovoltaic elements are being considered by methods other than the essential improvement of film quality as described above. As an example, a method using a so-called buffer layer in which the hand width is sloped at the bonding interface between a p-type semiconductor layer and/or an n-type semiconductor layer and a type 1 semiconductor layer is disclosed in U.S. Pat. No. 4,254,429. It is disclosed in the specifications of the same No. 4.377 and 723. The purpose of the buffer layer is to improve the internal electric field and open circuit voltage (■).

C) is the same as above.

Furthermore, as another method, a method of forming a so-called graded layer in which the composition distribution is set in the intrinsic layer (i-type semiconductor layer) and the characteristics are improved by changing the composition ratio of silicon and germanium has been disclosed. There is. For example, according to US Pat. No. 4,816,082, i
There is a method of widening the band gap of the type semiconductor layer, gradually narrowing the band gap toward the center, and then gradually widening the band gap toward the second valence-electron-controlled semiconductor layer. Disclosed. According to this method, carriers generated by light can be efficiently separated by the action of an internal electric field, and film properties are improved.

By the way, another problem with photovoltaic elements using amorphous semiconductors is that the conversion efficiency decreases over time due to light irradiation. This is caused by the film quality of amorphous silicon and amorphous silicon germanium being degraded by light irradiation, resulting in poor carrier mobility. This is a phenomenon unique to semiconductors.As a result, when used for power applications, reliability is poor, which is currently an obstacle to practical application.

A lot of research is being done to improve the photodegradation of amorphous semiconductors, and while some are looking into preventing photodegradation by improving the film quality, others are also looking into reducing photodegradation by improving the cell structure. . As a specific example of such a method, using the tandem cell structure described above is an effective means, and by making the intrinsic layer thinner (and shortening the travel distance of carriers), the deterioration of cell characteristics due to light irradiation is minimized. It has been confirmed that this will happen.

FIG. 11 is a cross-sectional view schematically showing the structure of the above-mentioned general pin-type amorphous photovoltaic element. This photovoltaic element 300 is one in which light enters from above in the figure, and above the substrate 301 (two lower electrodes 302, an n-type semiconductor layer 3
A 03.1 type semiconductor layer 304, a p type semiconductor m305, a transparent electrode 306, and a current collecting electrode 307 are laminated in this order.

Next, the bandgap profile of the above-mentioned conventional photovoltaic device will be explained using FIGS. 12(a) to 12(c). In the figure, the lower solid line Ev represents the top of the valence band, and the upper solid line Ec represents the bottom of the conduction band.

FIG. 12(a) shows the photovoltaic element 30 shown in FIG.
0 hunt profile. In the case of such a band profile, the open circuit voltage is determined by the band gap of the i-type semiconductor layer 304, so if a material with a narrow band gap is used here, the open circuit voltage will become small. In addition, free electrons and free holes generated in the type 1 semiconductor layer 304 in contact with the n-type semiconductor layer 305 are recombined by the high concentration recombination level existing at the pi interface, so they cannot be effectively extracted. It will be lost. Furthermore, a large amount of photoelectricity is absorbed on the light incident side, and the amount of absorbed light decreases exponentially toward the inside. The energy of the light absorbed on the light incident side is greater than the band gap, and this means that this extra energy cannot be used effectively.

Therefore, as described above, near the interface between the n-type semiconductor layer 303 and the l-type semiconductor layer 304 and the interface between the n-type semiconductor layer 305 and the l-type semiconductor layer 304, on the l-type semiconductor layer 304 side, In order to give a slope to the bandgap in the waveform, a sofa [308, 309] is provided.

The buffer layers 308 and 309 reduce the density of localized levels at each of these interfaces, allowing free electrons and free holes generated in the l-type semiconductor layer 304 to be taken out effectively. The band profile in this case is the first
This is shown in Figure 2(b).

Furthermore, assuming that light is incident from the n-type semiconductor layer 305 side, as described above, the n-type and n-type semiconductor layers of the l-type semiconductor layer 304
Buffer layers 311 and 314 are formed near the interfaces with the L-type semiconductor layers 303 and 305 and have a wide bandgap without containing a bandgap adjusting agent, and the bandgap gradually increases from the buffer layer 311 to the center of the L-type semiconductor layer 304. The sloped layer 312 becomes narrower, and the l-type semiconductor layer 30
A gradient layer 313 is formed in which the bandgap is narrower in the central portion of 4 than in the other portions, and the bandgap gradually widens from there toward the buffer layer 314. The bandgap is adjusted by changing the content of the bandgap adjusting agent. In the case of amorphous silicon germanium, germanium acts as a band gap adjusting agent for silicon. The band profile in this case is shown in FIG. 12(C). By doing so, light can be used effectively, the open circuit voltage increases, and the fill factor (FF) also improves. Here, the fill factor (FF) is the maximum power that can be extracted from the photovoltaic element under certain incident light conditions at the open circuit voltage (■
. C) and (■.C) divided by the product of short circuit current (1, e). Such methods are generally being studied with a focus on improving initial efficiency.
No particular consideration is given to photodegradation.

[Problems to be solved by the invention] In the conventional amorphous photovoltaic device described above, the conversion efficiency is not sufficient because the film quality is inferior to that of the crystalline silicon photovoltaic device, and the power generation cost per watt is low. The problem is that it is more expensive than existing nuclear power, thermal power, and hydropower. In order for amorphous silicon-based photovoltaic elements to be used for electric power applications along with existing power generation methods, it is required to further improve conversion efficiency.

In addition, as mentioned above, despite continued efforts to elucidate the mechanism of photodegradation and to investigate ways to prevent photodegradation, the problem of photodegradation has not been completely resolved, and the photovoltaic power of amorphous silicon Elements are required to be even more reliable. In the method of preventing deterioration using stacked cells described above, the overall amount of light absorbed by the photovoltaic element is reduced by making the intrinsic layer thinner, so although it may be effective in preventing photodeterioration, the initial photovoltaic Since the characteristics deteriorate, a problem arises if the requirements of high reliability and high efficiency cannot be met at the same time.

An object of the present invention is to solve the above-mentioned problems and provide a photovoltaic element with high reliability and high photoelectric conversion efficiency.

[Means for Solving Problem y] In order to solve the above-mentioned problems, the present inventors investigated a photovoltaic element with little photodeterioration and high conversion efficiency, and found that the initial efficiency was improved by improving the band profile. In addition to the improvement, we found that amorphous silicon germanium exhibits less photodegradation in certain germanium compositions. In other words, we found that by controlling the stoichiometric ratio of silicon-based gas and germanium-based gas used for film formation, it is possible to obtain a high-quality film with very little increase in conductivity and defects in the film when exposed to continuous light irradiation. Furthermore, we obtained the knowledge that a photovoltaic device with high initial efficiency can be obtained by controlling the composition ratio of silicon and germanium in the film thickness direction in the amorphous silicon germanium region. The present invention is based on this knowledge and has conducted further research to complete a photovoltaic element with little photodeterioration and high conversion efficiency.

The photovoltaic device of the present invention has at least one pin junction structure in which a p-type semiconductor layer, a type 1 semiconductor fake, and an n-type semiconductor layer are sequentially laminated, and the ] type semiconductor layer is a germanium-containing region. In a photovoltaic element having at least a germanium content of The germanium content increases monotonically in the film thickness direction toward the interface, and the rate of increase in the germanium content in the film thickness direction is 0.02 to 0.0. 10 atomic%/
Å.

The i-type semiconductor layer has a structure in which a germanium-containing layer and a buffer layer, which is a germanium-free region, are laminated, and the buffer layer is formed at the interface between the i-type semiconductor layer and the n-type semiconductor layer and/or the buffer layer. Alternatively, it may be provided at the interface with the p-type semiconductor layer. Further, the content of germanium in the germanium-containing layer near the interface on the p-type semiconductor layer side is preferably 20 to 70 atomic % when expressed as the composition ratio of germanium in the semiconductor constituent elements.

Note that the semiconductor constituent elements referred to here include C, Si, Ge, etc. in the ■ group raw conductors, and AI2. in the m-rv group semiconductors. It refers to elements essential to the structure of a semiconductor, such as P, Ga, As, In, and Sb. Therefore, hydrogen (H) and fluorine (F), which are added to amorphous silicon to destroy dangling bonds and improve film quality, are not included in the semiconductor constituent elements referred to here. Also,
A valence electron control agent is also not included in the semiconductor constituent elements referred to here. For example, phosphorus (P) becomes a semiconductor constituent element in III-V group raw conductors, but does not become a semiconductor constituent element when added to silicon as a valence electron control agent.

[Function] Next, the function of the photovoltaic device of the present invention will be described by describing reference experiments conducted by the inventors to complete the present invention.

(Reference Experiment 1) An amorphous silicon germanium film was formed on a glass substrate 701 in the following manner using a known high frequency plasma CVD film forming apparatus shown in FIG.

In the high frequency plasma CVD film forming apparatus shown in FIG. 13, a reaction chamber 700 includes an exhaust pipe 708 and a gas introduction pipe 71
is connected to the exhaust pipe 708, and the air is exhausted by an exhaust pump 709 provided at the other end of the exhaust pipe 708. Also, inside the reaction chamber 700 is an anode electrode 70 connected to a high frequency power source 702 via a matching box 706.
2 and a cathode electrode 703 which is grounded via a mutual connection terminal 705 are provided so as to face each other.

A glass substrate 701 is attached to the surface of a cathode electrode 703, and a heater 704 for heating the substrate 701 is provided inside the cathode electrode 703. A large number of gas cylinders (not shown) are connected to the gas introduction pipe 710 via a valve 711, a mass flow controller (MFC) 712, and a valve 713 provided for each gas cylinder.

First, a glass substrate 701 (made by Corning Co., Ltd. No. 7059 glass) made of alkali-free glass with a size of 5 cm square.
is attached to the cathode electrode 703 in the reaction chamber 700, the reaction chamber 700 is sufficiently evacuated by the exhaust pump 709, and the reaction chamber 700 is evacuated using an ion gauge (not shown).
The degree of vacuum inside was set to 10-'' Torr. Next, the substrate 701 was heated to 30oC using the substrate heating heater 704. After the substrate temperature became constant, the valves 711 and 713 were opened and the mass flow The controller 712 is controlled to supply S i Ha gas from an unillustrated S i Ha gas cylinder.
30 accm of gas was introduced into reaction chamber 700 via inlet tube 710 . Similarly, H2 gas 30
0 sccm and G e H4 gas 1 sccm
introduced. The internal pressure of the reaction chamber 700 is set to 1.5 Torr.
After adjusting a pressure controller (not shown) to keep the temperature at A crystalline silicon germanium film was deposited on the substrate 701 to a thickness of 1 μm. The gas supply is stopped and the substrate 70 is removed from the reaction chamber 700.
Sample M was taken out and designated as S-1. Similarly, G
e H4 gas amount 5 sccm, 10 sccm.

Samples were prepared by changing 2 Q sccm and S
-2, S-3, and S-4. Note that sccm is a flow rate expressed in cm3/s converted to standard conditions (0° C., 1 atm).

The elements that make up these samples are silicon, germanium, and hydrogen. Some of these samples were subjected to elemental analysis using Auger electron spectroscopy to determine the ratio of germanium element to the sum of germanium and silicon, that is, the composition ratio. Here, germanium and silicon are semiconductor constituent elements. Furthermore, the optical bandgap Eg0pt of a part of the sample was measured by measuring the spectral absorption coefficient using a visible spectrometer. These results are shown in Table 1. Furthermore, some of these samples and similarly prepared samples were deposited in an electron beam vacuum evaporation apparatus (U), not shown.
The sample was transferred to an EBX-6D type (manufactured by LBAC) evaporator, the interior of the evaporator was evacuated, and 1,000 Cr electrodes with a gap width of 250 μm and a length of 1 cm were deposited on the sample surface. Next, these samples were placed on a temperature-controllable sample stage and heated for 25 minutes.
AM-
1.5 of the solar spectrum at 100 mW/cm”
The change over time in the prior conductivity ap was measured when continuous irradiation was performed at an intensity of . The conductivity σp of the helium neon laser is
The measurement was performed by irradiating the sample with light (wavelength: 632.8 nm). Note that the AM value is an index expressing the quality of sunlight in relation to the zenith distance of the sun, and is expressed as 5ecZ when the zenith distance is 2 under a standard atmosphere. The results are shown in FIG. In the figure, the photoconductivity opO value is normalized by the initial value σp(○). As shown in the figure, it can be seen that the film with a high composition ratio of germanium element has less photodeterioration. In particular, the composition ratio of germanium is 2.
Those with a content of 0 atm% or more are stable to light irradiation.

(Reference Experiment 2) As in Reference Experiment 1, using the apparatus shown in FIG. 13, a 5 cm square substrate 701 made of Corning No. 7059 glass was heated to 300° C., and Si H4 gas 3 was heated to 300° C.
5i2He gas 10 sccm instead of 0 sccm,
Amorphous silicon germanium was deposited on the substrate 701 under exactly the same conditions as in Experiment 1, except that the H2 gas was 1 OOsccm and the high frequency power was LOW, and sample S-5 was obtained. Next, the amount of G e Ha gas was increased to 3 sec as described above.
Samples were prepared by varying the length in the range of ~30 sec and named S-6 to S-5-12, respectively.

By measuring the film thickness of a portion of these samples using a needle-step film thickness measuring device to determine the film formation rate, and further measuring the spectral absorption coefficient of a portion of the sample using a visible spectrometer, The optical bandgap was measured.

Further, as in Experiment 1, a part of the sample was subjected to elemental analysis by Auger electron spectroscopy to determine the ratio of germanium element in the film to the sum of silicon and germanium, that is, the composition ratio.

The above results are shown in Table 2. As shown in the table, the film formation rate, optical band gap, and germanium composition ratio are G
It can be seen that there is a correlation with e Ha gas flow rate.

Table 2 In conventional device configurations, the internal electric field is created with a band width slope at the junction interface between the p-type semiconductor layer and/or the n-type semiconductor layer and the intrinsic layer (type 1 semiconductor layer). The characteristics of the photovoltaic device are improved by using a so-called Hetufa layer, which improves the band gap from about 17 eV to about 1 eV by continuously changing the composition ratio of silicon and germanium. .5ev
It was made by changing it. The composition ratio of germanium at this time varies from O to about 50 atm%.

Furthermore, even when creating a so-called graded layer that improves properties by changing the composition ratio of silicon and germanium in the intrinsic layer, when using a composition with a wide band gap, germanium It contained a portion with a composition ratio of 20 atm % or less. In such a photovoltaic device, the characteristics of the buffer layer and gradient layer described above are not fully utilized, and in fact, the characteristics of the amorphous silicon germanium photovoltaic device are The reality is that it has been decided.

According to the device structure of the present invention, by adjusting the composition ratio of the germanium element in the i-type semiconductor layer and making the band profile into the desired shape, a photovoltaic power source containing amorphous silicon germanium with a composition that is easily photodegradable can be generated. It is also possible to improve the initial efficiency of the device.

[Example] Next, an example of the present invention will be described with reference to the drawings.

FIG. 1 is a schematic cross-sectional view showing the structure of a photovoltaic device according to an embodiment of the present invention.

This photovoltaic element is a photovoltaic element using an amorphous semiconductor, and includes a lower electrode 102, an n-type semiconductor layer 103, an i-type semiconductor layer 104, and a p-type semiconductor layer 10 on a substrate 101.
5. Transparent electrodes 106 are sequentially laminated, and a current collecting electrode 107 is further provided on the top surface of the transparent electrodes 106. The structure of each part except the type 1 semiconductor layer 104 is as follows.
This is the same as the conventional photovoltaic element described with reference to FIG.

First, the substrate 101 will be explained.

Each of the semiconductor layers 103, 104, and 105 is a thin film of about 1 μm at most, and is therefore deposited on a suitable substrate. Such a substrate 101 may be single crystal or non-single crystal, and may be electrically conductive or electrically insulating. Further, although they may be translucent or non-transparent, it is preferable that they are less deformed and distorted and have desired strength. Specifically, Fe.

Ni, Cr, A1. Mo, Au, Nb, Ta.

Metals such as V, Ti, Pt, Pd, Pb or alloys thereof, such as brass, stainless steel thin plates and composites thereof, polyester, polyethylene, polycarbonate, cellulose acetate, polypropylene, polyvinyl chloride, polyvinylidene chloride, polystyrene Examples include films or sheets of heat-resistant synthetic resins such as polyamide, polyimide, and epoxy, and composites of these with glass fibers, carbon fibers, boron fibers, metal fibers, and the like. Further, a metal thin film of a different material and/or 5iOz is applied to the surface of these metal thin plates, resin sheets, etc.

Si3N4. A1□03. Examples include those obtained by surface coating an insulating thin film such as AIN by sputtering, vapor deposition, plating, etc., as well as glass and ceramics.

When the substrate is used as the substrate 101 for a photovoltaic element, if the strip-shaped substrate is made of electrically conductive material such as metal, it may be used as an electrode for direct current extraction, or it may be made of synthetic resin or the like. If the material is electrically insulating, A1. Ag, Pt, Au,
Ni, Ti.

Mo, W, Fe, V, Cr, Cu, stainless steel, brass, nichrome, SnO211n20s +ZnO, ■T
Surface treatment is performed in advance using a method such as plating, vapor deposition, or sputtering using a so-called metal element or alloy such as O (I nz Os +5nOz ) or a transparent conductive oxide (TCO) to form an electrode for current extraction. is desirable.

Of course, even if the strip-shaped substrate is made of electrically conductive material such as metal, it is possible to improve the reflectance of long-wavelength light on the substrate surface, or to improve the interaction of constituent elements between the substrate material and the deposited film. A different metal layer or the like may be provided on the side of the substrate on which the deposited film is formed, for the purpose of preventing diffusion or the like. In addition, when the substrate 101 is relatively transparent and the photovoltaic element has a layered structure in which light is incident from the side of the substrate 101, a conductive thin film such as the transparent conductive oxide or metal thin film is coated in advance. It is desirable to form a deposit.

Further, the surface of the substrate 101 may be a so-called smooth surface or may be a minutely uneven surface.

In the case of a minutely uneven surface, the uneven shape should be hemispherical, conical, pyramidal, etc., and the maximum height (Rma
By setting x) to preferably 500 to 5,000 people, light reflection on the surface becomes diffuse reflection, resulting in a substantial increase in the optical path length of the reflected light on the surface. Substrate 101
Depending on the purpose, the shape can be a plate with a smooth or uneven surface, a long belt, a cylinder, etc., and its thickness is determined as appropriate so as to form the desired photovoltaic element. However, if flexibility is required as a photovoltaic element, or if light is incident from the side of the substrate 101, it is necessary to make it as thin as possible within the range that allows the substrate to fully function. can. 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 this photovoltaic device, appropriate electrodes are selected and used depending on the configuration of the device. Examples of these electrodes include a lower electrode 102, a transparent electrode (upper electrode) 106, and a current collecting electrode 107. (However, the upper electrode here refers to one provided on the light incident side, and the lower electrode 102 refers to one provided opposite to the upper electrode 106 with the semiconductor layer in between.) These electrodes will be explained in detail below.

Lower electrode 102 is provided between substrate 101 and n-type semiconductor layer 103. However, if the substrate 101 is conductive, the substrate 101 can also serve as the lower electrode. However, even if the substrate 101 is conductive, if the sheet resistance value (surface resistance value) is high, it may be used as a low resistance electrode for current extraction or by increasing the reflectance on the substrate surface and making effective use of incident light. The electrode 102 may be installed for the purpose of achieving this.

The electrode materials include Ag, Au, Pt, and Ni.

Examples include metals such as Cr, Cu, AI, 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, a diffusion prevention layer made of conductive zinc oxide (ZnO) or the like may be provided between the lower electrode 102 and the n-type semiconductor layer 103. The effect of the diffusion prevention layer is not only to prevent the metal element constituting the lower electrode 102 from diffusing into the n-type semiconductor layer 103, but also to provide a slight resistance value so that the metal element constituting the lower electrode 102 can be prevented from diffusing into the n-type semiconductor layer 103. To prevent short circuits due to defects such as pinholes occurring between the lower electrode 102 and the transparent electrode 106 provided, and to generate multiple interference with a thin film to confine incident light within the photovoltaic element. can be mentioned.

The transparent electrode (upper electrode) 106 preferably has a light transmittance of 85% or more in order to efficiently absorb light from the sun, a white fluorescent lamp, etc. into the semiconductor M. It is desirable that the sheet resistance value is 100Ω or less so that it does not become a resistance component with respect to the output of the photovoltaic element.

As a material with such characteristics, Sn○2゜I n
20s, ZnO, CaO, CdSn0. .

Examples include metal oxides such as IT◯ (In, 03 +5nOz), and metal thin films made of extremely thin, translucent metals such as Au, Al, and Cu.

The transparent RI 106 is laminated on the p-type semiconductor layer 105, and as a manufacturing method, a resistance heating evaporation method, an electron beam heating evaporation method, a sputtering method, a spray method, etc. can be used. be selected as appropriate.

The current collecting electrode 107 is provided on the transparent electrode 106 for the purpose of reducing the surface resistance value of the transparent electrode 106. The electrode materials include Ag, Cr, and Ni.

Examples include thin films of metals such as AI, Au, Ti, Pt, Cu, Mo, and W, or alloys thereof.7 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 enters the semiconductor layer. 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.

Each semiconductor layer 103, 104, 105 is manufactured by a normal thin film manufacturing process, such as a vapor deposition method, a sputtering method, a high frequency plasma CVD method, or a microwave plasma CVD method.
It can be produced by using, as desired, a known method such as the method, ECR (electron cyclotron resonance) method, thermal CVD method, or LPCVD (low pressure CVD) method. The method adopted industrially is to decompose the raw material gas with plasma,
Plasma CVD methods for depositing on a substrate are preferably used. Furthermore, as a reaction device, a batch type device, a continuous film forming device, etc. can be used as desired. Semiconductor layers with valence electrons controlled (n-type semiconductor layer 103, n-type semiconductor layer 1
05) is produced by simultaneously decomposing PH, B, H6 gases, etc. containing phosphorus (P), boron (B), etc. as constituent atoms.

The i-type semiconductor layer 104 is made of an amorphous semiconductor containing silicon and germanium, and includes a buffer layer 111, a germanium-containing layer 112, and a germanium-containing layer 112 from the interface side with the n-type semiconductor layer 103.
It has a structure in which buffer layers 113 are sequentially stacked. The buffer layers 111 and 113 are portions of the i-type semiconductor N104 near the interfaces with the n-type semiconductor layer 103 and the n-type semiconductor layer 105, respectively, and are regions that do not contain a band gap adjuster (that is, germanium).

On the other hand, the germanium-containing layer 112 is a layer made of silicon and germanium, and the germanium content increases in the direction from the n-type semiconductor layer 103 to the p-type semiconductor layer l○5. Quantitatively expressing this change in germanium content, since silicon and germanium are semiconductor constituent elements as used in the present invention, when the content of germanium element is expressed as a ratio to the sum of germanium and silicon, that is, as a composition ratio. , n-type semiconductor layer 103
The composition ratio is O at the side interface, and the composition ratio is 0302% to 0.10% toward the n-type semiconductor layer 105 side in the film thickness direction.
/Å. This solid type 1 semiconductor layer 104 is formed by decomposing a gas containing a silicon element and a gas containing a germanium element, which are source gases during film formation, and is formed by decomposing a gas containing a silicon element and a gas containing a germanium element.
The change in the germanium composition ratio in is achieved by changing the flow rate ratio of the gas. Of course, when forming the buffer layers 111 and 11.3, the gas containing the germanium element is not allowed to flow.

When producing an i-layer of amorphous silicon germanium as a semiconductor material constituting the i-type semiconductor layer 104, a-3
iGe:H, a-3iGe:F.

Examples include so-called group IV amorphous alloy semiconductor materials such as a-3iGe:H:F. In addition, semiconductor materials constituting the i-type semiconductor layer other than amorphous silicon germanium in a tandem cell structure in which unit element configurations are stacked include a-3i:H, a-Si:F, and a-Si:H:F.
, a-SiC:H.

a-3i C: F, a-S i C: H: F,
po1y-3i:H, poly-8i:F,
Examples include semiconductor materials such as I group and TV group gold alloys such as poly-3i:H:F, as well as m-v and ll-Vl group compound semiconductor materials.

Silicon (Si) is used as a raw material gas for the CVD method.
A chain or cyclic silane compound is used as a compound containing the element, specifically, for example, S i H4,S
i F4 , (SiFz n s, (S
i F2) a.

(SiFz)4. Si□Fa,S :s Fa,S i
HF3゜S i H2F2. S1. ,t
H2F4. S i 2 H3Fs.

5iC14, (SiC12)s, SiBr4. (SiB
rz) a, Si□C16, 5iHC1s, 5iH2B
Examples include gases such as ra, 5iH2C12, and Siz C13F3, or those that can be easily gasified.

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 alkyl groups, specifically QeH4, Ge2Ha,G
ez Ha, n-Gea H+o, t Ge4 H+
o, Gea H+o.

GeHsCl, GeH4F2. G, e (CHs)
4°Ge (Cm H8)4. G e (Ce
H5)4. G e(CHs )2 F2. Examples include GeFn.

The semiconductor material constituting the n-type semiconductor layer 103 or the n-type semiconductor layer 105 is obtained by doping the semiconductor material constituting the i-type semiconductor layer 104 described above with a valence electron control agent. The manufacturing method includes the above-described i-type semiconductor layer 10.
A method similar to the manufacturing method of No. 4 can be suitably used. Further, as a raw material, in the case of obtaining a deposited film of group Ⅰ of the periodic table, a compound containing an element of group m of the periodic table is used as a valence electron control agent for obtaining the n-type semiconductor layer 105.

Examples of the Group Ⅰ elements include B, Al, Ga, and In. Specifically, as a compound containing a group element, B
F3, 'B2 Ha, B4HIO, B5He,
Bs H++, Ba J(+o, B (CH3)3
, B(C21(6)s, Be He2.
A1:Xs, Al(CHs)2 C1, Al
(CHs) 3. Al(OCHs)2C1, Al
(CH3)C12,A1(C2Ha)! , A
l (QC2Hs) s, A12(CH3)3
C13, Al (i C4H9)3. AI (C3
H7)s, A1 (QC489)3, GaX
5.

Ga (OCH3) 3. Ga (QC2Ha)s.

Ga (QC3H7)3, Ga (CH3)
3.GazH,,GaH(C2Hs)2,
Ga (QC2Ha) (C2HJ2. I n (
CH3)3.1 n (C3H7)3.

Examples include I n (C4He) 3. However, X is any halogen element.

As the valence electron control agent for obtaining the n-type semiconductor layer 103, a compound containing an element of Group Ⅰ of the periodic table is used. Part ■
Group elements include P and N.

Examples include As and Sb. A specific example of a compound containing a Group Ⅰ element is NH3HN3.

N2H51'J3. N2H4, NH4N3. PX3゜P
(OCH3)3. P(○C2H3)3. P(C3H7)
! , P (QC-Ha)s, P(CHs)3. P (
C2Ha)3, P(CsH7)II.

P (C4H9)3. P (OCH3)3. P (QC
2Hs) 3゜P(○C3H7)s, P(○c-He)
s.

P (SCN)s, P2H4, PH3, AsH3゜A
S XS, A S (OCH3) 3. A S (QC
2Ha)3.

As (QC3H7) 3. , AS (QC4He
) 3.

A5(CH3)3. A S (C2H5)3. A.S.
(CsHs)3, 5bX3, s-b (
○ CH3)3. S b (OC28B) 3
, S b (QC3H7) 3 ,
Sb(QC4He)s, Sb(CH3
) 3, 5b(C3H7) 3. Sb (C4H
e) 3 etc. However, X is any halogen element.

Of course, these raw material gases may be one type, but 9
．． Two or more types may be used in combination.

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

Next, the band profile of this photovoltaic element will be explained.

FIG. 2(a) is a diagram showing the band profile of this photovoltaic device. It is assumed that light enters from the left side in the figure, that is, from the n-type semiconductor layer 105 side. In an amorphous silicon germanium semiconductor, the band gap becomes narrower by increasing the germanium content, so the band gap is approximately 1.30 eV near the interface between the germanium-containing layer 112 and the buffer layer 113.
The bandgap near the interface with 1.113 is approximately 1.7
It is 0eV. 5 In the present invention, the buffer layer 1
11. ．． 113 is not necessarily necessary, and the buffer layer 111 . ．． 113 may be removed. The bunt profiles obtained when either or both of the buffer layers 111 and 113 are removed are shown in FIGS. 2(b) to 2(d).
are shown respectively.

Next, the function of the photovoltaic element having the band profile shown in FIG. 2(a) will be explained.

The irradiation light enters from the side of the p-type semiconductor layer 105, and is applied to the buffer layer 113, germanium-containing layer]12, buffer layer 1]
1, optical carriers are generated. buffer layer 113
Because it has a wide bandgap, it absorbs only short-wavelength light and allows long-wavelength light to pass through it, allowing the light to reach the inside of the photovoltaic element and make effective use of the light. If here, the p-type semiconductor layer 10 of the i-type semiconductor layer 104
If the vicinity of the interface with 5 is formed with an amorphous silicon germanium film, the essential film quality of the film is poor,
Due to the lattice spacing mismatch between the 9th layer and the amorphous silicon germanium, the number of recombination levels increases and the photovoltaic device characteristics deteriorate. However, 1. Since the buffer layer 113 made of amorphous silicon is provided near the interface with the p-type semiconductor layer 105, there are relatively few recombination levels, and the high concentration of carriers generated at the pi interface is It can be effectively taken out without recombination, and since there are few recombination levels, the open circuit voltage increases and the photoelectric conversion efficiency improves. In addition, the buffer layer 111
is a layer having the same function as the buffer layer 113, and alleviates lattice mismatch at the Ni interface.

Furthermore, the effect that this band profile has on the collection of photogenerated carriers is explained as follows.

The optical bandgap in the i-type semiconductor layer 104 is p
Since the type semiconductor layer 105 side is thicker (and the n-type semiconductor layer 103 side is smaller, most photoexcited carriers are generated near the pFI side. At this time, if the slope of the band gap in the germanium-containing layer 113 is too steep, free Since the electric field acts to increase the electric field for holes, free holes generated near the n-layer side are accelerated toward the p-type semiconductor layer 105 and effectively extracted, but the energy at the bottom of the conduction band is absorbed by the p-layer. Since it is small on the side and large on the n-layer side, it acts to reduce the electric field for free electrons (7).Furthermore, the amount of germanium in the i-layer increases and the film quality deteriorates, so in order to improve the conversion efficiency, it is necessary to make the film thinner. If the slope of the bandgap in the germanium-containing layer 112 is too gradual, the slope of the energy at the bottom of the conduction band in the i-layer becomes gradual, which increases the probability of a shunt. Although the electric field acts to become relatively large for electrons, for free holes there is a relatively small difference in the energy at the top of the iM medium electron band between the p side and the n side. works to reduce the electric field, and the amount of germanium also decreases, so when trying to obtain a larger amount of current, the film thickness must be thicker (which is disadvantageous for holes with low mobility, The rate of deterioration is also high.

In general, the mobility of electrons is better than that of holes, so the effects of the electric field and film thickness act effectively on free holes (in general, the characteristics of photovoltaic devices are determined by the mobility of holes). Therefore, if the structure is such that the electric field is large relative to the holes, the characteristics of the photovoltaic device will be improved.

However, if you take into consideration the effects of film thickness and film quality, there is an appropriate band gap profile.

By using the device structure of the band file as shown in FIGS. 2(a) to 2(b) described above, a highly efficient and reliable photovoltaic device can be manufactured.

Next, examples of the photovoltaic device of the present invention will be described in more detail by citing specific numerical values. Note that the present invention is not limited in any way by these Examples.

(Example 1) A photovoltaic device using a pin-type heterojunction shown in FIG. 1 was manufactured using a deposited film forming apparatus shown in FIG. 3.

First, the deposited film forming apparatus used will be explained.

This deposited film forming apparatus has a plurality of film forming chambers connected in series, and the first one of the connected film forming chambers is a film forming chamber 501; , a load lock chamber 513 for introduction is connected, and the last one is connected to a load lock chamber (not shown) for taking pressure. Each film forming chamber has the same configuration, and a film forming chamber 502 is adjacent to a film forming chamber 501. The film forming chambers and the film forming chamber and the load lock chamber are separated by gate valves 507 that can be opened and closed.

The substrate 101 of the photovoltaic element is placed in a cassette 5 for transporting the substrate.
03 so that it is held downward. The substrate transport cassette 503 can be moved between each film forming chamber and each load lock chamber while holding the substrate 101 by a substrate transport means 506 provided in each film forming chamber and each load lock chamber. There is. In addition, the substrate transport cassette 5
03 is made of a conductive member, is grounded when in the film forming chamber, and operates as a cathode electrode for high frequency discharge for high frequency plasma CVD.

The details of the structure of the film forming chamber will be explained using the film forming chamber 501.

The film forming chamber 501 can be evacuated by being connected to an exhaust pump 515 via an exhaust valve 514, and is also connected to a gas supply source (not shown) such as a gas cylinder via a mass flow controller (not shown). Gas inlet pipes 508 and 509 are connected. The mass flow controller (not shown) is controlled by a microcomputer and can instantaneously change the gas flow rate. Typically, the mass flow controller responds to a change in set flow rate by reaching the new set flow rate within one second without overshoot. The error between the actual flow rate and the set flow rate was within ±2%. The pressure inside the film forming chamber 501 is measured by a pressure gauge 517 attached to the film forming chamber 501.
Measured by

Further, in the film forming chamber 501, a substrate transport means 5 is provided as described above.
06 is provided and connected to one end of a high frequency power source 510 via a matching box 511 at a position opposite to the substrate transport cassette 503 that has been transported by the substrate transport means 506 and moved to an approximately central position in the film forming chamber 501. An anode electrode 512 is provided. High frequency type @1s
The other end of 1o is grounded. At this time, as described above, the substrate transport cassette 503 is grounded and operates as a cathode electrode, so the pressure inside the film forming chamber 501 is reduced and a predetermined gas is introduced, and a high frequency type! By operating 510, high frequency plasma discharge is generated within the film forming chamber 501.

Further, a heater 505 is provided to heat the substrate 101 held in the substrate holding cassette 503 that has been moved to the center position of the film forming chamber 501, and the heating status is monitored by a thermocouple 504. It looks like this.

Next, a film forming method will be explained.

First, mirror polish the surface and reduce the maximum surface height (Rmax) to 0.
．． A stainless steel (SUS304) substrate 101 with a size of 50 mm x 50 mm and a size of 0.05 μm or less is placed in a sputtering device (not shown), and the inside of the device is filled with 10-IITo.
After evacuation to below rr, a lower electrode 102 (FIG. 1) is formed on the substrate 101 using Ar as a sputtering gas.
A thin Ag film with a thickness of approximately 1000 nm was deposited. This substrate 101 is taken out and placed on the substrate transfer cassette 503 on the substrate transfer means 506 in the load lock chamber 513.
The surface on which the lower electrode 102 was deposited was fixed facing downward as shown in FIG. 3, and the inside of the load lock chamber 513 was evacuated to a pressure of 10-5 Torr or less using an exhaust pump (not shown). During this time, the film forming chamber 501 is operated by the exhaust pump 515.
It is evacuated to a pressure below 0-'Torr. When the pressures in both chambers became almost equal, the gate valve 507 separating the two chambers was opened, the substrate transport cassette 503 was moved into the film forming chamber 501 using the substrate transport means 506, and the gate valve 507 was closed again.

Next, the surface temperature of the substrate 101 is raised to 20°C using the heater 505.
Heating was performed to a temperature of 0°C. When the substrate temperature stabilizes, 5izes gas 10 stored in a cylinder (not shown) is added.
105c and PH3 gas (1% diluted with hydrogen gas) 12s
ccm and 300 Sccm of H2 gas were introduced through gas introduction pipes 508 and 509 while being mixed. Next, the opening degree of the exhaust valve 514 is adjusted to reduce the internal pressure of the film forming chamber 501 to 1.5.
It was kept at Torr. A high frequency power source 510 is connected to an anode electrode 512 via a matching box 511, and by immediately applying 20 W of high frequency power of 13.56 MHz from the high frequency power source 510, an n-type semiconductor made of an amorphous n-type S1H film is produced. Deposition of layer 103 is started.

After the n-type semiconductor layer 103 was formed to a thickness of 200 layers, the introduction of gas and the supply of high frequency power were stopped, and the inside of the film forming chamber 501 was evacuated to 10-' Torr or less using the exhaust pump 515. At this time, the inside of the film forming chamber 502 adjacent to the film forming chamber 501 was also evacuated to 10-5 Torr or less.

Next, the substrate 101 and the substrate transport cassette 503 were moved from the film forming chamber 501 to the film forming chamber 502 by the substrate transport means 506. The film forming chamber 502 is for forming the n-type semiconductor layer 104.

The deposition chamber 501 and the deposition chamber 502 are separated by a gate valve 507, and by opening the gate valve only when transporting the substrate 101, it is possible to prevent n-type impurities from entering the n-type semiconductor layer 104. .

The substrate 101 is heated in the same manner as described above, and once the substrate temperature is stabilized, 5izHa gas is introduced into the film forming chamber 502 for 10 seconds.
, H2 gas was introduced for 100 seconds, and 12 W of high frequency power (13.56 MHz) was applied for 20 seconds to form a buffer layer 111 made of amorphous i-type Si:H with a thickness of 50 nm on the n-type semiconductor layer 103. was formed.

Subsequently, Si2H6 gas and H2 gas are introduced, and GeHa gas is introduced into the film forming chamber 502 by controlling a master flow controller (not shown) while supplying high-frequency power to form amorphous i-type 5iGe:H. A germanium-containing layer 112 having a thickness of 1000 layers was formed. In this case, during the formation of the germanium-containing layer 112, G
e By changing the flow rate of H4 gas, the content of germanium in the germanium-containing layer 112 was made to increase in the direction from the n-type semiconductor layer 103 side to the n-type semiconductor layer 105 side, which will be described later.

To describe the formation of the germanium-containing layer 112 in more detail, when starting the formation of the germanium-containing layer 112, G
e With the flow rate of H4 gas set to 0, germanium-containing layer 1
The mass flow controller was controlled so that the flow rate of G e H4 gas increased as the film formation of No. 12 progressed. In this case, from the results of the reference experiment mentioned above, the flow rate of the G e H4 gas and the germanium composition ratio in the film to be formed (the ratio of the germanium element content to the sum of silicon and germanium) are known. The flow rate of the G e H4 gas was controlled by a microcontroller so that the composition ratio in the film thickness direction increased at a constant rate. FIG. 4 schematically shows the relationship between the position in the film thickness direction and the germanium composition ratio. When the film thickness of the germanium-containing layer 112 reached a predetermined thickness (here, 1000 people), the introduction of the G e H4 gas was immediately stopped, and the film formation of the germanium-containing layer 11.2 was completed. Germanium-containing layer 112
At the moment when the film thickness of G e H4 reaches the predetermined thickness, that is, immediately before the film formation of the germanium-containing layer 112 is completed,
The gas flow rate was I 5 CCDI. Further, the film-forming time for the germanium-containing layer 112 was 8 minutes. Also,
G e When stopping the introduction of H4 gas, the G
e) (a) Since it was necessary to set the flow rate of gas to O, a solenoid valve was connected in series to a mass flow controller (not shown) in advance, and this solenoid valve was closed.

After the film formation of the germanium element 112 is completed, the introduction of Si2H6 gas and H2 gas and the input of high-frequency power are continued for 20 seconds, and a buffer made of amorphous i-type Si:H with a thickness of 50 μm is formed on the germanium-containing layer 112. Layer 113 was formed. After the formation of the buffer layer 113 was completed, the supply of gas and high frequency power was stopped, and the inside of the film forming chamber 502 was evacuated to below 10-5 Torr.

Note that the n-type semiconductor layer 104, that is, the buffer layer 111,
While forming the germanium-containing layer 112 and the buffer layer 113, the pressure inside the film forming chamber 502 was controlled to be stable at 1.1 Torr.

Next, the film is placed adjacent to the film forming chamber 502 and is heated to 10-5 Torr in advance.
The substrate transport means 5 is placed in a film forming chamber (not shown) that is evacuated to a temperature below r, while the substrate 101 is held in the substrate transport cassette 503.
Transported by 06. This film forming chamber has an n-type semiconductor layer 105.
It is intended to form a After heating the substrate 101 to 230° C. and stabilizing the substrate temperature, the Si Ha gas 5
sccm, B F 3 gas (2% diluted with hydrogen gas)
A p-type semiconductor layer made of microcrystalline p-type Si with a thickness of 100 μm was formed by introducing 5 sec. 105 as buffer layer 1
It was formed on top of 13.

After the p-type semiconductor layer 105 was formed, the substrate 101 on which the nip-type semi-conductor layer was formed was taken out through a load-lock chamber for taking out (not shown).

This substrate 101 is mounted on a vacuum evaporation apparatus equipped with an evaporation boat filled with metal particles of In and Sn at a weight ratio of 1.1, and after evacuating the apparatus to 10"@Torr or higher, An ITO thin film (I
n203-5n02) was deposited on the p-type semiconductor layer 105 to a thickness of about 700 nm.

The substrate temperature at this time was 170"C. After cooling, the substrate 101 was taken out, a permalloy mask for forming a current collecting electrode pattern was closely attached to the upper surface of the transparent electrode 106, and the substrate was placed in a vacuum evaporation apparatus at 10-!ITorr. After vacuum evacuation, Ag was evaporated to a thickness of about 08 μm by resistance heating method.
Twenty-five comb-shaped current collecting electrodes 107 were formed so that each could be evaluated individually as a subcell.

The photovoltaic device produced as described above was used as sample Al-
It was set to 1.

Next, the flow rates of G e H4 gas immediately before the end of film formation of the germanium-containing layer 112 are 3°4.5 and 10°, respectively.
, 20, 25, and 30 sccm, and in the same manner as sample A1-1, samples A1-2 to A]-8 were prepared.

Of course, the change in the germanium composition ratio of the germanium-containing layer 112 of each of samples Al-2 to A1-8 in the film thickness direction is similar to that of sample A1-1. It increases linearly from the interface on the 103 side toward the n-type semiconductor layer 105 side.

Regarding Tomoshiki materials A1-1 to A1-8, the distribution of germanium element in the germanium-containing layer 112 was investigated by Auger electron spectroscopy. FIG. 5 shows the results expressed as changes in the germanium composition ratio in the film thickness direction.

Next, photovoltaic element samples Al-1 to A! −8, thick f4 light (100 mW/c
m”), and the photoelectric conversion efficiency η and photocurrent values were determined for each subcell of the photovoltaic element. Find the number of subcells, 11
The shunt rate, which is the ratio of subcells shunting among the subcells (25) included in the photovoltaic element m1, was calculated. That is, a shunt rate of 4% indicates that 91 out of 25 subcells were shunting. Furthermore, the collection efficiency of photocarriers was determined for each wavelength of irradiation light, and a collection efficiency curve was obtained.

Table 3 shows the results regarding conversion efficiency, photocurrent, and shunt rate. In this table, the G e H4 flow rate (seem) is the G e H4 flow rate (seem) just before the completion of the formation of germanium cargo N112.
e is the flow rate of Ha gas, and the conversion efficiency and photocurrent are the averages of those obtained for each subcell (calculated including the shunted subcells), and the conversion efficiency and photocurrent in sample Al-1 are calculated as the relative It is displayed as a value. Also, Ge
The column for rate of change in composition ratio shows the magnitude of change in the germanium composition ratio in terms of rate of change in the film thickness direction. For reference, these values for sample C-1 according to a comparative example to be described later are also listed.

From the results shown in Table 3, the rate of change in the composition ratio of germanium in the germanium-containing N112 in the film thickness direction is 0.02 to 0.
If it is 10 (%/Å) (sample A1-3 to Al-8),
It was found that at least one of conversion efficiency and photocurrent was 10 or more, and that the device was good as a photovoltaic device. Also, by comparing the results in Figure 5 and Table 4,
It has also been found that if the composition ratio of germanium at the interface on the p-type semiconductor N112 side of the germanium-containing layer 112 is in the range of 20 to 70 at %, a good photovoltaic element can be obtained.

Incidentally, as the germanium element increases, the amount of light absorption increases, so although it was expected that sufficient characteristics would be leaked even if the film thickness was thin, shunts occurred frequently and the conversion efficiency deteriorated accordingly.

(Comparative Example 1) A photovoltaic element was produced in the same manner as in Example 1, but in which germanium was uniformly contained in the layer corresponding to the germanium-containing layer 112 of Example 1.

First, a method for manufacturing this photovoltaic element will be explained.

The surface was mirror polished in the same manner as in Example 1. 50mmX50
As in Example 1, a lower electrode and an n-type semiconductor layer were formed on a substrate made of stainless steel (SUS304) with a size of mm.

Next, without breaking the vacuum, the substrate was moved from the deposition chamber for depositing the n-type semiconductor layer to the deposition chamber for depositing the i-type semiconductor layer, and the substrate surface was heated to 200°C. While heating to 5i2Hs gas 10105e, H2 gas 100
Introducing sccm and applying 12W of high frequency power to
By performing film formation for 0 seconds, a buffer layer made of amorphous i-type Si 2 :H with a thickness of 50 nm was formed on the n-type semiconductor layer.

Subsequently, 5izHa gas and H2 gas were introduced, and while applying high frequency power, G e H4 gas 1 sc was added.
EM was introduced and film formation was performed for 8 minutes to form a layer of amorphous i-type 5iGe with a thickness of 1000 nm. During the deposition of this layer, the flow rate of G e H4 gas is constant, so the distribution of germanium elements in this layer is uniform. After the film formation of this layer is completed, the introduction of G e Ha gas is stopped and the 5i
Open film formation was performed for 20 seconds by introducing only 2Ha gas and H2 gas, and a buffer layer made of amorphous i-type Si:H was formed to a thickness of 50 layers, thereby completing the film formation of the i-type semiconductor layer. The pressure during film formation was 1.1 torr.

Next, in the same manner as in Example 1, an n-type semiconductor layer, a transparent electrode, and a current collecting electrode were formed to complete a photovoltaic device. This photovoltaic device is referred to as sample C-1.

Furthermore, the flow rates of G e H4 gas when forming a layer consisting of amorphous i-type 5iGe in the i-type semiconductor M were set to 3, respectively.
Each photovoltaic device of samples C-2 to C-8 was produced in the same manner at 4, 5, 10, 20, 25, and 30 sccm.

Conversion efficiency, shunt rate, and collection efficiency were measured for samples C-1 to C-8 in the same manner as in Example 1. The results are shown in Table 4. In the table, the G e Ha flow rate is the G e Ha flow rate when the above-mentioned amorphous i-type 5iGe layer is formed.
The flow rate of H4 gas is the value at 800 nm, and the collection efficiency is the value at 800 nm. Conversion efficiency and collection efficiency are expressed as relative values with the value for sample C-1 being 1.0.

Table 4 From this result, the flow rate of G e H4 gas is 5i28e
4 to 20 sec for gas flow rate 10105e
It can be seen that the conversion efficiency increases when the value is within the range of . Since the deposition conditions for the layer consisting of amorphous i-type 5iGe are the same as in the reference experiment described above, when compared with this reference experiment,
It can be seen that the conversion efficiency exhibits a good value when the composition ratio of germanium is in the range of 22 to 67 atomic %.

In addition, if the content ratio of germanium is large (the optical band gap becomes narrower, the collection efficiency for long wavelength light improves, and the photocurrent increases, but if the content ratio becomes too large, other factors As a result, photoelectric conversion efficiency decreases.

(Comparative Example 2) In Comparative Example 1 described above, SigH was used to form the i-type semiconductor layer.
S gas is used, but S gas is used instead of 5izHs gas.
A photovoltaic device was created using iH4 gas. To explain in more detail, in Comparative Example 1, 5i2Ha gas 10
1100 sccm of H2 gas and 12 W of high frequency power were introduced, but here, 30 sccm of S i Ha gas and 300 sccm of H2 gas were introduced and 20 W of high frequency power was input. Other conditions were the same as in Comparative Example 1, and the flow rates of G e H4 gas were set to 1, 3,
4.5°10.20, 25, and 30 sccm, photovoltaic device samples C-9 to C-16 were prepared.

Regarding these samples C-9 to C-16, the photoelectric conversion efficiency, shunt rate, and collection efficiency were measured in the same manner as in Comparative Example 1. The results are shown in Table 5. Note that in this table, the conversion efficiency and collection efficiency are the values of sample C-9 of 1. It is displayed as a relative value marked with ○.

Table 5 From this result, the flow rate of G e H4 gas is 4 to 20 se
It has been found that good conversion efficiency can be obtained within a range of cm.

In other words, if the germanium composition ratio is too small, it will not be possible to obtain the desired photocurrent; on the other hand, if the germanium composition ratio is too large, germanium atoms will act as impurities.
Conversion efficiency decreases due to the influence of other characteristic determining factors.

(Example 2) In Example 1 described above, the film thickness of the germanium-containing layer 112 was constant for 1000 people, but here, the film thickness of the germanium-containing layer 112 was changed to create a photovoltaic element.

First, a method for producing this photovoltaic device will be explained.

As in Example 1, a lower electrode 102 and an n-type semiconductor layer 103 were formed on a stainless steel substrate 101 measuring 50 mm x 50 mm. Next, the substrate 101 was moved from the deposition chamber for depositing the n-type semiconductor N103 to the deposition chamber for depositing the i-type semiconductor layer 104 without breaking the vacuum, and the substrate 101 was heated. . Then, in the same manner as in Example 1, 5iz Hs gas 10 sccm, H2 gas 1
01005c was introduced, 12 W of high-frequency power was applied, and the film was formed for 12 seconds to form amorphous i-type Si with a thickness of 50 mm.
:H buffer N111 is connected to the n-type semiconductor layer 103.
formed on top of.

Subsequently, Si, H, gas, and H2 gas were introduced, and while high frequency power was being applied, GeH+ gas was introduced to form the germanium-containing layer 112. In this case, when the germanium-containing layer 112 is formed, the flow rate of the G e Ha gas is set to O, and as the germanium-containing layer 112 is formed, the flow rate of the G e H4 gas is increased, and the germanium-containing layer 112 is finished forming. Immediately before, the flow rate of G e H4 gas was set to 1 sec. Further, the flow rate of the GeH4 gas was controlled by a mass flow controller (not shown) so that the germanium composition ratio in the germanium-containing layer 112 increased linearly in the film thickness direction. The thickness of the germanium-containing layer 112 was determined through experiments so that the photocurrent of the photovoltaic device to be produced was approximately equal to the photocurrent of the photovoltaic device A1-4 of the above-mentioned example. Here, the film thickness is about 1900 people. The film thickness was controlled by changing the film formation time.

After completing the film formation of the germanium-containing layer 112, 5i
Introducing zHs gas and H2 gas, applying high frequency power, and performing open film formation for 20 seconds to form amorphous i-type Si to a thickness of 50 mm.
A buffer layer 113 consisting of H was formed. Thereafter, in the same manner as in Example 1, the p-type semiconductor layer 105. Transparent electrode 106
, a current collecting electrode 107 was formed. The photovoltaic device created in this manner is referred to as sample A2-1.

Next, G immediately before the completion of film formation of the germanium-containing layer 112
e The flow rate of H4 gas is 3 and 4.5°10, respectively.
20, 25, 30 sccm,
Samples A2-2 to A2-8 of photovoltaic devices were created. The film thickness of the germanium-containing layer 112 of each of these samples A2-2 to A2-8 is such that the photocurrent value of each sample is the same as that of sample A.
It was experimentally determined to be approximately equal to the photocurrent of No. 1-4. Of course, in the germanium-containing 11112 samples A2-2 to A2-8, the germanium composition ratio changes linearly in the film thickness direction.

Next, as in Example 1, the distribution of the germanium element in the germanium-containing layer 112 of each sample A2-1 to A2-8 in the film thickness direction was investigated by Auger electron spectroscopy. The results are shown in FIG. Note that the white circle in the figure indicates the interface of the germanium-containing layer 112 on the p-type semiconductor layer 105 side, and its position corresponds to the film thickness of the germanium-containing layer 112.

In addition, as in Example 1, photoelectric conversion efficiency, shunt rate,
Photocurrent was measured. The results are shown in Table 6. In addition, in the table, the conversion efficiency and photocurrent are calculated by setting the value of sample A2-1 to 1.0.
It is expressed as a relative value. Further, for reference, the values of sample A1-4 of Example 1 are also listed.

(Example 3) In Example 2, Si2H was used to form the i-type semiconductor layer 104.
6 gas, H2 gas, and Q e Ha gas is used to contain germanium in the germanium-containing layer 112, but here S s Ha gas, H2 gas, G
A photovoltaic element was constructed using e Ha gas and without providing the buffer layer 111 on the n-type semiconductor layer 103 side. That is, compared to the second embodiment, S i H4 is used instead of 5izHs, and the buffer layer 111 is missing.

First, a method for producing this photovoltaic device will be explained.

The process up to the formation of the n-type semiconductor layer 103 on the substrate 101 is the same as in Examples 1 and 2. Next, the substrate 101 is moved to a film formation chamber for forming the i-type semiconductor layer 104, and the SiH
4 gas at 30 sccm, H2 gas at 300 sccm, 20 W of high frequency power, and G e
Formation of the germanium-containing layer 112 was started by introducing H4 gas. The flow rate of the G e H4 gas was O at the start of the film formation of the germanium-containing layer 112, and was increased as the film formation progressed, and the flow rate of the G e H4 gas immediately before the end of the film formation of the germanium-containing layer 112 was set to 1 sec. . In the germanium-containing layer 112, the germanium composition ratio changes linearly in the film thickness direction, and the film thickness is such that the obtained photocurrent is equal to that of sample A1-4 of Example 1.
The photocurrent was determined experimentally to be approximately the same as the photocurrent of . Thereafter, high frequency power was continuously applied while the 5iHi gas and H2 gas were being introduced, and film formation was performed for 20 seconds to form a buffer layer 113 made of amorphous i-type Si:H with a thickness of 50 μm.

Next, a p-type semiconductor 105, a transparent electrode 106, and a current collecting electrode 107 were formed in the same manner as in Example 1.2 to create a photovoltaic device. This is designated as sample A3-1. In addition, the flow rates of G e H4 gas immediately before the end of film formation of the germanium-containing layer were set to 3, 4°, 5.10, 20, and 25.30 sec, respectively.
A photovoltaic device was prepared as sample A3.
-2 to A3-8.

FIG. 7 shows the results of examining the distribution of germanium in the germanium-containing layer 112 for samples A3-1 to A3-8 in the same manner as in Example 2. Furthermore, Table 7 shows the results of measuring the conversion efficiency, shunt rate, and photocurrent for samples A3-1 to A3-8. In addition, in the table, the conversion efficiency and photocurrent are relative values with the value of sample A3-1 being 1.0, and the value of sample Al-4 is also shown for reference.

(Example 4) In Example 3, S I was used to form the i-type semiconductor layer 112.
While using H4 gas, 50 sccm of SiF4 gas was introduced instead of SiH4, the amount of H2 gas introduced was 500 sec, and the high frequency power input was 50 sccm.
Samples A4-1 to A4-8 of photovoltaic devices were prepared in the same manner as in Example 3 except that the power was set to 0W. However, the film thickness of the germanium-containing layer 112 of these samples A4-1 to A4-8 is selected in Example 3 so that the photocurrent is approximately the same as that of Sample A1-4 of Example 1. On the other hand, here, the photocurrent of the photovoltaic element to be created is determined by experiment so as to be approximately equal to the photocurrent of sample A1-3 of Example 1.

Then, as in Example 3, the distribution of germanium in the germanium-containing layer 112 was determined. The results are shown in FIG. We also measured conversion efficiency, shunt rate, and photocurrent.

The results are shown in Table 8. In the table, the conversion efficiency and photocurrent are expressed as relative values, with the value of sample A4-1 being 1.0, and the values of sample A1-3 are also listed for reference.

(Example 5) In Example 3, S I was used to form the i-type semiconductor layer 104.
In contrast to using H4 gas, 81□F gas was introduced instead of SiH4 at a rate of 25 sec, the amount of H2 gas introduced was 300 sec, and the high frequency power input was 300 sec.
Samples A3-1 to A3-8 of photovoltaic devices were prepared in the same manner as in Example 3 except that the power was set to 0W. However, the film thickness of the germanium-containing layer 112 of these samples A3-1 to A3-8 is as follows.
Here, the photocurrent of the photovoltaic element to be created is selected to be approximately equal to the photocurrent of sample A1-2 of Example 1.
Determined by experiment.

Then, as in Example 3, the distribution of germanium in the germanium-containing layer 112 was determined. The results are shown in FIG. We also measured conversion efficiency, shunt rate, and photocurrent.

The results are shown in Table 9. In the table, the conversion efficiency and photocurrent are shown as relative values, with the value of sample A3-1 being 1.0, and the values of sample A1-2 are also listed for reference.

(Example 6) In Example 3, G e Ha gas was used to contain germanium in the germanium-containing layer 112, but here G e F gas was used instead of G e H4 gas.
4 gases were used, and the flow rate of the gas for forming the i-type semiconductor layer 104 was set to SiH.

6-1 to 86- were applied to the photovoltaic element samples in the same manner as in Example 3, except that the gas was changed to 20 sccm, the H2 gas was changed to 200 sec, and the high-frequency power input was 30 W.
8 was produced. However, these samples A6-1 to A6-8
The thickness of the germanium-containing layer 112 in Example 3 is selected so that the photocurrent is approximately the same as that of sample Al-4 in Example 1, whereas here, the thickness of the created light It has been experimentally determined that the photocurrent of the electromotive force element is approximately equal to the photocurrent of sample Al-1 of Example 1.

Then, as in Example 3, the distribution of germanium in the germanium-containing layer 112 was determined. The results are shown in FIG. We also measured conversion efficiency, shunt rate, and photocurrent. The results are shown in Table 9. In the table, the conversion efficiency and photocurrent are shown as relative values, with the value of sample A1-1 being 1.0, and the values of sample A1-1 are also listed for reference.

Table 6 Table 7 Table 8 Table 9 Table 1 0 In each of the above embodiments, the structure of the i-type semiconductor layer 104,
Even when the raw material gas, film formation conditions, and film thickness of the germanium-containing layer 112 are varied, the composition ratio of germanium in the germanium-containing layer 112 remains the same as that of the n-type semiconductor layer 1.
0.03 in the film thickness direction toward the n-type semiconductor layer 105.
It was found that the photovoltaic device having an intensifier with a ratio of 0.02 to 0.0 and 10 at. Comparing the conventional example and the example, in the above-mentioned example, the thickness of the i-type semiconductor layer required to obtain the same photocurrent can be reduced in the germanium composition ratio region where the conversion efficiency is high. The influence of deterioration is reduced and reliability as a photovoltaic element is improved.

Furthermore, the n-type semiconductor layer 10 of the germanium-containing layer 112
The composition ratio of germanium near the interface on the 5th side is 20 to 70.
It has been found that a better photovoltaic device can be obtained if the amount is atomic %. As the content of germanium in the germanium-containing layer 112 increases, the amount of light absorption increases, so it is expected that sufficient characteristics can be obtained even if the film thickness is thin, but if the content is too high, the actual As is clear from the above-mentioned examples, shunts occurred frequently and the conversion efficiency decreased accordingly.For this reason, the p of the germanium-containing layer 112
This means that it is desirable that the germanium composition ratio near the interface of the type semiconductor layer 105 (where the germanium composition ratio is maximum) is 70 atomic % or less.

Although the embodiments of the present invention have been described above, the present invention is not limited to the above embodiments.

For example, multiple pins, such as a so-called tandem cell.
The present invention can also be applied to a photovoltaic element in which junctions are arranged vertically.

[Effect of the Invention 1 As explained above, the present invention provides a photovoltaic element with a pin junction structure, in which a germanium-containing layer is provided in an i-type semiconductor layer, and the composition ratio of germanium in the germanium-containing layer is on the n-type semiconductor layer side. By increasing O at 0.02 to 1.0 atomic %/in in the film thickness direction toward the interface on the n-type semiconductor layer side, the conversion efficiency is improved. This has the effect that the thickness of the i-type semiconductor layer can be reduced and reliability is improved. Furthermore, by setting the composition ratio of germanium in the vicinity of the interface on the n-type semiconductor layer side of the germanium-containing layer to 20 to 70 atomic %, a superior photovoltaic device with high conversion efficiency and low shunt rate can be obtained. There is an effect.

[Brief explanation of the drawing]

FIG. 1 is a schematic cross-sectional view showing the configuration of a photovoltaic device according to an embodiment of the present invention, and FIGS. 2(a) to (d) are explanatory diagrams illustrating band profiles of the photovoltaic device according to the present invention, respectively. , FIG. 3 is a schematic cross-sectional view of a main part showing the configuration of an apparatus used for film formation of a photovoltaic element of the present invention, FIG. 4 is an explanatory diagram showing the distribution of germanium in a germanium-containing layer, and FIGS. FIG. 10 is a characteristic diagram showing changes in the germanium composition ratio in the germanium-containing layer of each sample of the photovoltaic devices prepared in Examples 1 to 6, respectively, and FIG. 11 is a configuration of an example of a conventional photovoltaic device. 12(a) to (C) are explanatory diagrams illustrating the band profile of a conventional photovoltaic device, and FIG. 13 is a film forming method used for forming a film of a conventional photovoltaic device. A schematic sectional view showing the configuration of the device,
FIG. 14 is a characteristic diagram showing the correlation between the germanium composition ratio and the prior conductivity ratio. 101... Substrate, 102... Lower electrode,
103... N-type semiconductor layer, 104... I-type semiconductor layer, 105... P-type semiconductor layer, 106... Transparent electrode,
107... Current collecting electrode, 111.113... Buffer layer 112... Germanium-containing layer, 501.502... Film forming chamber, 503... Substrate transport cassette, 504... Thermocouple,
505... Heater, 506... Substrate transport means, 507... Gate valve, 508.509... Gas introduction pipe, 510... High frequency power supply, 511... Matching box, 512... Anode electrode, 513...Load lock chamber 514...Exhaust valve, 515...Exhaust pump, 517...Pressure gauge.

Claims (1)

[Claims] 1. At least one set of pin junction structure in which a p-type semiconductor layer, an i-type semiconductor layer, and an n-type semiconductor layer are sequentially stacked, and the i-type semiconductor layer is a region containing germanium. In a photovoltaic element having at least a germanium-containing layer, the germanium content in the germanium-containing layer is O at the interface on the n-type semiconductor layer side,
The germanium content increases monotonically in the film thickness direction toward the interface on the side of the p-type semiconductor layer, and the rate of increase in the germanium content in the film thickness direction is expressed as the rate of change in the composition ratio of germanium in the semiconductor constituent elements. When, 0.02 to 0.10 atomic%/Å
A photovoltaic element characterized by: 2. The i-type semiconductor layer has a structure in which a germanium-containing layer and a buffer layer, which is a germanium-free region, are laminated, and the buffer layer is located at the interface between the i-type semiconductor layer and the n-type semiconductor layer. The photovoltaic element according to claim 1, wherein the photovoltaic element is provided at an interface with the p-type semiconductor layer. 3. The content of germanium in the germanium-containing layer near the interface on the side of the p-type semiconductor layer is 20 to 70 atomic % when expressed as the composition ratio of germanium in the semiconductor constituent elements. photovoltaic device.