JPS6317344B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a solar cell using a condensed heat-resistant polymer imparted with electrical conductivity by thermal decomposition.

Conventional configurations and their problems Solar cells are a next-generation energy source, and research is being carried out both domestically and internationally to improve their efficiency and reduce costs. New materials include cadmium sulfide-copper sulfide junction cells, or junctions of merocyanine, phthalocyanine, and polyacetylene with indium and aluminum, but the most promising are solar cells based on silicon or gallium arsenide. It seems likely. Silicon has undergone significant improvements in performance and cost in recent years due to lower cost production of monocrystalline or polycrystalline silicon and the production of low cost solar cells using amorphous silicon. It is said that the simplest structure of silicon and gallium arsenide solar cells with the potential for high efficiency is a metal-semiconductor or metal-oxide-semiconductor junction structure. In this case, in order to increase the open-circuit voltage of the solar cell, for example, if the semiconductor is p-type, the work function of the metal should be as low as possible, and conversely, if the semiconductor is n-type, the work function should be as high as possible. However, existing metals have a limited work function, and extremely large or small work functions have not been obtained. Metals with high work functions include platinum and gold, but these are extremely expensive and are an impediment to lowering the cost of solar cells. In this way, creating new metals with different work functions is one of the keys to the emergence of high-efficiency solar cells. This is M.
Solar Energy Materials, 1 by Mr. Schoijet
It is described in detail in Volume 43 (1979).

Purpose of the Invention The present invention utilizes the property that the novel organic conductive material invented by the present inventors behaves similarly to a metal having a high work function, and utilizes the property that the organic conductive material and the n-type semiconductor are bonded to each other. The purpose of this invention is to provide a solar cell with higher efficiency and lower cost.

Structure of the Invention First, in order to facilitate understanding of the present invention, an organic conductive material will be explained.

Recently, many attempts have been made to impart electrical conductivity to organic polymers, which are originally electrically insulating, and to create unique properties that cannot be achieved with inorganic metals, semiconductors, and oxides. One important method in this effort is the formation of pyrolytic polymers. The idea is to start from a specific material and obtain a polycondensed compound through thermal decomposition. The product is mainly carbonaceous, but its electrical conductivity is controlled by the starting materials. Pyrolytic polymers have a long history and are a material that attracted attention in the early 1960s. For example, the thermal decomposition of polyacrylonitrile by the Russian school is famous;
An electrical conductivity of 2Scm ^-1 has been obtained. (AV
Airapetjanc et al. Dokl.Akad.Nauk SSSR magazine,
148, p. 605, 1963). Also in 1964, IBM
Mr. SDBruck of the company thermally decomposed polyimide (DuPont's Kapton H film) at 800℃, and 20Scm ^-1
It was discovered that an electrical conductor having a conductivity of . The conductivity of polyimide film is 10 ^-18 S at room temperature.
cm ^-1 , and the conductivity changes as much as 10 to ²⁰ due to thermal decomposition, so this method is considered to be an excellent method for imparting conductivity to organic polymer materials. Good fit. In this way, the types of materials that can be made highly conductive through thermal decomposition are limited, but as a result of research by the same applicant, some condensation polymers can similarly become highly conductive through thermal decomposition. It was discovered that. The polymer belongs to the class of heat-resistant polymers developed from recent advances in polymer chemistry, such as aromatic polyamide, polyamideimide, polyesterimide, polybenzimide, polyoxadiazole, polythiadiazole, and polybenzthiazole. It is. These materials are heated at 400-1000℃ in vacuum or inert gas flow.
When pyrolyzed at temperatures of , it gives conductivities up to 500 Scm ^-1 .

The pyrolytic polymer applied by the same applicant is characterized by high conductivity and thermal and chemical stability, so in addition to being a conductive film for resistors and conductors, it can also be used as a heater and actuator. Applications can be considered in a wide variety of fields, including sensors for pressure, strain, and wind speed, oxidation-resistant and corrosion-resistant electrodes, and absorption films for solar heat collectors. One method for realizing the solar cell according to the present invention is to use the flexible film obtained by the above process as a substrate and physically deposit a thin film of silicon or gallium arsenide thereon. Since a solar cell having this structure is irradiated with light from the semiconductor side and an electromotive force is generated, a decrease in efficiency due to attenuation of light in the semiconductor portion is unavoidable to some extent. However, the built-in potential of the junction in this configuration is 0.6V for silicon and 1.2V for gallium arsenide, so there is ample potential for higher efficiency.

There are various methods for physical vapor deposition of silicon or gallium arsenide, and for example, physical vapor deposition can be performed using an ion beam sputtering apparatus shown in FIG. In the figure, reference numeral 11 denotes a target consisting of the material to be deposited, in this case silicon or gallium arsenide. As the silicon in this case, amorphous silicon is suitable.
12 is a discharge chamber, which has a built-in heater 13.
14 is a discharge gas inlet, 15 is a substrate, and 16 is a lens for the ion beam 17.

Amorphous silicon is placed as the target 11 and the conductive pyrolytic polymer film described above is placed as the substrate 15. Hydrogen gas is introduced from the gas inlet to heat the heater 13 and discharge the hydrogen gas. When a DC voltage of 1 KV or more is applied to the target 13 and the target 11, the target 11 is sputtered by a hydrogen ion beam, a silicon film is deposited on the substrate 15, and a bond between the silicon and the pyrolytic polymer is formed. This junction forms a so-called shot barrier, which generates an electromotive force when exposed to sunlight and operates as a solar cell. The same applies if gallium arsenide is used instead of silicon.

A second method for realizing a solar cell according to the present invention is to use silicon or gallium arsenide as a substrate and physically deposit a thin film of the pyrolytic polymer thereon. In order to make it possible to manufacture solar cells with stable characteristics at low cost using thin films obtained by physical vapor deposition of pyrolytic polymers, a strong and highly heat-resistant conductive thin film with a thickness of several thousand angstroms or less is needed. needs to be manufactured.

The simplest physical vapor deposition method is vacuum thermal evaporation, but the pyrolytic polymer used in the present invention
This method was found to be unusable, as it was never deposited at a rate greater than 100 angstroms. Next, we considered a method (plasma CVD) in which the polymer is decomposed in PF plasma and precipitated on the substrate, but in this case, only a highly transparent and insulating film of 10 ^-10 Scm ^-1 or less could be deposited. I couldn't get it. After investigating various other physical vapor deposition methods, it was found that it is effective to deposit a thin film on a substrate by bombarding a pyrolytic polymer with high-energy particles. High-energy particles include discharged argon, nitrogen, and the like, and as a physical vapor deposition method using these particles, high-frequency sputtering or the above-mentioned ion beam sputtering method is optimal. However, these methods do not allow the deposition of all polymers; for example, polymers that do not undergo thermal decomposition may not evaporate at all, or
Polymers pyrolyzed at temperatures below 0.degree. C. either did not evaporate or had a significantly low evaporation rate. Therefore, the thermally decomposable polymers used in this case include fused systems having nitrogen-containing heterocycles such as polyimide, polyamideimide, polyesterimide, polybenzimide, polyoxadiazole, polybenzthiazole, and polythiadiazole. Preferred are aromatic polyamides obtained by a condensation reaction of polymers and either m-phenylenediamine and isophthalic acid or p-phenylenediamine and terephthalic acid.

Description of examples Next, examples of the present invention will be described.

[Example 1] A polyimide film (Kapton H film, 150 microns) sold by DuPont was thermally decomposed at 1000°C for 1 hour in a nitrogen stream to obtain a film with a thickness of approximately 100 microns.
A conductive thin film was deposited on a single crystal silicon substrate using the apparatus shown in FIG. 1, targeting a micron conductive film. Silicon is diffusion doped n/n ⁺
On a type wafer, first sputter silicon with an ion beam for 10 minutes to clean the surface, then heat a pyrolytic polymer on top of it with a voltage of 1KV and a beam current of 1KV.
Precipitation was performed at 10 mA/cm ² for 30 minutes. The film was a semi-transparent, uniform oil with a similar color and a thickness of 500 angstroms. Figure 2 shows the cross-sectional structure. 2 in the diagram
1 is an n ⁺ type silicon layer, 22 is an n type silicon layer,
23 is a pyrolytic polymer vapor deposited film. Attach electrode lead wires to the silicon substrate 21 and the deposited film 23,
When the current-voltage characteristics were measured, the results shown in FIG. 3 were obtained. The voltage is plotted so that the deposited film is positive, and from the results in this figure it can be seen that the deposited film is of P type. The forward current-voltage characteristic is J∝exp
It was found that the built-in potential V _b reached a maximum of 0.9 V by matching the equation [-q/kT (V-V _b )]. This value is higher than that of the junction of gold, platinum and n-silicon, and higher than that of polyacetylene-n-silicon (0.72 V), which has recently attracted attention for forming a high barrier. This indicates that the conductive thin film obtained by the present invention has high electronegativity and therefore a high work function, and forms a large barrier to n-type semiconductors.

[Example 2] Using n ⁺ /n gallium arsenide as a semiconductor, n
When a pyrolytic polymer obtained from polyoxadiazole was sputtered on gallium arsenide in the same manner as in Example 1, a built-in potential of 1.3V was obtained. In addition, the solar cell composed of an 800 Å sputtered film gave an open-circuit voltage of 0.7 to 0.75 V to sunlight.

Effects of the Invention As described above, the present invention provides a shot key with a high built-in potential, in which a bond formed between an n-type silicon or gallium arsenide semiconductor and a condensation polymer thermally decomposed in a vacuum or an inert gas is used. It provides a solar cell that utilizes the formation of a bond, and in order to construct a solar cell with a particularly stable structure, a condensed polymer with a nitrogen-containing heterocycle and an aromatic compound that has been given conductivity by thermal decomposition is used. One of the group polyamides is precipitated onto the n-type semiconductor by physical vapor deposition in vacuum. A solar cell with such a configuration has a high open-circuit voltage, and a solar cell with high efficiency and low cost can be obtained.

The most suitable physical vapor deposition method is to use high-energy particles such as ion beam sputtering, but regular high-frequency sputtering using argon can also produce thin films in the same way, except for the possibility of contamination with impurities. It can be precipitated. Further, the polymer raw materials used here are not limited to those listed in the examples, but include condensed polymers having a nitrogen-containing heterocycle that can be imparted with high conductivity by thermal decomposition, such as polyamideimide,
Polyesterimide, polythiadiazole or aromatic polyamide can be used as well.

[Brief explanation of the drawing]

FIG. 1 is a conceptual diagram of an ion beam sputtering apparatus for manufacturing solar cells according to the present invention, and FIG.
The figure is a cross-sectional side view showing an embodiment of the solar cell according to the present invention, and FIG. 3 is a current-voltage characteristic diagram of the solar cell having the configuration shown in FIG. 2. 11...Target, 12...Discharge chamber, 13...
...Heater, 14...Gas inlet, 15...Substrate,
16...Lens, 17...Ion beam, 21...
...n ⁺ type silicon layer, 22...n type silicon substrate,
23...Pyrolytic polymer membrane, 24...Lead wire.

Claims (1)

[Scope of Claims] 1 A pyrolyzed polymer obtained by thermally decomposing n-type silicon or gallium arsenide and either a condensed polymer having a nitrogen-containing heterocycle or an aromatic polyamide in vacuum or in an inert gas. A solar cell characterized in that the n-type silicon or gallium arsenide and the pyrolytic polymer are each provided with an electrode. 2. The solar cell according to claim 1, wherein a thin film of n-type silicon or gallium arsenide is physically deposited on a film of pyrolytic polymer. 3. The solar cell according to claim 1, wherein a pyrolytic polymer film is physically vapor deposited on an n-type silicon or gallium arsenide substrate. 4 The condensed polymer having a nitrogen-containing heterocycle is polyimide, polyamideimide, polyesterimide, polybenzimide, polyoxadiazole,
The solar cell according to claim 1, which is either polybenzthiazole or polythiadiazole. 5. The solar cell according to claim 1, wherein the aromatic polyamide is obtained by a condensation reaction of m-phenylenediamine and isophthalic acid, or p-phenylenediamine and terephthalic acid. 6. The solar cell according to claim 2 or 3, wherein the physical vapor deposition method is an ion beam sputtering method.