JPH0320073B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to an amorphous silicon solar cell, and more particularly to a solar cell in which an amorphous silicon thin film is provided as a photovoltaic element on a specific flexible substrate. Solar cells that have an amorphous silicon thin film on a non-flexible substrate such as a stainless steel or glass plate are well known, and solar cells that have an amorphous silicon thin film on a flexible substrate made of a resin thin film such as polyimide are well known. Solar cells are also well known. A feature of manufacturing an amorphous solar cell using a flexible substrate is that the necessary amorphous silicon layer can be continuously provided on the substrate, which is non-trivial in terms of manufacturing cost and ease of manufacturing. This is extremely advantageous compared to the case where a flexible substrate is used. Furthermore, unlike conventional solar cells that have an amorphous silicon thin film on a non-flexible substrate, amorphous solar cells that have an amorphous silicon thin film on a flexible substrate are sheet-shaped, so the product The shape can be made arbitrary, and it is expected that its applications will expand with future application development. As mentioned above, in the conventionally developed solar cells in which an amorphous silicon thin film is provided on a flexible substrate, a polyimide film is used for the flexible substrate. However, in solar cells in which an amorphous silicon thin film is provided on a polyimide film, depending on the type of polyimide selected, the lower electrode of the solar cell may corrode or the substrate, that is, the film and the lower electrode may separate. , which may lead to a decrease in the output of the solar cell, which poses a serious problem in practical use. In order to create an amorphous silicon solar cell by providing an amorphous silicon thin film on a flexible substrate, the flexible substrate needs to be heat resistant to at least 250°C, and it is also difficult to separate it from the bottom electrode. It is necessary to use a substrate that does not cause corrosion of the lower electrode or the lower electrode. At present, most such amorphous silicon solar cells are realized by forming an amorphous silicon thin film using a glow discharge method, so when selecting the material for the substrate of such a solar cell, the aforementioned As shown in Figure 2, heat resistance of 250℃ or higher is required, and the range of substrate material selection is becoming extremely narrow. Therefore, an object of the present invention is to form an amorphous silicon thin film on a flexible substrate by forming the amorphous silicon thin film while keeping the temperature of the substrate relatively low. An object of the present invention is to provide an amorphous silicon thin film solar cell in which the stability of solar cell characteristics over time is improved by appropriately selecting materials. In summary, the amorphous silicon solar cell of the present invention has an amorphous silicon thin film on a flexible substrate, and the flexible substrate has a gel fraction of 30% or more as measured by the following measuring method. It is characterized by being a crosslinked polyester film having a value. Gel fraction measurement method: 1 g of film was mixed with phenol/sym-tetrachloroethane = 60/40 (weight ratio) mixed solvent at 120°C.
After heating in 250 c.c. for 2 hours, the insoluble part is filtered with a No. 2 glass filter, washed with the above mixed solvent and chloroform in that order, dried, and the gel fraction is determined from the weight of the insoluble part. . Preferably, by cluster ion beam method,
A thin amorphous silicon film is formed on the crosslinked polyester film. By the way, the crosslinked polyester film used for the flexible substrate of the present invention has heat resistance and heat resistance depending on the degree of crosslinking.
Dimensional stability is different. When forming an amorphous silicon thin film, it is necessary to heat the substrate as mentioned above, so heat resistance and dimensional stability at the temperature during the formation of this amorphous silicon film are issues. We actually fabricated a prototype solar cell device using a cross-linked polyester film with a
Here, the degree of crosslinking was determined based on the gel fraction described above, and substrates having various values of this gel fraction were created. A substrate that achieves desirable characteristics as a solar cell preferably has a gel fraction of 30% or more. The reason why the gel fraction is set at 30% or more is because if the gel fraction is less than 30%, the heat resistance and dimensional stability at the temperature during formation of the amorphous silicon film will be insufficient, and the effects of this invention will not be fully exhibited. This is because it will no longer be possible. Further, in the present invention, the gel fraction of the flexible substrate is more preferably 50% or more, particularly preferably 70 to 95%. Preferred examples of the crosslinked polyester film will be described below, but the crosslinked polyester film of the present invention is not limited to these examples. That is, 0.01 to 50 per 100 moles of total ester bonds in saturated linear polyester as a crosslinked polyester film.
A crosslinked or high molecular weight polyester film obtained by irradiating a molded product of a polymer containing a structural unit represented by the following general formula 1 with moles of active light is used. This film is preferred as it provides a gel fraction of 70-95%. However, R ¹ and R ² represent substituents on the benzene nucleus, are monovalent organic acid groups, and may be the same or different. m and n represent the number of substituents, and are numbers from 0 to 3, and may be the same or different. The crosslinked polyester film thus obtained has an excellent thermal shrinkage rate of 0.7% or less at a substrate temperature of about 200°C, which is required when forming an amorphous silicon thin film thereon according to the present invention. Furthermore, the obtained amorphous silicon thin film solar cell of the present invention does not cause corrosion or peeling of the lower electrode. In this way, the crosslinked polyester film has extremely excellent properties as a substrate for amorphous silicon thin film solar cells. Furthermore, the crosslinked polyester film substrate has extremely excellent electrical properties such as dielectric breakdown resistance, dielectric constant, and volume resistivity. In this way, when a crosslinked polyester film is used as a substrate for the amorphous solar cell of the present invention, the thickness of the film is 10 to 10%.
500μ is suitable from the viewpoint of flexibility, and the thickness is 500μ
If it exceeds this value, it will lack flexibility, making it difficult to employ the roll-to-roll method, which is one of the advantages of using a flexible polymer film as a substrate. On the other hand, if the thickness of the film becomes less than 10 μm, the thermal stress during the formation of the amorphous silicon thin film may not be fully alleviated and the substrate may deform, so it is not preferable to reduce the thickness of the substrate too much. The crosslinked polyester film is produced by melt film forming, and the film having a thickness of 10 to 500 μm as described above can be easily obtained. Next, the cluster ion beam method will be mentioned. In the cluster ion beam method, the constituent elements of the substance to be formed are stored in a closed crucible with at least one injection nozzle, heated and vaporized, and the vapor is heated to a high pressure sufficiently lower than the vapor pressure. In a vacuum, for example, a pressure of 1/100 or less than the above pressure in the crucible is injected from an injection nozzle into a high vacuum, and a supercooling phenomenon based on adiabatic expansion at the time of injection usually produces about 100 to 2000 atoms. forms a loosely bound mass of atoms, a so-called cluster, due to the Van der Waals force, and further ionizes at least a portion of this cluster, which is then accelerated by the kinetic energy imparted during nozzle injection or, if necessary, by electrolysis. The purpose is to allow the component elements to reach the surface of the substrate and form a thin film of the component elements on the substrate. In this case, by setting various parameters such as the temperature of the substrate, the degree of vacuum in the space around the crucible, the ionization rate of clusters, or the acceleration voltage when accelerating cluster ions, the material to be grown on the substrate can be changed to an amorphous state or a polymorphous state. It has the characteristic that it can be arbitrarily controlled to a crystalline state or a single crystalline state, and a thin film of the compound can be easily formed by introducing a reactive gas into the cluster formation space. In a preferred embodiment of the present invention, this cluster ion beam evaporation method is used to form an amorphous silicon thin film as a photoelectric change element. As a result of various studies on the method of forming an amorphous silicon thin film as a photoelectric change element using this cluster ion beam evaporation method, we found that by appropriately selecting the conditions, the quality is extremely high even at substrate temperatures below 200℃. We were able to obtain a thin amorphous silicon film. In the conventional glow discharge method,
In order to obtain a high-quality amorphous silicon thin film, a substrate temperature of at least 250°C is required.
Although the range of material selection for the flexible substrate was extremely limited, by using the cluster ion beam evaporation method, a high-quality amorphous silicon thin film could be obtained even at a substrate temperature of 200°C or less, specifically around 180°C. I confirmed that it is possible. When a polymer film is used as a substrate for a solar cell having an amorphous silicon thin film, there is a problem in that gas released from the substrate has an adverse effect on the amorphous silicon thin film itself as a photovoltaic element. By employing the cluster ion beam method, the required substrate temperature can be lowered and problems associated with outgassing can also be resolved. In other words, with the conventional glow discharge method, a high temperature of 200°C or higher is required to obtain a high-quality amorphous silicon thin film, but by adopting the cluster ion beam method, the necessary substrate temperature can be lowered. is possible,
For the first time, it has become possible to use a crosslinked polyester film as a substrate for a solar cell with an amorphous silicon thin film. Cross-linked polyester film is
It can be used without problems up to substrate temperatures of around 230°C. When a crosslinked polyester film is used as a substrate for a solar cell having an amorphous silicon thin film, an electrode is formed on the surface of the film, but the material for the electrode is not particularly limited and may include aluminum, iron, stainless steel, nickel, Vapor deposition, sputtering, and ion plating (including cluster ion deposition) of thin films such as tungsten
Alternatively, the electrodes may be formed by laminating aluminum foil, stainless steel foil, etc. via a heat-resistant adhesive. A cluster ion beam method is used to form an amorphous silicon thin film as a photoelectric conversion element on a crosslinked polyester film substrate on which electrodes are formed. The figure shows an example of an apparatus used for forming an amorphous silicon thin film according to the present invention. Referring to the figure, a crucible 1 having a nozzle 2 with a hole diameter of about 0.5 to 2.0 mm is provided in a vacuum container (not shown) and supported by a suitable support member.
This vacuum vessel is evacuated to a high vacuum atmosphere of at least 10 ⁻⁵ Torr or less. Crucible 1 is filled with silicon crushed to an appropriate size. Then,
A gas introduction pipe 3 is provided to the vacuum vessel, and hydrogen, phosphine, and diborane are supplied from the gas introduction pipe 3 into the vacuum vessel, and the pressure within the vacuum vessel is increased to 10 ^-6 to ^{10 -3} Torr. maintain it. A substrate 6 is provided in front of the nozzle 2 of the crucible 1 in the vacuum vessel, and this substrate 6 is also heated to an appropriate temperature. Next, heat crucible 1,
The silicon filled inside is heated and melted to generate silicon vapor. In this case, the heating temperature of the silicon is set according to the surrounding space of the crucible 1, that is, the pressure inside the vacuum container, and when the pressure inside the crucible 1 is P and the pressure inside the vacuum container is P ₀ ,
It is set so that P/P ₀ ≧10 ² , preferably P/P ₀ ≧10 ⁴ . Actually, the heating temperature is 1400 ~
A range of about 2300°C is selected. Due to this pressure difference, silicon vapor is ejected from the nozzle 2 to the outside of the crucible 1, and at this time, a supercooling phenomenon based on adiabatic expansion forms clusters in which vaporized silicon atoms and molecules are loosely bonded by Van der Waals forces. be done. By the way, the vapor flow of silicon atoms and molecules that have gained kinetic energy by being ejected out of the crucible 1 enters the ionization chamber, where at least a portion of them is ionized, forming so-called cluster ions. A heating electrode 5 is provided between the crucible 1 and the substrate 6 in the vacuum container, and a suitable acceleration voltage is applied to the heating electrode 5, so that the cluster ions are accelerated and bombarded with the substrate 6. . At this time, the cluster ions are separated into individual atoms in the vacuum container due to the impact of the bombardment and move on the surface of the substrate in an atomic state, resulting in the so-called surface migration effect and the ionization created by partially ionized silicon ions. This effect promotes bonding between silicones. On the other hand, some of the H ₂ introduced into the vacuum container is ionized by the bombardment of atoms, or becomes a single atom H, hits the substrate 6 together with the silicon vapor flow mentioned above, and moves on the surface. do. H is applied to the unbonded portion of the silicon that is bonded while moving the substrate 6.
This means that an amorphous silicon thin film having a structure in which the dangling bonds are closed by H is formed. More specifically, in order to form an i-type amorphous silicon thin film, only H ₂ gas is introduced as the introduced gas, and in order to form a p-type amorphous silicon thin film, H ₂ gas and diborane gas (B ₂ H ₅ ) and to form an n-type amorphous silicon thin film
Introduce _H2 gas and phosphine gas ( _PH3 ). Therefore, as an amorphous silicon layer serving as a photoelectric conversion element, an n layer, an i layer, and a p layer are appropriately formed on a crosslinked polyester film substrate on which a back electrode is formed. In the case of shot cell, a crosslinked polyester film substrate with laminated n-layers and i-layers is placed in a vacuum container, and platinum, gold, palladium, etc. are deposited as shot barrier metals by sputtering, vacuum evaporation, or ion plating (cluster). (including ion beam method), etc.
Deposit to a film thickness of approximately 100 Å. In the case of a hetero (face) junction cell, a film of indium oxide, tin oxide, or tin oxide-indium oxide with a thickness of 200 to 5000 Å is applied on a cross-linked polyester film substrate on which the n-layer, i-layer, and p-layer are laminated. The surface electrode is formed by depositing the film by sputtering, vacuum evaporation, ion plating (including cluster ion beam method), etc. to a certain film thickness. A collection electrode is then provided on the shot barrier metal, heteroface surface to complete the amorphous silicon solar cell device. As described above, in the amorphous silicon solar cell according to the present invention, a back electrode is formed on a crosslinked polyester film substrate, and an amorphous silicon film is formed on the back electrode, preferably by an ion cluster beam method. It has a basic structure in which a short barrier metal or a hetero (face) electrode is provided thereon, and a collecting electrode is further provided thereon. In producing the amorphous silicon solar cell of the present invention, the cluster ion beam method is used as a preferred method for forming an amorphous silicon thin film on a crosslinked polyester film, thereby providing the following advantages. (1) By using the cluster ion beam method, extremely high-quality amorphous silicon thin films can be formed even at substrate temperatures of 200°C or lower, specifically 180°C. (2) Since the required substrate temperature is significantly lowered, the selection range of substrate materials is expanded, and crosslinked polyester films that cannot be used with the glow discharge method can now be used. (3) By using the cluster ion beam method, an amorphous silicon thin film with excellent adhesion to the substrate can be obtained. As described above, according to the present invention, an amorphous silicon thin film is formed on a crosslinked polyester film, preferably using a cluster ion beam method, so that the dimensional stability of the substrate is excellent, and furthermore, the dimensional stability of the substrate is excellent. A solar cell substrate that does not cause peeling from the electrode or corrosion of the lower electrode can be realized, and a solar cell that provides stable electrical output over a long period of time can be provided. Naturally, solar cells are placed in harsh natural environments where they are exposed to direct sunlight and exposed to wind and rain. Therefore, the above-mentioned excellent dimensional stability under high temperature and high humidity conditions and no deterioration in the performance of the lower electrode can be said to be a great advantage as a substrate for solar cells. Thus, in this invention, by using a crosslinked polyester film as a solar cell substrate, it has become possible to realize a highly practical solar cell. Examples of the present invention will be described below. Examples and Comparative Examples First, an example of a method for producing a crosslinked polyester film used in the amorphous silicon solar cell of the present invention will be described. Dimethyl terephthalate 1144 was added to a 4 liter autoclave equipped with a distillation device and a stirrer.
(5.89 mol), ethylene glycol 720 c.c. (13 mol), zinc acetate, 418 mg of dihydrate, and 358 mg of antimotine trioxide were charged, and transesterification was carried out at 145°C to 215°C for 120 minutes. Methanol was distilled off. 126.6g of N,N'-bis(β-hydroxyethyl)benzophenonetetracarboxylic acid imide (abbreviated as HEBTI) was added thereto, and the mixture was heated at 215°C.
After stirring at normal pressure for 15 minutes, while gradually increasing the temperature,
The pressure was gradually reduced to 275°C and 0.014 mmHg after 50 minutes. Further, polycondensation was carried out at this temperature and pressure for 30 minutes to obtain a polymer having an intrinsic viscosity of 0.637 dl/g.
The melting point of this polymer was 252°C. However, the temperature is expressed as the melting peak temperature of a thermogram obtained by measuring at a heating rate of 20° C./min using a differential scanning calorimeter. After drying the polyester produced as above, it was melted and formed into a film at a melting temperature of 285 to 290℃ to a thickness of 30μ.
An unstretched original film was obtained. 3 vertically
The film was stretched 3.1 times in the transverse direction and then heat-treated at 200°C to form a 54μ film. This film was heated to 120° C. and irradiated with a 450 W high-pressure mercury lamp from a distance of 10 cm onto the film surface for 10 minutes to obtain a photo-crosslinked polyester film with a gel fraction of 86%. Next, another example of the method for producing the crosslinked polyester film used in the amorphous silicon solar cell of the present invention will be described. Terephthalic acid 42.8Kg, ethylene glycol 32.0
Kg, ammotine trioxide 18.8g, triethylamine
A mixture consisting of 78.3 g, 4.7 g of trimethyl phosphate, and 25 g of Thyroid #150 was heated at 240° C. and 3.5 Kg/cm ² to react for 105 minutes, and then brought to normal pressure to obtain a reaction mixture. Meanwhile, 415.5 g of benzophenone tetracarboxylic dianhydride, monoethanolamine
A mixture of 158.3g of ethylene glycol and 1289ml of ethylene glycol was reacted at 140°C for 3 hours under a nitrogen atmosphere, added to the above reaction mixture, and further reacted at 240°C and normal pressure for 15 minutes, and the reaction mixture was transferred to a 150 polycondensation reaction vessel. The temperature was increased from 240°C to 280°C in 75 minutes while the pressure was gradually reduced to a final value of 0.1 mmHg. Furthermore, the reaction was continued for 70 minutes at 280°C and 0.1 mmHg. The obtained polymer has a melting point of 260°C and a reduced specific viscosity (100 mg of 25 ml polymer is dissolved in a mixed solvent with a weight ratio of phenol/sym-tetrachloroethane of 6/4,
(measured at 30°C) was 0.652 dl/g. After drying the polyester produced as described above, it was melt-formed at a melting temperature of 290 to 295°C to obtain a 500μ unstretched raw film. 3 vertically
The film was stretched 3.1 times in the transverse direction and then heat-treated at 200°C to form a 54μ film. this film
Heat to 120℃, and from 10cm above the film surface.
Irradiated with a 450W high-pressure mercury lamp for 10 minutes to achieve a gel fraction of 83%.
A photocrosslinked polyester film was obtained. Next, the production of a solar cell using the above-mentioned crosslinked polyester film will be described. The crosslinked polyester film having a thickness of 54 μm obtained as described above was dried at 150° C. for 2 hours under a vacuum of 10 ⁻² Torr. This dried substrate was mounted on a sputtering device, and a 1.5 μm thick tungsten thin film was formed as a back electrode using tungsten as a target. The substrate on which the back electrode was formed was then set under tension on a substrate support stand of a cluster ion beam device, and heated to 200° C. while exhausting to 5×10 ^-7 Torr. 5×10 ^-7 Torr
When the vacuum level is reached, phosphine gas (PH ₃ ) diluted to 1% with H ₂ gas and hydrogen gas is added to the vacuum container.
was introduced at a flow rate ratio of 5:1, and the inside of the vacuum vessel was
Maintained at 10 ^-4 Torr. Set the heating temperature of the crucible to 2000
~2200℃, ionization current 100mA (150V)
An n-type amorphous silicon thin film with a thickness of 200 Å was formed under conditions of an accelerating voltage of 3 kV. Next, in the same manner as above, only hydrogen gas was introduced, and n
An i-type amorphous silicon thin film with a thickness of 3000 Å was formed on the type-type amorphous silicon thin film. Next, hydrogen gas and 1% diborane gas diluted with hydrogen gas were introduced into the vacuum container to form a p-type amorphous silicon thin film with a thickness of 100A, and a pin-type amorphous silicon film was formed on the crosslinked polyester film substrate. A thin film of high-quality silicon was laminated. The pin-type amorphous silicon thin film thus obtained was mounted on a sputtering device, and a tin oxide-indium oxide thin film was deposited to a thickness of 1000 Å to form a heteroface layer. Finally, palladium is deposited in a comb shape to a thickness of 1000 Å on top of this heteroface layer, and then deposited on a cross-linked polyester film substrate.
A pin heteroface type solar cell device was realized. Next, a comparative example will be described. As the substrate, thickness
A 125μ polyimide film was selected and dried under a vacuum of 10 ⁻² Torr at a temperature of 150° C. for 2 hours. This dried polyimide film was mounted on a sputtering device, and a 1.5 μm thick tungsten thin film was formed as a back electrode using tungsten as a target. The amorphous silicon thin film was produced using a capacitively coupled high frequency (13.56 MHz) glow discharge device. 10 ^- While exhausting to Torr
The substrate was heated to 300°C. Then _N2 gas
500c.c./min was introduced in a 1.0Torr _N2 gas atmosphere.
The substrate was cleaned by applying 200 W of high frequency power and ion bombarding the substrate for 20 minutes. Next, 10% silane gas diluted with hydrogen gas and phosphine gas diluted with hydrogen gas to 0.1% were introduced into the glow discharge device, and in this gas atmosphere of 0.6 Torr, 100 W of high frequency power was applied, and a 200 Å n-type An amorphous silicon thin film was formed. Next, an i-type amorphous silicon thin film was formed on the n-type amorphous silicon thin film using hydrogen gas and silane gas in the same manner as above.
It was formed with a thickness of 3000 Å. Then with hydrogen gas 10
% silane gas and diborane gas diluted to 0.1% with hydrogen gas are introduced into the glow discharge device to form a 300 Å p-type amorphous silicon thin film on the i-type amorphous silicon thin film, and then to form a 300 Å p-type amorphous silicon thin film on the polyimide film.
A pin-type amorphous silicon thin film was provided. The pin-type amorphous silicon thin film obtained in this way was attached to a sputtering device, and a tin oxide-indium oxide thin film was formed.
It was deposited to a thickness of 1000 Å to form a heterophasic layer. Finally, palladium was deposited in the shape of a comb to a thickness of 1000 Å on top of this heteroface layer as a collector electrode, and a pin heteroface type solar cell device was realized on a polyimide film substrate. Next, a comparison between a solar cell device according to an example of the present invention and a solar cell device according to a comparative example will be described. AM
The measurement was performed using a solar simulator manufactured by Oriel Co., Ltd., which was adjusted to =1. The results are shown in Table 1.

[Table] After the solar cells whose initial characteristics were measured in the comparison described above were left in a temperature-controlled and humidified room at 20°C and 75% RH for 30 days, the exchange efficiency was measured. The results are shown in Table 2.

[Table] It can be seen that when the crosslinked polyester film according to the present invention is used as a solar cell substrate, a solar cell with extremely excellent practical performance can be obtained.

[Brief explanation of the drawing]

The figure is an explanatory diagram of the basic configuration of a cluster ion beam device. In the figure, 1 is a crucible, 2 is a nozzle, 3 is a gas introduction tube, 4 is an ionization electrode, 5 is an acceleration electrode, and 6
is the substrate.

Claims (1)

[Claims] 1. A solar cell having an amorphous silicon thin film on a flexible substrate, characterized in that the flexible substrate is a crosslinked polyester film having a gel fraction of 30% or more. Amorphous silicon solar cells. (Here, the gel fraction is 1g of film at 120°C.
phenol/sym-tetrachloroethane=
After heating in 250c.c. of a 60/40 (weight ratio) mixed solvent for 2 hours, the insoluble portion was filtered with a No. 2 glass filter.
The weight ratio was determined by washing with the above mixed solvent and chloroform in that order, drying, and then measuring the weight of the insoluble portion. ) 2 The crosslinked polyester film contains 0.01 to 50 per 10 moles of total ester bonds of the saturated linear polyester.
Claim 1, which is a crosslinked or high molecular weight polyester film obtained by irradiating a molded product of a polymer containing a structural unit represented by the following general formula (1) in terms of moles with actinic rays. The amorphous silicon solar cell described. (However, R ¹ and R ² represent substituents to the benzene nucleus, are monovalent organic acid groups, and may be the same or different. m and n represent the number of substituents, and 0
~3, and may be the same or different. ) 3 As a means of forming an amorphous silicon thin film,
The amorphous silicon solar cell according to claim 1, characterized in that a cluster ion beam method is used.