JPH0671091B2 - Thin film solar cell - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a thin film solar cell, and more particularly to amorphous silicon characterized by using a specific flexible thermoplastic polymer film as a substrate. Type thin film solar cell.

[Prior Art] An amorphous silicon film forming a photovoltaic layer of a solar cell is disclosed in JP-A-52-16990, JP-A-56-104433 and JP-A-56-104.
No. 477, it is formed by a plasma glow discharge method, a sputter deposition method or an ion plating method as disclosed in each publication, and contains at least 10 to 30 atomic% of hydrogen atoms in the film. Fluorine atom as component atom,
Representative ones include those containing carbon atoms or nitrogen atoms. Here, the term "amorphous silicon film" is used to include a silicon film made of fine crystals having a grain size of about 100Å or less.

The above-mentioned amorphous silicon film has an absorption coefficient for visible light larger than that of a single crystal silicon film by one digit or more, and accordingly, the film thickness necessary for effectively absorbing and utilizing sunlight can be 3 μm or less. is there. This suggests that by providing the photovoltaic generation layer made of the amorphous silicon film on the flexible substrate, it is possible to manufacture a thin film solar cell having a wide applicable range that can be arbitrarily bent.

In fact, an amorphous silicon type thin film solar cell based on a flexible plastic film has already been disclosed in JP-A-54-
No. 149489, No. 55-4994, and No. 55-154726.

However, in order to form a high-quality amorphous silicon film as a solar cell, it is necessary to use 200 as a flexible plastic film.
It is said that heat resistance of ~ 300 ° C is required, and from this point of view, it has been proposed to use a heat-resistant polyimide film as a base, but since these films contain a solvent and adsorbed water, they are amorphous. When the silicon film is heated to a temperature range where the silicon film is laminated, the solvent or adsorbed water is released, which contaminates the formed amorphous silicon film and interferes with the formation of a good quality amorphous silicon film. Further, since these films are generally colored, it is difficult to adopt a mode in which light is incident from the film side for use, and there is a problem that the application form is limited.

Therefore, the present inventors have selected the polyethylene terephthalate film as the transparent plastic film without causing troubles such as the release of the solvent or the adsorbed water even in the temperature range where the amorphous silicon film is laminated, and select the amorphous silicon type thin film solar cell. Attempts were made to manufacture a battery, but at the temperature at which the amorphous silicon film is stacked (for example, around 200 ° C.), the film has insufficient heat resistance and thus heat shrinks, causing cracks in the formed amorphous silicon film. It was practically difficult to use as a solar cell.

[Object of the Invention]

An object of the present invention is to solve the above drawbacks, to provide a thin film solar cell which is rich in flexibility and excellent in light conversion efficiency. In particular, we have found a new flexible material that has no problems in transparency, heat resistance, and workability (silicon film formation, etc.), which is a substrate for thin-film solar cells, and has practical utility. Is provided.

[Structure of Invention]

The present invention is a thin film solar cell in which a photovoltaic generation layer made of amorphous silicon is formed on a flexible substrate,
As the substrate, a biaxially oriented polyethylene-2,6-naphthalenedicarboxylate film having a heat shrinkage at 200 ° C of 2.0% or less in all directions, if necessary, a density of 1.355 to 1.390 g / cm ³ is used. Is a thin film solar cell.

The polyethylene naphthalate in the present invention means polyethylene-2,6-naphthalenedicarboxylate (hereinafter referred to as PEN
Abbreviated) homopolymer, other isomer polymer, PEN 70
The term "copolymer" or "mixture" means that the content of polyethylene 2,6-naphthalenedicarboxylate is not lost.

In the present invention, the substrate may include a PEN film as a base on which an electrode layer necessary for a solar cell is laminated. As such an electrode layer, an ordinary metal layer can be mentioned.

The biaxially oriented PEN film in the present invention is a known sequential 2
It may be manufactured by an axial stretching method, and the simultaneous 2
It may be manufactured by the axial stretching method or manufactured by a simple test device.

The PEN film having such characteristics can be obtained by heat-treating the film obtained by a usual production method. As the temperature of the heat treatment (depending on the time and the method, when fixed to a stenter or a frame and performed under a fixed length), 210 ° C to 250 ° C, preferably 230 ° C to 245 ° C,
The processing time is several seconds to several minutes.

The heat treatment can be carried out under tension or under tension, but it is preferable to carry out tension on all sides.
It is sufficient to fix the four sides of the film to the small scale film with a frame. Industrially, this can be achieved by applying a tension in the traveling direction with a take-up roll, a take-up roll, etc. and fixing it in the width direction with a tenter. It is also possible to fix it with a spring or the like to maintain a constant tension.

The above-mentioned PEN film may be provided with various undercoat layers on one side or both sides before or after the heat treatment, if necessary.

The thickness of the film is preferably 25 to 500 μm for manufacturing and handling.

Next, the present invention will be described in detail with reference to the drawings.

A typical structure of a thin film solar cell obtained by the present invention using a flexible film as a substrate is shown in FIGS. In the figure, 1 is a PEN film, and 2 is a metal layer in ohmic contact with the amorphous silicon film. This layer is made of metal such as iron, chromium, titanium, tantalum, niobium, molybdenum, nickel, aluminum and cobalt, and alloy such as nichrome and stainless steel. These are provided as thin layers by physical or chemical methods. 3, 4 and 5 are amorphous silicon films (including those made of fine crystals having a grain size of 100Å or less, as described above). These are provided by the glow discharge method, the sputtering method, and the ion plating method. 3 is 100 ppm of phosphorus (P) or arsenic (As) which is a group V atom
It is an n-type silicon layer containing ˜20,000 ppm and makes an ohmic contact with the metal layer 2. 5 is a group III atom such as boron (B), gallium (Ga) or aluminum (Al)
It is a P-type silicon layer containing 100 ppm to 20,000 ppm. First
In FIGS. 3 and 4, the n-type silicon layer and the P-type silicon layer 5 may be replaced with each other.

In order to provide the silicon layers 3 to 5, in the glow discharge method, silane (SiH ₄ ) gas or disilane (Si ₂ H ₆ ) is used as a starting material and glow discharge decomposition is performed to form a film. The n-type silicon layer of No. ₃ is glow-discharged by using a mixed gas in which about 1% PH ₃ or AsH ₃ is added to SiH ₄ . In this case, it may be diluted with a gas such as H ₂ , Ar ₂ or He ₂ . On the other hand, in the case of the P-type silicon layer of 5, for example, when adding boron, 1 is added to SiH _4.
A glow discharge may be performed using a mixed gas containing about ₂ % of B ₂ H ₃ . In this case as well, it can be diluted as in the above case. The RF power in the glow discharge and the pressure during the discharge are appropriately selected according to the required silicon film, but they can be performed under known conditions of usually 10 Torr or less, preferably 5 Torr or less. Substrate temperature is 100-300 ℃, preferably
200 to 270 ° C, particularly preferably 230 to 260 ° C.

In the sputtering method and the ion plating method, hydrogen gas is introduced into the atmosphere in order to contain 10 to 30 atomic% of hydrogen in the film, and hydrogen atoms compensate dangling bonds in the silicon film to improve the electrical characteristics. Try to improve.

When introducing a fluorine atom as a third component atom, fluorine gas or silane trifluoride (SiF ₄ ) gas is used; when introducing a carbon atom, the number of carbon atoms of methane, ethylene, ethane, etc. is 1 to 2 When a nitrogen atom is introduced, the hydrocarbon molecule (1) may be deposited by mixing nitrogen gas or ammonia gas into silane gas or hydrogen gas.

Reference numeral 6 in the figure denotes a potential barrier forming layer, which is a thin metal film such as gold, platinum or palladium having a thickness of 10 to 200Å or a thickness of 100 to 5
It is a transparent conductive film of 000Å tin oxide, indium oxide, cadmium stannate, etc. The potential barrier forming layer is preferably a layer which transmits incident sunlight well and has a small surface resistance, and a gold or platinum layer having a thickness of 50 to 150Å and an indium oxide layer of tin dove having a thickness of 300 to 1500Å are preferable.

In the figure, 7 is a collecting electrode, which is a vapor deposition method, a sputtering method, a printing method,
A variety of methods such as a method for plating can be used.

In the figure, reference numeral 8 is a non-reflective coating layer, and an inorganic layer such as silicon oxide, titanium oxide, or tungsten oxide or an appropriate organic layer can be used.

〔The invention's effect〕

Since the thin film solar cell of the present invention uses a biaxially oriented PEN film, there is no solvent or adsorbed water in the film and, unlike polyethylene terephthalate film, much less residual gas, oligomers, etc. The amorphous silicon film is not contaminated over time.

In addition, the thermal shrinkage ratio measured under the condition of standing at 200 ° C for 1 hour is as small as 2.0% or less, the dimensional change is small, and the amorphous silicon film is not cracked. The characteristics of a thin film solar cell with excellent efficiency can be exhibited.

Since the substrate temperature during silicon film formation can be raised up to about 250 ° C, the selection range of film forming conditions is widened and high conversion efficiency is achieved compared to the case where other thermoplastic polymer films are used as the base material. One solar cell is obtained.

〔Example〕

 The present invention will be described below based on examples.

The heat shrinkage ratio and the density in the present invention are measured as follows.

(1) Thermal shrinkage rate This is the rate of change in length when held at 200 ° C for 1 hour without tension. For example, strip-shaped samples in the length direction (MD) and width direction (TD) of the film, respectively. Each sample is clamped on one side with a clip or the like, held at 200 ° C. for 1 hour in a tension-free state, and the length before and after the test is measured to obtain the shrinkage rate. When the length before the test is Lo and the length after the test is L, the contraction rate is Required by.

(2) Density Using a mixed solution of heptane and carbon tetrachloride, the density gradient tube method was used to measure at 25 ° C. The unit is [g / cm ³ ].

Example 1, Comparative Example 1 A pellet of polyethylene 2,6 naphthalene dicarboxylate (homopolymer) having an intrinsic viscosity of 0.65 was dried at 170 ° C for 5 hours. This pellet was melt extruded at 300 ° C. by a T-die according to a conventional method, and further, by a known sequential biaxial stretching method, the longitudinal direction was 130 ° C. at 3.7 times, and the transverse direction was 135 ° C. at 3.9 times.
Double stretching and heat setting at 240 ℃ for 30 seconds, thickness 100μ
m PEN biaxially oriented film was obtained. A part of this film was cut out and kept in a dryer at 200 ° C for 1 hour, and the heat shrinkage was measured. The shrinkage in the film length direction (MD) was 1.8%, and the shrinkage in the film width direction (TD). The rate was 1.3%. The density was 1.359 g / cm ³ . On the remaining PEN film, stainless steel (SUS 304) was provided as a metal electrode by a spattang method to a thickness of about 4000Å. SU this film
After mounting on an S 304 metal frame fixed on all sides, it was set in a glow discharge reactor, the substrate temperature was 200 ° C, and the pressure was 0.6 Torr.
It was cleaned by high frequency discharge of 13.56MHZ for 15 minutes in Argon atmosphere. Next, after exhausting to 10 ^-3 Torr, hydrogen diluted 10% silane gas (SiH ₄ ) and 2% phosphine gas (PH ₃ ) (1% amount of PH _{3 with} respect to SiH ₄ ) were introduced to about 1 Torr.
High frequency discharge was performed at a substrate temperature of 200 ° C, and an n-type silicon layer was provided on the stainless steel layer to a thickness of about 350Å. Next, after evacuating the inside of the apparatus, only a silane gas diluted with hydrogen was supplied to form a silicon layer of about 0.5 μm.

Further, diborane (B ₂ H ₆ ) was mixed in silane to a concentration of about 0.5%, and the mixture was introduced into the reactor, and a P-type silicon layer of about 150 Å was provided by using high frequency discharge.

Then, on the P-type silicon layer, indium oxide having a thickness of about 700 Å was provided by a reactive vapor deposition method. Further, silver was comb-deposited on the indium oxide film to form a collecting electrode.

For comparison, a polyethylene terephthalate (homopolymer) (abbreviated as PET) pellet having an intrinsic viscosity of 0.65 was used, and a biaxially oriented poly (ethylene terephthalate) film having a thickness of 100 μm was obtained by appropriately changing temperature conditions. However, the heat setting temperature was 210 ° C.

In this case, the heat shrinkage of the PET film at 200 ° C for 1 hour was 6.3% in the film length direction (MD) and 5.9% in the width direction (TD). The density was 1.396 g / cm ³ . Further, after the amorphous silicon layer was provided, many cracks were found on the film surface.

When the indium oxide layer was provided, 300 3 × 3 m square cells were provided on the same film using a mask, and the cells having up to 85% of the maximum conversion efficiency among them were counted as surviving cells. PEN
Table 1 shows the survival numbers in the case of the biaxially oriented film of PET and PET.

Example 23, Comparative Example 23 The biaxially oriented PEN film having a thickness of 100 μm of Example 1 was cut into a square shape, fixed on all four sides, and heat-treated at 245 ° C. for 3 minutes. It was measured. The heat shrinkage rate of the film is 1.0% in both the length and width directions,
The density was 1.372 g / cm ³ .

Similarly, the PET film of Comparative Example 1 was heat-treated at 240 ° C. for 3 minutes to obtain a heat shrinkage ratio of 1.4% in the length direction, 1.6% in the width direction, and a density of 1.40.
A film of ² g / cm ² was obtained.

Regarding these two kinds of films, the temperature of the substrate at the time of forming the silicon layer was changed to 200 ° C. and 240 ° C. respectively, and the other conditions were the same as in Example 1.
A solar cell was manufactured in the same manner as, and the cell characteristics were measured. Table 2 shows the survival rate of 100 3 mm square cell plates and the average conversion rate of 5 surviving cells.

By using the PEN film as the substrate, the selection range of film forming conditions can be widened and the conversion efficiency can be increased.

Example 4 and Comparative Example 4 In Example 1, the heat treatment temperature was 230 ° C. (Example 4) or 220.
C. (Comparative Example 4) was changed to heat-setting the PEN film, a sample was prepared in the same manner as in Example 1, and the same kind of evaluation was performed. The results are shown in Table-3. Even the PEN film having a density of less than 1.355 g / cm ³ (Comparative Example 4) has a small number of viable cells.

[Brief description of drawings]

1 to 4 are examples of the structure of the thin film solar cell obtained by the present invention. In the figure, (1) is polyethylene-2,6-naphthalenedicarboxylate film, (2) is a metal layer, (3),
(4) and (5) are amorphous silicon layers, (6) is a potential barrier forming layer, (7) is a collecting electrode, and (8) is an antireflection coating layer.

Continued Front Page (72) Inventor Shigeyoshi Shimoda 3-37-19 Oyama, Sagamihara City, Kanagawa Prefecture Teijin Ltd. Plastics Research Institute (72) Junji Kobayashi 3-37-19 Oyama, Sagamihara City, Kanagawa Prefecture Teijin Inside Plastics Research Laboratory (72) Inventor Tomoyuki Nakamura 3-37-19 Oyama, Sagamihara-shi, Kanagawa Inside Plastics Research Laboratory, Teijin Limited (56) Reference JP-A-56-152276 (JP, A) JP-A-58 -194377 (JP, A)

Claims (1)

[Claims] 1. A thin film solar cell comprising a substrate made of a flexible thermoplastic polymer film and a photovoltaic generation layer made of amorphous silicon formed thereon.
Thermal shrinkage measured in 1 hour at ℃ is 2.0% in all directions
A thin film solar cell characterized by using a biaxially oriented polyethylene-2,6-naphthalene dicarboxylate film having the following density of 1.355 to 1.390 g / cm ³ .