JP5378809B2 - Film for solar cell substrate - Google Patents

Description

  The present invention relates to a film for a solar cell substrate used as a substrate for a solar cell, and more particularly to a film for a solar cell substrate suitably used as a substrate for a flexible type thin film solar cell.

  Solar cells include a rigid type using glass as a base material and a flexible type using a plastic film. In recent years, flexible solar cells have come to be frequently used as auxiliary power sources for mobile communication devices such as mobile phones and mobile terminals.

  The rigid type has higher energy conversion efficiency in the solar cell than the flexible type, but there are limits to the reduction in thickness and weight of the solar cell module. May break and damage the solar cell module.

  Thin and light weight can be expected, and it is strong against impacts. Therefore, a flexible type is advantageous as a solar cell module and has been attracting attention for some time. For example, Japanese Patent Application Laid-Open No. 1-198081 discloses a thin film solar cell having a structure in which an amorphous silicon layer is sandwiched between electrode layers on a polymer film substrate. In addition, JP-A-2-260577, JP-B-6-5782, JP-A-6-350117, and JP-A-62-84568 disclose solar cell modules using a flexible substrate. Has been.

Japanese Patent Laid-Open No. 1-198081 JP-A-2-260577 Japanese Patent Publication No. 6-5782 JP-A-6-350117 JP-A-62-84568

  The solar cell has a super straight type that allows sunlight to pass through the solar cell substrate before it enters the power generation layer provided on the solar cell substrate (in this type, the power generation layer is incident on the solar cell substrate) And a substrate type that does not pass through the solar cell substrate (in this type, the power generation layer is on the light incident side of the solar cell substrate). In any type, although the process temperature required for the formation of the photovoltaic layer differs depending on the type of the solar cell, in order to obtain a reliable solar cell, as a film for the solar cell substrate, the thermal dimensions It is necessary to use a film that is highly stable and has a controlled release of water at the process temperature.

  An object of the present invention is a film for a solar cell substrate, which is suitably used as a substrate for a super straight type solar cell having high thermal dimensional stability and suppressed release of water at a process temperature during solar cell production. Is to provide.

That is, the present invention comprises polyethylene-2,6-naphthalene dicarboxylate, and the coefficient of thermal expansion when heated at a rate of temperature increase of 20 ° C./min from 30 ° C. to 120 ° C. is both in the longitudinal direction and in the width direction. 20.1ppm / ℃ less, and any thermal expansion coefficient in the longitudinal direction and the width direction when the temperature was raised at a heating rate of 20 ° C. / min up to 200 ° C. from 120 ° C. or less 50 ppm / ° C., at 200 ° C. The heat shrinkage rate when heat-treated for 10 minutes is 1% or less, the total light transmittance in the wavelength range of 400 to 900 nm is 70% or more on average, and the water absorption is 1% or less. It is a film for solar cell base materials.

  According to the present invention, a film for a solar cell substrate, which is preferably used as a substrate for a super straight type solar cell with high thermal dimensional stability and suppressed release of water at the process temperature during solar cell production. Can be provided.

  Hereinafter, the present invention will be described in detail.

[Thermoplastic resin]
The film for a solar cell substrate of the present invention is made of a thermoplastic resin. This thermoplastic resin is a thermoplastic resin that can be melt-extruded. For example, polyester, polyether ether ketone, polyphenylene sulfide, and polyamide can be used. Polyester is preferable as the thermoplastic resin because a film having high mechanical strength and low water absorption can be obtained. For example, polyethylene terephthalate, polyethylene-2,6-naphthalenedicarboxylate can be used, and among them, high mechanical strength Polyethylene-2,6-naphthalenedicarboxylate is particularly preferable because it has strength and heat resistance.

[Thermal expansion coefficient]
The film for solar cell base material of the present invention has a coefficient of thermal expansion of not more than 50 ppm / ° C. in both the longitudinal direction and the width direction when heated from 120 ° C. to 200 ° C. at a rate of temperature increase of 20 ° C./min, preferably 45 ppm / ° C. or less. The thermal expansion coefficient is a linear expansion coefficient measured using a thermomechanical analyzer (hereinafter sometimes referred to as “TMA”) at a temperature increase rate of 20 ° C./min.

  In the present invention, the longitudinal direction is the traveling direction of the film when the film is continuously formed. This is sometimes referred to as the vertical direction or the MD direction. The width direction is a direction orthogonal to the longitudinal direction in the film plane, and may be referred to as a transverse direction or a TD direction.

  When the thermal expansion coefficient when the temperature is increased from 120 ° C. to 200 ° C. at a rate of temperature increase of 20 ° C./min exceeds 50 ppm / ° C. in either the longitudinal direction or the width direction, the temperature is increased in the photovoltaic layer lamination process. When this occurs, the photovoltaic layer will be laminated on the expanded film, shrinking when returning to room temperature, causing distortion in the photovoltaic layer, and in some cases, cracks may occur, causing short circuits and leaks. It becomes.

  In order to prevent these cracks more reliably, the coefficient of thermal expansion when the temperature is increased from 120 ° C. to 200 ° C. at a rate of temperature increase of 20 ° C./min is 45 ppm / ° C. or less in both the longitudinal direction and the width direction. In addition, the coefficient of thermal expansion when the temperature is increased from 30 ° C. to 120 ° C. at a rate of temperature increase of 20 ° C./min is preferably 30 ppm / ° C. or less in both the longitudinal direction and the width direction.

[Light transmittance]
The total light transmittance of the film for solar cell substrate of the present invention in the range of 400 to 900 nm is 70% or more, preferably 75% or more, more preferably 80% or more, and particularly preferably 85% as an average value in this range. That's it. When the average value is less than 70%, the amount of light transmitted through the solar cell substrate is small, and high power generation efficiency cannot be obtained when used as a substrate for a super straight type solar cell. In addition, the total light transmittance in this invention is the sum of parallel light transmittance and scattered light light transmittance.

[Water absorption rate]
The water absorption rate of the solar cell substrate film of the present invention is 1% or less, preferably 0.8% or less. If the water absorption rate exceeds 1%, water vapor is released during the photovoltaic layer forming process, and a photovoltaic layer with high conversion efficiency cannot be laminated. In addition, it takes a long time to perform drying at the process temperature in advance before the photovoltaic layer is formed, resulting in poor productivity of the solar cell.

[Additive]
In order to improve the weather resistance of the film, the thermoplastic resin preferably contains an ultraviolet absorber. As the ultraviolet absorber, a compound having a large effective absorption coefficient in a small amount is preferable. For example, a benzoxazine-based, benzophenone-based, benzotriazole-based, or benzotriazine-based ultraviolet absorber can be used. Among these, those having high durability are preferable, and benzotriazine-based, benzotriazole-based, or benzoxazine-based ultraviolet absorbers are preferable. One type or a plurality of types of ultraviolet absorbers may be combined.

  In addition to the ultraviolet absorber, additives such as an antioxidant, a heat stabilizer, a lubricant, a flame retardant, an antistatic agent, and an ultraviolet absorber may be added.

[Thickness]
The thickness of the film for a solar cell substrate of the present invention is preferably 25 to 250 μm, more preferably 50 to 50% from the viewpoint of maintaining the stiffness as a support substrate of the solar cell and ensuring the flexibility of the solar cell module. It is 200 μm, particularly preferably 60 to 125 μm.

[Heat shrinkage]
The film for solar cell base material of the present invention has a heat shrinkage rate of preferably 1% or less when heat-treated at 200 ° C. for 10 minutes from the viewpoint of suppressing dimensional changes in the heating step in the processing step for solar cells. Preferably it is 0.8% or less, Most preferably, it is 0.6% or less.

[Coating layer]
You may provide an application layer in the single side | surface or both surfaces in the film for solar cell base materials of this invention. Examples of the coating layer include a layer composed of a polymer binder and fine particles, and a layer composed of an inorganic substance.

  When providing a layer composed of a polymer binder and fine particles, a polyester resin and / or an acrylic resin is used as the polymer binder from the viewpoint of obtaining good adhesion between the coating layer and the thermoplastic resin film of the substrate. preferable. In this case, the thickness of the coating layer is preferably 0.01 to 8.0 μm, more preferably 0.02 to 6.0 μm.

  In the case of providing an inorganic layer, a layer made of SiO, SiN or SiCN is preferable from the viewpoint of imparting high barrier properties. The thickness of the inorganic layer is at most 500 nm, preferably 50 to 500 nm. If it exceeds 500 nm, it will break easily, and on the contrary, stable production of solar cells will not be possible.

[Film Production Method]
The manufacturing method of the film for solar cell base materials of this invention is demonstrated below for the case where polyethylene-2,6-naphthalenedicarboxylate is used as a thermoplastic resin.

  The film for a solar cell substrate of the present invention can be produced by melting polyethylene-2,6-naphthalenedicarboxylate, melt-extruding these to form an unstretched sheet, and stretching the sheet. In order to obtain a practical mechanical strength, biaxial stretching is preferred.

  Here, a method for producing a film will be described in detail by taking as an example a method for producing a film by sequential biaxial stretching after melt extrusion. If necessary, the resin is melted at a normal melt extrusion temperature, that is, a melting point (referred to as “Tm”) or more and (Tm + 50 ° C.) or less after removing moisture by ordinary heating or drying under a reduced pressure atmosphere. The amorphous unstretched sheet is obtained by extruding from a slit of the die and rapidly solidifying on a rotary cooling drum cooled to a temperature lower than the glass transition temperature (referred to as “Tg”) of the resin. The obtained unstretched sheet is stretched at a stretch ratio of 3.1 to 5.0 times in the machine direction at a temperature of Tg or more and (Tg + 50 ° C.) or less, and then is Tg or more and (Tg + 50 ° C.) or less in the transverse direction. The film is stretched at a stretching ratio of 3.1 to 5.0 times. The draw ratio is preferably 3.5 to 5.0 times in the longitudinal direction and 3.3 to 5.0 times in the transverse direction. By increasing the draw ratio, the film of the present invention having a low coefficient of thermal expansion can be obtained. Note that simultaneous biaxial stretching in which longitudinal stretching and lateral stretching are performed simultaneously is also a preferred stretching method because it is easy to balance longitudinal and lateral mechanical properties.

  The biaxially stretched film stretched longitudinally and laterally is preferably heat-set at a temperature not lower than the resin crystallization temperature (hereinafter referred to as “Tc”) and not higher than (Tm−10 ° C.). When the thermal expansion coefficient is lowered by increasing the draw ratio and increasing the orientation of the molecular chain for the above-mentioned purpose, distortion tends to remain in the film, and as a result, the thermal shrinkage rate tends to increase. Therefore, in order to remove the distortion, it is preferable to perform thermal relaxation treatment in the range of 0.5 to 15% in the longitudinal direction and / or the lateral direction. The heat relaxation treatment may be performed at the time of film production, or may be heat-treated in another step after winding. The heat treatment method after winding is not particularly limited, but for example, a method of relaxing heat treatment in a suspended state as shown in JP-A-1-275031 can be used.

Hereinafter, the present invention will be further described by examples.
Each characteristic value was measured by the following method.

(1) Thermal expansion coefficient (αt)
Two types of samples were cut from the film. One is a rectangular sample having a length of 20 mm and a width of 5 mm along the longitudinal direction of the film, and the other is a rectangular sample having a length of 20 mm and a width of 5 mm along the width direction of the film.
Each of these samples was pretreated at 200 ° C. for 30 minutes with a load of 140 g / mm ² at a distance between chucks of 15 mm using a TMA / SS120C manufactured by Seiko Instruments Inc., and then at room temperature. The sample was measured for the thermal expansion coefficient in the longitudinal direction and the sample for thermal expansion coefficient measurement in the width direction.
Each measurement sample was heated from 30 ° C. to 200 ° C. at a rate of temperature increase of 20 ° C./min, and a chart of temperature and sample length was obtained. L ₁ , L ₂ , and L ₃ are read from this chart, and the coefficient of thermal expansion αt (120) when the temperature is increased from 30 ° C. to 120 ° C. at a temperature increase rate of 20 ° C./min, and 20 ° C. from 120 ° C. to 200 ° C. The coefficient of thermal expansion αt (200) when the temperature was increased at a rate of temperature increase per minute was calculated by the following formula.
αt (120) = {[(L ₂ −L ₁ ) × 10 ⁶ ] / (L ₁ × ΔT ₁ )} + 0.5
αt (200) = {[(L ₃ −L ₂ ) × 10 ⁶ ] / (L ₂ × ΔT ₂ )} + 0.5
here,
L ₁ ; sample length at 30 ° C. (mm)
L ₂ ; sample length at 120 ° C. (mm)
L ₃ ; sample length at 200 ° C. (mm)
ΔT ₁ ; 90 ° C. = (120 ° C.-30 ° C.)
ΔT ₂ ; 80 ° C. = (200 ° C.-120 ° C.)
It is. In the above, 0.5 is the coefficient of thermal expansion (ppm / ° C.) of the quartz glass used in the sample tube for this measurement.

(2) Water absorption Measured according to JIS K7209-1984 A method. That is, the sample is pretreated at 50 ° C. for 24 hours, the weight (M1) of the sample before the immersion treatment is measured, and then immersed in ion-exchanged water in an environment of 23 ° C. × 55 RH% for 24 hours. Went. Thereafter, the sample was taken out, the weight (M2) of the sample after the immersion treatment was measured, and the water absorption rate of the sample was calculated by the following formula from the change in the weight of the sample before and after the immersion.
Water absorption (%) = (M2−M1) / M1 × 100

(3) Intrinsic viscosity The viscosity of the solution in an orthochlorophenol solvent was measured and determined at 35 ° C.

(4) Film thickness Ten points of film samples were measured with an electric micrometer (Anritsu K-402B), and the average value was obtained to obtain the film thickness.

(5) Heat shrinkage rate Marks are applied to film samples at intervals of 30 cm, heat treatment is performed for a predetermined time in an oven at a predetermined temperature without applying a load, and the interval between the heat marks after the heat treatment is measured. In the direction (MD direction) and the direction (TD direction) perpendicular to the film forming direction, the thermal shrinkage rate was calculated by the following formula. The inter-marker distance L ₀ before the heat treatment and the inter-marker distance L after the heat treatment were measured, respectively, and the dimensional change rate after the heat treatment was calculated as the thermal contraction rate S (%) by the following equation.
S (%) = ((L ₀ −L) / L ₀ ) × 100

(6) Operation rate and photoelectric conversion efficiency of thin film solar cell An Ag thin film having a thickness of 200 nm was formed on the surface of the film sample by a sputtering method, and an AZO thin film having a thickness of 50 nm was further formed thereon. Then, the film sample in which these thin films were formed was put into a plasma CVD apparatus, the substrate temperature was set to 190 ° C., and the photoelectric conversion layer (three layers of n, i, p type amorphous silicon (a-Si) layers) Three layers having a total thickness of 0.4 μm) were formed. Then, after forming an AZO thin film with a thickness of 100 nm at a temperature of 190 ° C. at a temperature of 190 ° C. by a sputtering method in a state where the mask is placed on the mesh, an Ag thin film is formed with a thickness of 300 nm by sputtering using a comb-shaped mask, A thin film solar cell was obtained.
A 500 W xenon lamp (USHIO INC.) Is equipped with a solar simulation correction filter (AM 1.5 Global manufactured by Oriel), and simulated solar light with an incident light intensity of 100 mW / cm ² is applied to the above thin film solar cell. Irradiated so as to be perpendicular to the horizontal plane. The system was left indoors at a temperature of 25 ° C. and a humidity of 50%. Using a current / voltage measuring device (type Keithley source measure unit 238), the DC voltage applied to the system was scanned at a constant speed of 10 mV / sec, and the IV curve characteristics were measured. From the short-circuit current (Jsc) and open circuit voltage (Voc) FF (fill factor: fill factor) obtained from this result, the photoelectric conversion efficiency η was calculated by the following formula.
η (%) = Jsc × Voc × FF
Also, during these measurements, the operating rate (%) was calculated from these ratios A / B, where A was the number of cells that generated power without causing a short circuit or current leakage, and B was the total number of cells created.
Operating rate (%) = A / B × 100

(7) Total light transmittance Using a spectrophotometer (MPC3100 manufactured by Shimadzu Corporation), light transmittance in the wavelength range of 400 nm to 900 nm was measured at intervals of 2 nm. The total light transmittance at 400 to 900 nm was calculated from the average of the measured light transmittance of each wavelength.
Total light transmittance = ΣTn / m
Here, ΣTn is the total light transmittance of each wavelength, and m is the number of measurement points.

[Example 1]
Polyethylene-2,6-naphthalene dicarboxylate (amorphous density 1.33, intrinsic viscosity: 0.65) was dried at 170 ° C. for 6 hours and then supplied to an extruder, and a slit die at a melting temperature of 305 ° C. The film was further extruded and rapidly solidified on a rotary cooling drum maintained at a surface temperature of 50 ° C. to obtain an unstretched film.
Next, the film was stretched 4.0 times in the longitudinal direction at 140 ° C., then stretched 4.0 times in the transverse direction at 145 ° C., heat-set at 245 ° C. for 5 seconds, and contracted 2% in the width direction, and the thickness was 100 μm. Film was obtained. Further, the obtained film was heated and relaxed at 200 ° C. for 10 minutes to remove internal strain. The characteristics of the obtained film are as shown in Table 1, and the characteristics of the super straight type thin film solar cell prepared using the obtained film are as shown in Table 1.

[Example 2]
A film and a super straight type thin film solar cell were prepared using the same method as in Example 1 except that the draw ratio was 3.8 times in the longitudinal direction and 3.8 times in the transverse direction. The characteristics are shown in Table 1.

[Example 3]
A film and a super-straight type thin film solar cell were prepared in the same manner as in Example 1 except that a silicon oxide layer was laminated on both surfaces with a thickness of 150 nm on both surfaces of the film after relaxation. Each characteristic is as shown in Table 1.

[Comparative Example 1]
A film and a super straight type thin film solar cell were prepared using the same method as in Example 1 except that the stretching ratio was 3.1 times in the vertical direction and 3.3 times in the horizontal direction. The characteristics are as shown in Table 1, and the solar cell operating rate was low.

  The film for solar cell substrate of the present invention can be suitably used as a substrate for flexible type solar cells, particularly as a film for super straight type solar cell substrates.

Claims (3)

It consists of polyethylene-2,6-naphthalenedicarboxylate, and the coefficient of thermal expansion when heated from 30 ° C. to 120 ° C. at a heating rate of 20 ° C./min is 20.1 ppm / ° C. in both the longitudinal and width directions. or less and either 120 ° C. from heating and thermal expansion coefficient when the at a heating rate of 20 ° C. / min to 200 ° C. is in the longitudinal direction and the width direction is at 50 ppm / ° C. or less, when the heat-treated 10 minutes at 200 ° C. The solar cell base material is characterized in that the heat shrinkage ratio is 1% or less, the total light transmittance in the wavelength range of 400 to 900 nm is 70% or more on average, and the water absorption is 1% or less. Film.   The film for solar cell base materials according to claim 1, which is used as a super straight type solar cell base material. The film for solar cell base materials according to claim 1 or 2, which is for thin film solar cells.