JP2008041746A - Substrate of solar cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide the substrate for a solar cell, which has a surface of moderate roughness for attaining light confinement effect and useful for manufacturing a solar cell exhibiting excellent photoelectric conversion efficiency, when it is used as the substrate of a flexible solar cell, especially a thin film solar cell. <P>SOLUTION: The substrate for a solar cell consists of: a surface roughness layer composed of a thermoplastic crystalline resin composition containing inert particles, and having surface roughness Ra in the range of 30-500 nm; and a support layer composed of a thermoplastic crystalline resin contacting the surface roughness layer. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a solar cell substrate, and more particularly to a solar cell substrate capable of producing a solar cell exhibiting excellent photoelectric conversion efficiency when used as a flexible thin-film solar cell substrate.

  Solar cells generally include a rigid type using glass as a substrate material and a flexible type using a plastic film as a substrate material. Recently, flexible solar cells have come to be widely used as auxiliary power sources for mobile communication devices such as watches or mobile phones and mobile terminals. The conventional rigid type has higher energy conversion efficiency in solar cells than the flexible type, but there is a limit to making the device thinner and lighter, and the solar cell module will be damaged when subjected to an impact. Cases are also conceivable.

  For this reason, the usefulness of the flexible type 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, among which a polyethylene terephthalate film, a polyethylene naphthalate film, and a polyimide film are disclosed. Etc. are illustrated. In addition, JP-A-2-260577, JP-B-6-5782, and JP-A-6-350117 disclose solar cell modules using a flexible substrate.

  On the other hand, in thin film solar cells such as amorphous silicon, it is important to increase the amount of light absorption within the thickness of the light absorption layer in order to improve the photoelectric conversion efficiency. It has been conventionally performed to increase the optical path length of light in the light absorption layer by forming and diffusing light. However, in order to form an uneven metal layer or metal oxide layer, a high temperature of 350 ° C. or higher is generally required, and application is difficult when a plastic film is used as a substrate. On the other hand, Japanese Patent Application Laid-Open No. 1-119074 describes a method in which a filler is added to a resin solution and cast on a support to make the surface uneven, and a conductive layer is formed thereon. . However, this method cannot effectively form unevenness unless the filler concentration is increased, and if the filler concentration is increased, the substrate becomes brittle and cannot be put into practical use. Japanese Patent Application Laid-Open No. 4-196364 discloses a method of forming a film by coating a resin solution on a support substrate and further forming a film by applying a resin solution containing particles thereon. Describes a technique in which an ultraviolet curable resin is applied to the surface of a base film, pressed against a mold and cured to form irregularities, and a conductive layer is formed thereon. However, since these methods are performed in a separate process after film production, not only the cost increases, but also the residual solvent used in the coating of the resin for forming the concavo-convex layer is degassed when the transparent conductive layer is formed. There is a problem that the shape formed by the uneven layer is not reflected well. Although this unevenness forming technique using degassing is described in Japanese Patent Publication No. 7-50794, it is difficult to quantitatively control the residual solvent and degassing amount in the resin. Control is very difficult.

Japanese Patent Laid-Open No. 1-198081 JP-A-2-260577 Japanese Patent Publication No. 6-5782 JP-A-6-350117 Japanese Patent Laid-Open No. 1-119074 JP-A-4-196364 Japanese Patent No. 3749015 Japanese Patent Publication No. 7-50794

  The object of the present invention is to solve the problems of the technology, and have an appropriately rough surface for obtaining a light confinement effect, and is excellent when used as a base material for flexible solar cells, particularly thin film solar cells. Another object of the present invention is to provide a solar cell substrate useful for producing a solar cell having photoelectric conversion efficiency.

  That is, the present invention is a support composed of a composition of a thermoplastic crystalline resin containing inert particles, a rough surface layer having a surface roughness Ra of 30 to 500 nm, and a thermoplastic crystalline resin in contact therewith. It is the base material of the solar cell which consists of layers.

  According to the present invention, a solar cell having an appropriately rough surface for obtaining a light confinement effect and having excellent photoelectric conversion efficiency when used as a base material for a flexible solar cell, particularly a thin film solar cell. Solar cell substrates useful for manufacturing can be provided.

Hereinafter, the present invention will be described in detail.
[Surface roughness]
The rough surface layer of the solar cell substrate of the present invention has a surface roughness Ra of 30 to 500 nm, preferably 35 to 300 nm, and more preferably 40 to 200 nm. When Ra is less than 30 nm, the light scattering effect is reduced, and the effect of improving the photoelectric conversion efficiency of the solar cell is reduced. On the other hand, if it exceeds 500 nm, the protrusions on the surface are too large, and it becomes difficult to form a uniform conductive layer thereon.

The rough surface layer in the base material of the solar cell of the present invention is composed of a thermoplastic crystalline resin composition containing inert particles. The surface roughness can be obtained by adjusting the particle size and content of the particles to an appropriate range. For example, when using particles having an average particle size of 0.3 μm, the surface roughness can be obtained by incorporating particles into the resin in the range of 1% by volume to 20% by volume. For example, when using particles having an average particle diameter of 3 μm, the surface roughness can be obtained by incorporating particles into the resin in the range of 0.1% by volume to 3% by volume.
In addition, when manufacturing a film by a biaxial stretching method, surface roughness can be adjusted also by the manufacturing conditions of a film as mentioned later.

[Other physical properties]
The substrate of the solar cell of the present invention preferably has a total light transmittance of 80% or more. If it is less than 80%, the amount of incident light entering the solar cell is reduced, so that high power generation efficiency cannot be obtained. By providing this light transmittance, the base material of the solar cell of the present invention can be used as a base material of a super straight type solar cell or a surface electrode side base material.

  The base material of the solar cell of the present invention preferably has a thermal shrinkage rate of 1% or less, more preferably 0, when treated at 200 ° C. for 10 minutes from the viewpoint of suppressing dimensional changes in the heating step in the solar cell manufacturing process. 0.8% or less, particularly preferably 0.6% or less.

  The thickness of the substrate of the solar cell of the present invention is preferably 40 to 150 μm, more preferably 50 to 130 μm, and particularly preferably 60 to 125 μm. If the thickness is less than 40 μm, the stiffness as a base material that is a support for the solar cell is small and the solar cell may not be supported, and if it exceeds 150 μm, the thickness of the solar cell module becomes too thick and flexible. Is not preferable because it is lost.

  In order to obtain the substrate of the solar cell of the present invention, a rough surface layer comprising a composition of a thermoplastic crystalline resin containing particles, having a surface roughness Ra of 30 to 500 nm, and a thermoplastic crystal in contact therewith A film composed of a support layer made of a conductive resin may be used.

[resin]
The substrate of the solar cell of the present invention is a resin that can be melt-extruded, and is composed of a thermoplastic crystalline resin film. In the present invention, an unstretched laminated sheet is obtained by melt extrusion and stretched to obtain a film. However, if a film is produced by a solution method, a process of providing a conductive layer on the film in a later step for processing into a solar cell. In this case, degassing derived from the residual solvent is generated, and the uneven structure formed on the film before the step of providing the conductive layer is disturbed, and the uneven structure is not accurately reflected in the solar cell, which is not preferable. In order to maintain the mechanical strength, the film is preferably a film biaxially stretched.

  Considering the process of processing into a solar cell, a resin having high heat resistance is preferable. As the thermoplastic crystalline resin, a biaxially stretchable resin is preferably used, and polyether ether ketone, polyphenylene sulfide, polyamide, polyethylene terephthalate, and polyethylene-2,6-naphthalate are preferable. Polyethylene-2,6-naphthalate is particularly preferred.

  The thermoplastic crystalline resin that constitutes the support layer and the thermoplastic crystalline resin that constitutes the rough surface layer may be the same or different. From the viewpoint of facilitating stretching when biaxially stretching, it is preferable to use the same kind of thermoplastic crystalline resin.

[Rough surface layer particles]
In the present invention, in order to obtain a surface having a surface roughness Ra of 30 to 500 nm, a thermoplastic crystalline resin composition containing 0.5 to 20% by volume of particles having an average particle diameter of 0.05 to 10 μm is used. A rough surface layer is formed.

  As the particles, inert particles having sufficient heat resistance during melt extrusion of the resin are used. For example, inorganic particles such as spherical silica, porous silica, calcium carbonate, alumina, titanium dioxide, kaolin clay, barium sulfate, and zeolite are used. Particles, crosslinked polymer particles such as silicone resin particles and crosslinked polystyrene particles, or organic salt particles can be used.

  The average particle diameter of the particles is 0.05 μm to 10 μm, preferably 0.1 μm to 8 μm, more preferably 0.2 μm to 6 μm. If the average particle size of the particles is less than 0.05 μm, a surface shape that sufficiently scatters light cannot be formed. On the other hand, if the average particle size exceeds 10 μm, protrusions formed on the surface become too large and uniform on the surface. It may be difficult to form a conductive layer.

  The average particle size of the particles was measured using a CP-50 type centrifuggle particle size analyzer manufactured by Shimadzu Corporation, and particles of each particle size calculated based on the obtained centrifugal sedimentation curve. The particle size corresponding to 50% by weight is read from the integrated curve of the amount and the abundance thereof (see “Particle Size Measurement Technology” published by Nikkan Kogyo Shimbun, pages 242 to 247 in 1975).

  The content of the rough surface phase particles is preferably 0.5 to 20% by volume, more preferably 1 to 15% by volume, and particularly preferably 2 to 10% by volume. Here, the volume% is obtained by calculation using the true density of the particles and the density of the amorphous state of the resin from the weight%. When the content of the particles is less than 0.5% by volume, it is not preferable because a surface shape that sufficiently scatters light cannot be formed. On the other hand, when the content exceeds 20% by volume, the resin layer becomes brittle. Practical mechanical strength may not be obtained, which is not preferable.

  The particles may be a single component selected from those exemplified above, or may be a multicomponent including two components or three or more components. Further, in the case of a single component, it may contain two or more kinds of particles having different average particle diameters.

  From the viewpoint of improving the total light transmittance, it is preferable not to contain these particles in the thermoplastic crystalline resin constituting the support layer, and even if it is contained, it is at most 0.5% by volume, preferably 0.3%. It should be kept below the volume percent.

[Additive]
In addition to the above particles, an antioxidant, a heat stabilizer, a lubricant such as a wax, a flame retardant, an antistatic agent, an ultraviolet absorber and the like may be added to the thermoplastic crystalline resin composition.
Among these, in order to improve the weather resistance of the film, it is preferable to contain an ultraviolet absorber. As an ultraviolet absorber, a compound having a large absorption coefficient which is effective in a small amount is preferable. 2,2′-p-phenylenebis (3,1-benzoxazin-4-one), 2,2 ′-(4,4 '-Diphenylene) bis (3,1-benzoxazin-4-one) and 2,2'-(2,6-naphthylene) bis (3,1-benzoxazin-4-one) are preferred.

When the ultraviolet absorber is contained, it is effective to improve the weather resistance by adding it to the resin arranged on the most surface layer on the light incident side of the film.
When an ultraviolet absorber is contained, it is effective to improve the weather resistance by adding it to the resin layer disposed on the most surface layer on the light incident side of the film. For example, when using as a surface electrode side base material of a thin film solar cell as a two-layer structure of a support layer and a rough surface layer, since the support layer becomes the light incident side outermost layer, the thermoplastic crystalline resin of the support layer is made of ultraviolet rays. An absorbent may be added.

[Film Production Method]
The base material of the solar cell of the present invention is obtained by melting the thermoplastic crystalline resin that constitutes the support layer and the thermoplastic crystalline resin composition that constitutes the rough surface layer. It is manufactured by melt-extrusion and lamination by an extrusion method. As a method of laminating, resin extruded from separate extruders is merged by a feed block and extruded from a die, and resin extruded from separate extruders is used in a die immediately before extrusion in a die. For example, a method of joining and extruding can be used. The laminated structure is preferably a two-layer structure of support layer / rough surface layer and a three-layer structure of rough surface layer / support layer / rough surface layer. In the case of a multilayer structure of three or more layers, the rough surface layer is always arranged at least on the outermost layer on one side.

Moreover, since practical mechanical strength is obtained even when particles are added at a high concentration to the resin of the rough surface layer, the film is preferably produced by biaxial stretching.
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 being laminated in two layers by coextrusion.

  The composition of the thermoplastic crystalline resin that constitutes the support layer and the thermoplastic crystalline resin that constitutes the rough surface layer is subjected to moisture by drying under a normal heating or reduced pressure atmosphere as necessary. After removal, the melt is melted in a separate extruder at a normal melt extrusion temperature, that is, a melting point (hereinafter referred to as Tm) or more and (Tm + 50 ° C.) or less, and laminated in a two-layer structure using a feed block. The amorphous sheet is obtained by extruding from the slit of the die and rapidly solidifying on a rotary cooling drum cooled to Tg of a resin having a low glass transition temperature (hereinafter referred to as Tg) out of the two resins. The obtained sheet is stretched at a stretch ratio of 2.5 to 4.5 times in the longitudinal direction within the range of (Tg + 50 ° C.) of Tg of the resin having a high Tg out of the two resins, and then in the transverse direction. The two resins are stretched at a stretch ratio of 2.5 to 4.5 times within the range of Tg of a resin having a high Tg and (Tg + 50 ° C.). Note that simultaneous biaxial stretching in which longitudinal stretching and transverse stretching are simultaneously performed is also a preferred stretching method because it is easy to balance longitudinal and lateral mechanical properties.

The aforementioned surface roughness Ra can be adjusted even under this stretching condition. For example, when particles that are not deformed by an external force are used, protrusions are formed on the surface during the stretching process of the film, but Ra increases as the internal stress generated by stretching increases, that is, the higher magnification is performed at a lower temperature.
A film that has been stretched vertically and horizontally and made into a thin film has a crystallization temperature (hereinafter referred to as Tc) of two resins that is higher than Tc or higher (Tm-20 ° C. of resin A or resin B having a lower Tm). Heat fixing at a temperature of Thereafter, for the purpose of reducing the thermal shrinkage rate, it is preferable to perform a thermal relaxation treatment in the longitudinal direction and / or the lateral direction within a range of relaxation rate of 0.5 to 15%. The thermal relaxation treatment may be performed in a separate step after winding, in addition to the method performed during film production. As a heat treatment method after winding, for example, a method of relaxing heat treatment in a suspended state as disclosed in JP-A-1-275031 can be used.

  In order to increase the total light transmittance of the film, the number of particles contained in the resin A is minimized as described above, and the thickness ratio (support layer / rough surface layer) between the support layer and the rough surface layer is increased. . The rough surface layer also needs a certain thickness to enhance the light diffusion effect, and the thickness ratio is preferably 1 to 30, more preferably 3 to 20, and particularly preferably 5 to 15.

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

(1) Intrinsic Viscosity The viscosity of a solution using an orthochlorophenol solvent was measured and determined at 35 ° C.

(2) Thickness of each layer A film sample was cut into a triangle, fixed in an embedded capsule, and then embedded in an epoxy resin. The embedded sample was made into a thin film section having a thickness of 50 nm with a microtome (ULTRACUT-S) and then observed and photographed at an acceleration voltage of 100 kV using a transmission electron microscope. The thickness was measured.

(3) Heat shrinkage rate Hold the film in an unstrained state for 10 minutes in an oven set at a temperature of 200 ° C., and measure the distance L ₀ between gauge points before heat treatment and the distance L between gauge points after heat treatment. The dimensional change rate was calculated as the thermal shrinkage rate S (%) by the following equation.
S = ((L ₀ −L) / L ₀ ) × 100

(4) Average particle diameter of particles Measured using a CP-50 type centrifuggle particle size analyzer manufactured by Shimadzu Corporation, and particles having respective particle diameters calculated based on the obtained centrifugal sedimentation curve The particle size corresponding to 50% by weight was read from the integrated curve with the abundance (see “Particle Size Measurement Technology”, published by Nikkan Kogyo Shimbun, 1975, pages 242-247).

(5) Centerline average surface roughness (Ra)
Using a non-contact type three-dimensional roughness meter (manufactured by Kosaka Laboratories, ET30HK) with a semiconductor laser having a wavelength of 780 nm, an optical stylus with a beam diameter of 1.6 μm, a measurement length (LX) of 1 mm, a sampling pitch of 2 μm, a cutoff of 0. The projection profile on the film surface was measured under the conditions of 25 mm, longitudinal magnification of 5000 times, lateral magnification of 200 times, and the number of scanning lines of 100 (thus, the measurement length LY in the Y direction was 0.2 mm). When the curved surface is expressed by Z = F (X, Y), a value (Ra, unit nm) obtained by the following formula was defined as the surface roughness of the film.

(6) Total light transmittance of film Total light transmittance Tt (%) was measured according to JIS standard K6714-1958.

(7) Photoelectric conversion efficiency of thin film solar cell Using a comb-shaped mask on the surface of the rough surface layer of the film, a substrate temperature was set to room temperature, and a 2000 Å Ag thin film was formed by DC sputtering. Next, an ITO layer (transparent conductive layer) was formed with a thickness of 2000 mm under a substrate temperature of 140 ° C. by RF sputtering. Thereafter, this film is put into a plasma CVD apparatus, the inside of the apparatus is depressurized until it reaches 1.33 × 10 ⁻⁵ Pa, subsequently the substrate temperature is set to 140 ° C., and monosilane gas containing hydrogen gas and a small amount of B ₂ H ₆ gas (Molar ratio, SiH ₄ : H ₂ : B ₂ H ₆ = 1: 181: 3 × 10 ⁻³ ), a p-type silicon film layer having a thickness of 0.2 μm was formed on the transparent conductive layer. Subsequently, an i-type silicon film layer having a thickness of 2 μm was formed on the p-type silicon film layer using hydrogen gas and monosilane gas (molar ratio, SiH ₄ : H ₂ = 1: 39). Further, a monosilane gas (molar ratio, SiH ₄ : H ₂ : PH ₃ = 1: 143: 8 × 10 ⁻³ ) containing hydrogen gas and a small amount of PH ₃ gas is used to form a thickness of 0. A 4 μm n-type silicon film layer was formed. Thereafter, a zinc oxide thin film was formed to a thickness of 500 mm at 200 ° C. by DC sputtering, and the Ag thin film (back electrode layer) was formed to a thickness of 2000 mm after serving the substrate to room temperature to obtain a thin film solar cell. .

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 18 ° C. and a humidity of 50%. Using a current-voltage measuring device (Keutley source measure unit 238 type), the DC voltage applied to the system is scanned at a constant speed of 10 mV / second, and the photocurrent output from the device is measured, so that photocurrent-voltage The characteristics were measured and the photoelectric conversion efficiency was calculated.

[Example 1]
Polyethylene-2,6-naphthalate (amorphous density 1.33, intrinsic viscosity) containing 0.05% by volume of bulk silica (true density 2.2) having an average particle size of 3.5 μm as the thermoplastic crystalline resin of the support layer : 0.65) and polyethylene-2,6- containing 1.2% by volume of bulk silica (true density 2.2) having an average particle size of 3.5 μm as the composition of the thermoplastic crystalline resin of the rough surface layer Naphthalate (amorphous density 1.33, intrinsic viscosity: 0.65) and each were dried at 170 ° C. for 6 hours and then fed to separate extruders. After melting at a melting temperature of 305 ° C, they are merged to form a two-layer structure using a feed block, extruded from a slit die, and rapidly cooled and solidified on a rotary cooling drum maintained at a surface temperature of 50 ° C, unstretched A film was obtained.

Next, the film was stretched 3.1 times at 140 ° C. in the longitudinal direction, then stretched 3.3 times at 145 ° C. in the transverse direction, heat-set at 245 ° C. for 5 seconds, and contracted 2% in the width direction, and the thickness was 75 μm. A two-layer film was obtained. The thickness of the support layer and the roughness layer of this two-layer film were 70 μm and 5 μm, respectively. Ra of the surface of the rough surface layer was 135 nm, and the thermal shrinkage rate of the film at 200 ° C. was 0.4%. The total light transmittance of the film was 81%.
A thin film solar cell was prepared using the obtained film and the photoelectric conversion efficiency was measured. As a result, the open-circuit voltage was 0.42 V, the short-circuit current density was 23.1 mA / cm ² , and the photoelectric conversion efficiency was 5.3%. It was.

[Example 2]
Polyethylene-2,6-naphthalate (amorphous density 1.33, intrinsic viscosity) containing 0.1% by volume of calcium carbonate (true density 2.9) having an average particle size of 2.2 μm as the thermoplastic crystalline resin of the support layer : 0.64) and polyethylene-2,6- containing 3.0% by volume of calcium carbonate (true density 2.9) having an average particle size of 2.2 μm as the composition of the thermoplastic crystalline resin of the rough surface layer Naphthalate (amorphous density 1.33, intrinsic viscosity: 0.62) was dried at 170 ° C. for 6 hours and then fed to a separate extruder. After melting at a melting temperature of 305 ° C, they are merged to form a two-layer structure using a feed block, extruded from a slit die, and rapidly cooled and solidified on a rotary cooling drum maintained at a surface temperature of 50 ° C, unstretched A film was obtained.

Next, the film was stretched 3.1 times at 140 ° C. in the longitudinal direction, then stretched 3.3 times at 145 ° C. in the transverse direction, heat-set at 245 ° C. for 5 seconds, and contracted 2% in the width direction, and the thickness was 75 μm. A two-layer film was obtained. The thickness of the support layer and the roughness layer of this two-layer film were 65 μm and 10 μm, respectively. Ra of the surface of the rough surface layer was 75 nm, and the thermal shrinkage rate of the film at 200 ° C. was 0.2%. The total light transmittance of the film was 84%.
A thin film solar cell was prepared using the obtained film and the photoelectric conversion efficiency was measured. As a result, the open circuit voltage was 0.43 V, the short-circuit current density was 24.5 mA / cm ² , and the photoelectric conversion efficiency was 6.3%. It was.

[Comparative Example 1]
Polyethylene-2,6-naphthalate (amorphous density 1.33, intrinsic viscosity: 0.65) containing 0.3% by volume of bulk silica having an average particle diameter of 2 μm (true density 2.2) at 170 ° C. After drying for 6 hours, it was supplied to an extruder, extruded from a slit die at a melting temperature of 305 ° C., and melt-extruded on a rotating cooling drum maintained at a surface temperature of 50 ° C. to obtain an unstretched film.

Next, the film was stretched 3.1 times at 140 ° C. in the longitudinal direction, then stretched 3.3 times at 145 ° C. in the transverse direction, heat-set at 245 ° C. for 5 seconds, and contracted 2% in the width direction, and the thickness was 75 μm. Film was obtained. Ra of the surface of the obtained film was 28 nm, and the thermal shrinkage rate at 200 ° C. of the film was 0.3%. The total light transmittance of the film was 78%.
A thin film solar cell was prepared using the obtained film and the photoelectric conversion efficiency was measured. As a result, the open-circuit voltage was 0.42 V, the short-circuit current density was 17.7 mA / cm ² , and the photoelectric conversion efficiency was 4.5%. It was.

  The substrate of the solar cell of the present invention can be suitably used as a substrate of a flexible type thin film solar cell.

Claims (2)

  A solar cell comprising a thermoplastic crystalline resin composition containing inert particles and a rough surface layer having a surface roughness Ra of 30 to 500 nm, and a support layer comprising a thermoplastic crystalline resin in contact with the rough surface layer Base material.   The base material of the solar cell of Claim 1 used as a base material of a flexible type thin film solar cell.