JP2011009302A - Method of forming back electrode for thin film solar cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of forming a back electrode for a thin film solar cell, the method efficiently and inexpensively forming a uniform back electrode layer even on a substrate having any size and any shape without wasting an expensive metal and resource.SOLUTION: Conductive ink containing metal particles is held on a flexographic printing plate 11 on the surface of which an ink holding part having a predetermined pattern is formed, the conductive ink held in the ink holding part is transferred on an insulating translucent substrate 1 on which a photoelectric conversion layer 3 is formed by being laminated on the backside of a transparent electrode layer 2, and then the transferred conductive ink is heated. Thus, a back electrode layer 4 having a predetermined pattern can be effectively formed at high speed on the backside surface of a substrate even when the substrate has any size and any shape.

Description

  The present invention relates to a method for forming a back electrode on the back surface of a thin film solar cell in which a photoelectric conversion layer made of a semiconductor junction is laminated on a light-transmitting substrate.

In recent years, thin film semiconductors that can be manufactured at low cost using amorphous (amorphous) silicon or compound semiconductors such as CdS and CuInSe ₂ instead of conventional solar cells using single crystal or polycrystalline silicon. Solar cells (hereinafter, thin film solar cells) are being researched and developed. In this thin film solar cell, for example, a transparent conductive film (transparent electrode layer) such as SnO ₂ , ITO, or ZnO is formed on the back surface of an insulating translucent substrate such as glass, and an amorphous semiconductor is formed on the back surface thereof. A thin film semiconductor film (photoelectric conversion layer) in which p layers, i layers, and n layers are sequentially stacked is formed, and Ag (silver), Al (aluminum), Cu (copper), etc. A structure in which conductive films (back electrode layers) using a metal with high reflectance are stacked is formed (see, for example, Patent Documents 1 and 2).

  The use of a metal with high reflectivity for the back electrode (back electrode layer) is a device for supplementing low conversion efficiency (photoelectric conversion efficiency) compared to a crystalline silicon solar cell, and the light enters from the translucent substrate side. By reflecting sunlight, the power generation efficiency of the solar cell can be increased by the “light confinement effect”.

JP-A-8-37317 JP 2005-175449 A

  By the way, when manufacturing the thin film solar cell of the said structure, formation of the transparent electrode layer on an insulating translucent board | substrate, formation of the photoelectric converting layer on a transparent electrode layer, and formation of the back surface electrode layer on a photoelectric converting layer In these methods, sputtering (sputtering), vapor deposition, CVD, etc. are used. In particular, sputtering for forming a back electrode layer using a metal ingot as a target in a vacuum chamber is often used. Yes.

  However, since the formation of the back electrode layer using sputtering as described above is performed except for the necessary portions, there are also portions where expensive metal ingots are wasted. Further, since sputtering is a batch process, the cycle time (idle time) is long, the chamber size of the sputtering apparatus to be used is limited, and it is difficult to increase the size of the substrate to be processed.

  Furthermore, since sputtering equipment has many vacuum equipment and incidental facilities, a large space is required to install the equipment, and it is supplied for continuous processing such as inert gas and electric power to collide with the target. There are many resources to continue and running costs are high. These problems are common to conventional methods for forming the back electrode layer, such as vapor deposition and CVD.

  The present invention has been made in view of such circumstances, and it is possible to efficiently form a uniform back electrode layer on a substrate of any size and shape without wasting expensive metal and resources. It is an object of the present invention to provide a method for forming a back electrode for a thin film solar cell that can be formed at low cost.

  In order to achieve the above object, the present invention provides a method for forming a back electrode in a thin film solar cell including a transparent electrode layer, a photoelectric conversion layer, and a back electrode layer sequentially formed on the back surface of an insulating translucent substrate. A step of holding a conductive ink containing metal particles on a flexographic printing plate having an ink holding portion of a predetermined pattern formed on the surface thereof, and a photoelectric conversion layer on the flexographic printing plate, a transparent electrode layer A process of transferring the conductive ink held on the ink holding portion onto the photoelectric conversion layer, and heating the transferred conductive ink after the transfer Then, a gist is a method for forming (manufacturing) a back electrode for a thin-film solar cell, comprising: forming a back electrode layer having a predetermined pattern on the photoelectric conversion layer.

  That is, the present inventor has pondered whether the problem can be solved by an easy method instead of the method requiring the large-scale device, and has come up with an idea whether it can be realized by printing. As a result, the printing is devised, and as a result, a conductive film containing a predetermined electrode pattern in the form of a thin film is formed on the surface of the substrate by a flexographic printing method using a conductive ink containing metal particles having excellent reflectance. It has been found that a back electrode having characteristics suitable for a thin-film solar cell can be constructed (manufactured) by forming a conductive film.

  In the method for forming a back electrode for a thin film solar cell of the present invention, a conductive ink containing metal particles is held on a flexographic printing plate having an ink holding portion of a predetermined pattern formed on the surface thereof. After the transferred conductive ink is transferred to an insulating translucent substrate having a photoelectric conversion layer laminated on the transparent electrode layer, the transferred conductive ink is heated to obtain any size and shape. Even in this substrate, a back electrode having a predetermined pattern can be formed on the surface thereof at high speed and efficiently.

  The method for forming a back electrode for a thin film solar cell of the present invention does not require a large vacuum device such as sputtering, and unnecessary conductive ink is not transferred to an unneeded region. Thus, a back electrode suitable for a thin film solar cell can be formed at low cost without wasting expensive metal and industrial resources.

  Furthermore, in the formation method of the present invention, among them, the metal particles are silver particles having an average particle size of 0.5 to 300 nm, when the conductive film (back electrode layer) is formed by the flexographic printing method, Although it is a thin film, it has an advantage that a flat and uniform back surface electrode can be formed in which surface roughness is small and silver particles do not protrude from the surface.

  When the average particle size of silver particles contained in the conductive ink is less than 0.5 nm, or when the average particle size of silver particles exceeds 300 nm, sometimes a uniform and smooth back electrode cannot be formed. There is a fear.

  Further, the average particle diameter of the silver particles is assumed to be spherical silver particles in which the shape of each particle is relatively neat, but the silver particles are indefinite shapes such as flakes and scales. It may be. In the case of these irregularly shaped silver particles such as flakes and scales, particles having a micron order size whose average particle size greatly deviates from the above range may be mixed depending on the measurement direction. However, even for such large particles, for example, if the particles are flat, if the thickness (flat thickness direction) is within the range of 0.5 to 300 nm, the thin film solar cell of the present invention. In the method of forming the back electrode, there is no particular problem.

  In the formation method of the present invention, in particular, when the viscosity of the conductive ink is adjusted to 0.5 to 1000 mPa · s, the surface is flat and uniform even though the surface is thin. The back electrode can be efficiently manufactured. Moreover, since the viscosity of the conductive ink is suitable for the flexographic printing method, the amount of the conductive ink used, and hence the amount of expensive silver (silver particles) contained in the ink, is reduced. The cost of the back electrode for a thin film solar cell can be reduced. In addition, when the viscosity of the conductive ink is less than 0.5 mPa · s or more than 1000 mPa · s, there is a possibility that a uniform back electrode cannot be applied.

It is the top view which looked at the thin film solar cell in the embodiment of the present invention from the back side. It is a side view of the thin film solar cell in embodiment of this invention. It is a schematic block diagram of the flexographic printing machine which manufactures the back surface electrode for thin film solar cells in embodiment of this invention.

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

  FIG. 1 is a plan view of a thin film solar cell according to an embodiment of the present invention as viewed from the back side, and FIG. 2 is a side view of the thin film solar cell as viewed from the side. These drawings are drawn with emphasis on thickness.

  The thin film solar cell in this embodiment includes an insulating translucent substrate 1, a transparent electrode layer 2 formed on the back side thereof by a sputtering method, and a photoelectric conversion layer 3 formed on the back side thereof by a similar sputtering method. And a back electrode layer 4 formed so as to cover the photoelectric conversion layer 3. When this thin-film solar cell is actually used, each cell of the solar cell is electrically connected and protected with a sealing agent made of resin or the like, and then paneled and unitized. Used as a solar cell panel group.

  As a typical example, the insulating translucent substrate 1 may be a plate glass having a thickness of about 0.5 to 10 mm. In addition, the back surface (film formation surface side) of the insulating translucent substrate 1 is subjected in advance to a surface treatment such as plasma treatment, UV treatment, or polishing treatment for improving adhesion of a film formed in a later treatment. It may be left.

The transparent electrode layer 2 is made of, for example, SnO ₂ , ITO, ZnO or the like, and is formed as a transparent conductive film having a thickness of about 500 to 1500 nm using a sputtering method or the like.

The photoelectric conversion layer 3 is made of a crystalline system such as single crystal silicon, polycrystalline silicon, single crystal germanium, or microcrystalline silicon, or an amorphous system such as amorphous (non-crystalline) silicon, GaAs, InP, CdS, CdTe, CuInSe ₂ or the like. The inside is a multilayer structure (not shown) constituting a pn junction, a pin junction, a heterojunction, a Schottky type, a multiple junction type, and the like. The layer thickness (film thickness) is, for example, 200 to 400 μm for a pn junction and 100 nm to 5 μm for a pin junction.

  And the back electrode layer 4 is a conductive film having a film thickness of 50 to 1500 nm by a flexographic printing method using a conductive ink containing metal particles such as Ag (silver), Al (aluminum), Cu (copper). It is formed as. Note that Ag (silver), Al (aluminum), and Cu (copper) are used as the metal because the light incident from the insulating translucent substrate 1 side is reflected and the short-circuit current of the thin-film solar cell is used. This is to increase the value, and in the present embodiment, among these, Ag (silver) is preferably used.

  Moreover, as the conductive ink used, “nanosilver conductive ink” containing silver particles having an average particle diameter of 0.5 to 300 nm is suitably used as the metal particles. This nano silver conductive ink is composed of a resin as a binder and a hydrocarbon solvent as a dispersion solvent in addition to the silver particles having the above average particle diameter, and the binder resin constituting this ink. For example, acrylic resins, epoxy resins, and the like are used, and tetradecane, tridecane, decanol, terpineol, and the like are used as the hydrocarbon solvent. Furthermore, the conductive ink is adjusted so that its solid content is 3 to 80 wt% and its viscosity is 0.5 to 1000 mPa · s.

  The above-mentioned “nanosilver conductive ink” is a ink containing silver particles, a binder and a solvent as main components (low temperature fired silver conductive ink). Here, the main component means a component that occupies a majority of the whole, and includes the case where the whole consists of only the main component.

  The silver particles are silver powder having a spherical shape, a flake shape, a scale (phosphorus) shape, etc., and an average particle size (or average equivalent circle diameter) before heating (firing) is 0.5 to 0.5. It is in the range of 300 nm. Furthermore, the average particle diameter of the silver particles is measured by photon correlation spectroscopy using a dynamic light scattering particle analyzer, and the conductivity decreases when the average particle diameter of the silver particles exceeds 300 nm. Otherwise, the fluidity of the ink may be hindered and the stability of the conductive ink may be reduced.

  In addition, various additives such as a plasticizer, a lubricant, a dispersant (a sea surface active agent), a leveling agent, an antifoaming agent, and an antioxidant may be added to the conductive ink in the present embodiment as necessary. . Furthermore, an organic / inorganic filler may be added as appropriate.

Next, a method for forming a back electrode (back electrode layer 4) on the back side of the thin film solar cell using a flexographic printing machine will be described.
FIG. 3 is a schematic configuration diagram of a flexographic printing machine for manufacturing a back electrode for a thin film solar cell according to an embodiment of the present invention, in which reference numeral 11 denotes a printing plate, 12 denotes a plate cylinder, 13 denotes an anilox roll, and 14 denotes A stage, 15 is a squeegee, and 16 is an ink tank.

  First, a flexographic printing plate used for forming a back electrode for a thin film solar cell in this embodiment will be described. This printing plate 11 is a convex printing formed by curing a mixture of a urethane acrylate prepolymer, an acrylate oligomer, an acrylate monomer, a photopolymerization inhibitor, a photopolymerization initiator, and the like by ultraviolet irradiation through a negative film. It is a version.

On the surface (ink holding surface) of the printing plate 11, fine irregularities are formed along a predetermined shape of the back electrode pattern, and the conductive property is formed in a recess (ink holding portion) formed therebetween. Ink is retained. Note that the ink holding amount per unit area held in the ink holding unit is set to about 1 to 50 ml / m ² .

  The method for forming a back electrode for a thin-film solar cell using the flexographic printing plate 11 having the above structure is basically the same procedure as in normal flexographic printing. First, the ink (nano silver conductive ink) supplied from the ink tank 16 is supplied to the printing plate 11 through the anilox roll 13 and is applied to the ink holding portion of the predetermined electrode pattern formed on the surface of the printing plate 11. A predetermined amount of nanosilver conductive ink is retained.

  Next, while rotating the printing plate 11 together with the plate cylinder 12, the substrate placed on the stage 14 (the insulating translucent substrate 1 on which the transparent electrode layer 2 and the photoelectric conversion layer 3 are laminated) is synchronized. The nano silver conductive ink held by the ink holding part is applied to the surface (printing surface) of the photoelectric conversion layer 3 on the substrate 1 by moving and bringing the substrate 1 into close contact (kiss touch) with the printing plate 11. Transcribed.

  Thereafter, the substrate 1 after the nano silver conductive ink has been transferred is put into a dryer such as an oven, and the ink is heated and baked (200 to 300 ° C., 30 to 60 minutes). As the solvent and the like evaporate, the silver particles in the ink are baked to form a conductive film to be the back electrode layer 4.

By the above method, it is suitable for a thin film solar cell having a film thickness of 100 to 1500 nm (surface roughness Ra: 10 to 150 nm) and a volume resistance value of the back electrode layer 4 of 1.0 × 10 ⁻⁴ Ω · cm or less. A back electrode could be manufactured.

  Moreover, since the various printing conditions are set optimally, the formation method of the back surface electrode for thin film solar cells in this embodiment manufactures the back surface electrode for thin film solar cells of the said structure by one process passage (one pass). In combination with the high speed of the flexographic printing process (printing speed: 20 m / min or more), this thin-film solar cell can be produced at low cost and at high speed and efficiency.

  Next, examples will be described together with comparative examples. However, the present invention is not limited to the following examples.

  In this example, a back electrode for a thin film solar cell produced by flexographic printing using nano silver conductive ink [Example 1] and a back electrode for a thin film solar cell produced by conventional sputtering [Comparative Example 1] The volume resistance value (specific resistance: Ω · cm), reflectance (%: at 400 nm), etc. of these back electrode layers were compared.

The following conductive ink (low-temperature baked nanosilver conductive ink) and a substrate were used for the back electrode for the thin-film solar cell of Example 1.
[Nanosilver conductive ink]
NPS-J-HTB manufactured by Harima Chemicals Co., Ltd.
Component: Silver particles-particle size 3 to 7 nm (average particle size: 5 nm)
Silver content: 53-58wt%
Binder resin
Solvent / Diluent-Tetradecane Solid Content: 55-60wt%
Viscosity: 8-12 mPa · s
〔Base material〕
Base glass Kuramoto Seisakusho thickness -0.7mm

Moreover, the flexographic printing machine used for the production of Example 1 was used under the following processing conditions. (See Fig. 3 for the schematic configuration of flexographic printing.)
[Flexo printing machine]
FC-33S manufactured by MT Tech
[Flexographic printing plate]
Made by Komuratec-Uses flexographic printing plate as detailed in the above embodiment.
Plate thickness-2.25mm 600 lines / inch Opening ratio 5 to 10%
Hardness: 40-70 degrees (Shore A hardness)
Swelling ratio with respect to ink solvent (tetradecane): 0.5 to 15% (weight change rate)
Ink holding amount of printing ink holding part: 4 ml / m ² (adjustment range: 1 to 5 ml / m ² )
[Anilox Roll]
200 lines / inch (100-600 lines / inch)
Cell capacity (cell volume): 8 ml / m ² (adjustment range: 1.5 to 50 ml / m ² )

[Flexo printing conditions]
・ Printing speed (moving amount of printing stage): 25m / min ・ Anilox roll speed: 200rpm
-Between anilox roll and printing plate Nip width: 8mm (adjustment width: 4-8mm)
-Between printing plate and substrate Nip width: 10 mm (adjustment width: 8-12 mm)
・ Environment (atmosphere) of printing chamber
Temperature: 15-30 ° C Humidity: 40-70% RH
-Drying conditions after printing Predrying: Temperature: 80 to 150 ° C Time: 30 seconds to 5 minutes Main baking: Temperature: 150 to 300 ° C Time: 20 minutes to 180 minutes

[Example 1]
Under the above processing conditions, the nano silver conductive ink was printed and transferred to the back surface of the substrate 1 on which the transparent electrode layer 2 and the photoelectric conversion layer 3 were previously formed on the glass substrate by a flexographic printing machine, After pre-drying for 5 minutes, main baking (heating from 70 ° C. to 300 ° C. in 30 minutes) was performed to obtain a back electrode for a thin film solar cell having a thickness of 0.4 μm.

[Comparative Example 1]
Using a general sputtering apparatus, a silver thin film layer (film thickness: 0.3 μm) similar to the conventional product is formed on a substrate 1 on which a transparent electrode layer 2 and a photoelectric conversion layer 3 are formed in advance on a glass substrate. Formed.

  Using the samples of Example 1 and Comparative Example 1 described above, the physical properties of the back electrode for a thin film solar cell were compared.

[Volume resistance value (specific resistance)]
The resistance value was measured by a four-terminal method using a digital multimeter (R6551 manufactured by Advantest). Moreover, the cross section was observed using an electron microscope (JSM-5500 manufactured by JEOL Ltd.), the thickness of the layer formed of silver particles was measured, and the volume resistance value (specific resistance) was calculated from these measured values. .

[Adhesion]
Evaluation was performed according to a JIS K5400-8.5 (JIS D0202) cross cut test. The interval between the cuts is 1 mm. After cutting the conductive film, 1 minute after attaching the adhesive tape, hold the edge of the tape and peel it off at right angles to the coating surface, and visually check the peeled state. evaluated. Cellophane tape CT-12 (manufactured by Nichiban Co., Ltd.) was used as the adhesive tape.
Evaluation criteria:
No peeling from the substrate is observed. − ○
Partial peeling from the substrate is observed. − △
Peeling from the substrate is generally observed. − ×

[Reflectance]
Using a UV-visible near-infrared spectrophotometer UV-3600 manufactured by Shimadzu Corporation, the 45 ° reflectance during light irradiation at 400 nm was measured.

The above test results are shown in “Table 1”.

  By the said formation method, the back surface electrode for thin film solar cells which has a performance equivalent to the back surface electrode formed by the conventional sputtering method was able to be produced at low cost.

  The formation method of this invention is suitable for manufacturing the back surface electrode used for the thin film solar cell which laminated | stacked the photoelectric converting layer which consists of semiconductor junctions on the back surface of a translucent board | substrate. In particular, the formation method of the present invention is suitable because it can efficiently produce the back electrode, and thus the thin film solar cell, at low cost.

DESCRIPTION OF SYMBOLS 1 Insulating translucent board | substrate 2 Transparent electrode layer 3 Photoelectric converting layer 4 Back surface electrode layer 11 Printing board

Claims (3)

In a thin film solar cell comprising a transparent electrode layer, a photoelectric conversion layer and a back electrode layer sequentially formed on the back surface of an insulating translucent substrate, a method for forming the back electrode,
A step of holding a conductive ink containing metal particles on a flexographic printing plate having an ink holding portion of a predetermined pattern formed on the surface thereof, and a photoelectric conversion layer formed on the transparent electrode layer on the flexographic printing plate A step of transferring the conductive ink held on the ink holding portion onto the photoelectric conversion layer, and heating the transferred conductive ink after the transfer, Forming a back electrode layer having a predetermined pattern on the photoelectric conversion layer, and forming a back electrode for a thin film solar cell.   The method for forming a back electrode for a thin film solar cell according to claim 1, wherein the metal particles are silver particles having an average particle size of 0.5 to 300 nm.   The method of forming a back electrode for a thin-film solar cell according to claim 1 or 2, wherein the viscosity of the conductive ink is adjusted to 0.5 to 1000 mPa · s.

Images