JP2018139329A - Manufacturing method for crystalline silicon-based solar battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a crystalline silicon-based solar battery excellent in its conversion efficiency.SOLUTION: A crystalline silicon-based solar battery includes a first transparent electrode layer 61 in contact with a p-type silicon-based layer 41 on one surface of a conductivity type single crystalline silicon substrate 1, and a second transparent electrode layer 62 in contact with an n-type silicon-based layer 42 on the other surface of the substrate 1. Both of the first transparent electrode layer 61 and the second transparent electrode layer 62 are deposited by a magnetron sputtering method of not higher than 100°C in substrate temperature.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a method for producing a crystalline silicon solar cell having a heterojunction on the surface of a single crystal silicon substrate.

  A crystalline silicon solar cell including a conductive silicon thin film on a single crystal silicon substrate is called a heterojunction solar cell. Among them, a heterojunction solar cell having an intrinsic amorphous silicon thin film between a conductive silicon thin film and a single crystal silicon substrate is known as one of the forms of crystalline silicon solar cells having the highest conversion efficiency. Yes.

  In the heterojunction solar cell, a transparent electrode layer is further formed on the surface of the conductive silicon thin film. The transparent electrode layer preferably has a high light transmittance and a low resistance. The material used is a transparent conductive metal oxide such as crystalline indium tin composite oxide (ITO) or zinc oxide. It is done. Moreover, a metal collector electrode is formed on the transparent electrode layer by printing a conductive paste or plating.

  In order to improve the conversion efficiency of a solar cell, it is important to increase the amount of light taken into the photoelectric conversion layer and to efficiently extract holes and electrons generated in the photoelectric conversion layer to an external circuit. In the heterojunction solar cell using a single crystal silicon substrate, in particular, the loss when extracting holes and electrons that have reached the surface of the conductive silicon thin film from the collector electrode to the external circuit through the transparent electrode layer is reduced. This is very important. For this reason, attempts have been made to improve the interface characteristics between the conductive silicon thin film and the transparent electrode layer and to improve the adhesion between the transparent electrode layer and the collector electrode.

  For example, Patent Document 1 discloses a heterojunction solar cell in which a pattern-shaped metal collector electrode is formed on the light incident side and a metal electrode is formed on substantially the entire back surface side. According to this configuration, the amount of incident light can be increased, and incident light that has not been absorbed by the silicon substrate can be reflected by the metal electrode on the back surface side and reused. In such a front-back asymmetric configuration, since the substrate is warped, in view of problems such as cracking of the substrate and peeling of the collector electrode film, Patent Document 1 discloses zinc oxide having predetermined crystal characteristics. A transparent electrode layer is used. According to such a configuration, the stress applied to the front and back surfaces of the silicon substrate is controlled to suppress the warpage of the substrate, and fine irregularities are formed on the surface of the zinc oxide layer. Adhesion with is improved.

  Patent Document 2 discloses that a transparent electrode layer composed of two layers of an ITO thin film having a high carrier density and an ITO thin film having a low carrier density is formed on the conductive silicon-based thin film. With this configuration, while improving the electrical bonding state between the conductive silicon thin film and the transparent electrode layer, the amount of carriers in the film of the transparent electrode layer can be reduced, and light absorption loss due to the transparent electrode layer can be reduced. .

WO2011 / 002086 International Publication Pamphlet WO2012 / 020682 International Publication Pamphlet

  As proposed in Patent Document 1, the method of adjusting the crystallinity of the transparent electrode layer suppresses the warpage of the substrate by adjusting the stress on the front and back of the substrate. The crystal characteristics may change due to a subtle change, and the substrate may be warped. Therefore, in order to increase the productivity of the solar cell and suppress problems such as peeling of the collector electrode, it is preferable that the collector electrode has a symmetrical configuration.

  On the other hand, in a solar cell in which a patterned collector electrode is formed on the back side as well as on the light incident side, incident light that is not absorbed in the silicon substrate cannot be reflected by the back side electrode and reused. For this reason, it is important to reduce the light absorption loss due to the transparent electrode layer, the silicon-based thin film, and the like, thereby increasing the light utilization efficiency. In particular, it is necessary to increase the light absorption amount on the back side in the silicon substrate.

  In a single crystal silicon substrate, light on the short wavelength side is preferentially absorbed on the light incident side in the thickness direction, and light on the long wavelength side that is not absorbed on the light incident side (mainly near-infrared light having a wavelength longer than 900 nm). Light) tends to be absorbed on the back side in the thickness direction. Therefore, in a heterojunction solar cell in which patterned collector electrodes are formed on both the light incident side and the back surface side, it is possible to reduce the absorption loss of light on the long wavelength side due to the transparent electrode layer or the silicon-based thin film. It becomes important in improving. Since long-wavelength light is easily absorbed by the carriers in the film, reducing the carrier density in the film of the transparent electrode layer or the silicon-based thin film is thought to lead to improved conversion efficiency.

  Patent Document 2 proposes to reduce the amount of carriers in the film by relatively increasing the thickness of the ITO thin film having a low carrier density. However, light absorption by the high carrier density ITO thin film provided for improving the interface bonding with the silicon-based thin film is a non-negligible amount, which is a barrier when the conversion efficiency is further improved. Further, the ITO transparent electrode layer used in the heterojunction solar cell of Patent Document 2 does not necessarily have sufficient adhesion to the metal electrode formed thereon, and from this point, the conversion characteristics are further improved. There is room for.

  In view of the above, an object of the present invention is to provide a heterojunction solar cell that is excellent in conversion efficiency and includes a transparent electrode layer that has low light absorption and that has excellent interfacial adhesion and adhesion to a silicon thin film and a collector electrode. And

  As a result of intensive studies in view of the above problems, it has been found that the conversion efficiency of a heterojunction solar cell is improved by forming an ITO transparent electrode layer having a carrier density and a surface free energy within a predetermined range.

  The present invention has a p-type silicon-based layer and a first transparent electrode layer in this order on one surface of a conductive single crystal silicon substrate, and an n-type silicon-based layer on the other surface of the conductive single crystal silicon substrate. And a method for manufacturing a crystalline silicon solar cell having a second transparent electrode layer in this order. The p-type silicon-based layer is in direct contact with the first transparent electrode layer, and the n-type silicon-based layer is in direct contact with the second transparent electrode layer. It is preferable to further have a collector electrode on each of the first transparent electrode layer and the second transparent electrode layer.

Each of the first transparent electrode layer and the second transparent electrode layer is made of indium tin composite oxide (ITO) in which the content of tin oxide is 7 wt% to 12 wt% with respect to the total of indium oxide and tin oxide. It is preferable to become. The first transparent electrode layer and the second transparent electrode layer both preferably have a carrier density of 1 × 10 ²⁰ cm ⁻³ to 4 × 10 ²⁰ cm ⁻³ , and the surface free energy is It is preferable that it is 80 mN / m or more.

Both the first transparent electrode layer and the second transparent electrode layer are preferably made of an amorphous indium tin composite oxide. In addition, the hole mobility of the first transparent electrode layer and the second transparent electrode layer is preferably 50 cm ² / Vs or more.

  The first and second transparent electrode layers are preferably formed by a magnetron sputtering method with a substrate temperature of 100 ° C. or lower.

  In one embodiment, the n-type silicon-based layer includes an n-type amorphous silicon-based thin film and an n-type microcrystalline silicon-based thin film in this order from the conductive single crystal silicon substrate side.

  In the present invention, the carrier density of the transparent electrode layer is set within a predetermined range, whereby the light absorption loss due to the transparent electrode layer is reduced, and the short-circuit current density of the crystalline silicon-based solar cell is improved. In addition, since the transparent electrode layer has a predetermined surface free energy, the adhesion between the transparent electrode layer and the collector electrode is improved, and the bondability with the silicon thin film is improved even at a low carrier density. Therefore, according to the present invention, a crystalline silicon solar cell with high conversion efficiency is provided. Further, by using an amorphous film as the transparent electrode layer, the warpage of the substrate is improved, the characteristic deterioration is suppressed, and the productivity of the solar cell is increased.

It is a typical sectional view of a crystalline silicon system solar cell concerning one embodiment of the present invention.

  FIG. 1 is a schematic cross-sectional view of a crystalline silicon solar cell according to an embodiment of the present invention. The crystalline silicon solar cell includes a first type between the conductive single crystal silicon substrate 1 and the p-type silicon layer 41 and between the conductive single crystal silicon substrate 1 and the n-type silicon layer 42. It is preferable to have an intrinsic silicon-based layer 21 and a second intrinsic silicon-based layer 22. In general, collector electrodes 71 and 72 are formed on the transparent electrode layers 61 and 62. It is preferable that a protective layer (not shown) is further formed on the collector electrode.

  The conductive single crystal silicon substrate includes an n-type single crystal silicon substrate containing an impurity that introduces electrons into Si atoms (for example, phosphorus atoms) and an impurity that introduces holes into Si atoms (for example, And a p-type single crystal silicon substrate having a boron atom). In this specification, “conductivity type” means either n-type or p-type. A texture (uneven structure) is preferably formed on the surface of the single crystal silicon substrate from the viewpoint of light confinement. The texture is formed, for example, by anisotropic etching applying the different etching rates of the (100) plane and the (111) plane of the crystalline silicon substrate.

  In a crystalline silicon solar cell using a conductive single crystal silicon substrate, it is preferable that the heterojunction on the light incident side where the light incident on the single crystal silicon substrate 1 is absorbed most is a reverse junction. If the heterojunction on the light incident side is a reverse junction, a strong electric field is provided, and electron / hole pairs can be efficiently separated and recovered. When holes and electrons are compared, in general, electrons have higher mobility. Therefore, the conductive single crystal silicon substrate 1 is preferably an n-type single crystal silicon substrate.

  In one embodiment, the thickness of the conductive single crystal silicon substrate is preferably 250 μm or less. By reducing the thickness of the silicon substrate, the amount of silicon used is reduced, so that the cost can be reduced and the silicon substrate can be easily secured. On the other hand, if the thickness of the silicon substrate is excessively small, the mechanical strength may decrease, or external light (sunlight) may not be sufficiently absorbed, resulting in a decrease in short-circuit current density. Therefore, the thickness of the conductive single crystal silicon substrate 1 is preferably 50 μm or more, and more preferably 70 μm or more. When a texture is formed on the surface of the silicon substrate, the thickness of the silicon substrate is represented by a distance between straight lines connecting the vertices of the convex portions of the concavo-convex structure on the light incident side and the back surface side.

  The crystalline silicon solar cell of the present invention has a p-type silicon-based layer 41 and a transparent electrode layer 61 in this order on one surface of the single crystal silicon substrate 1, and n on the other surface of the single crystal silicon substrate 1. A silicon-based layer 42 and a transparent electrode layer 62 are provided in this order. From the viewpoint of effectively performing passivation of the surface of the single crystal silicon while suppressing the diffusion of impurities into the single crystal silicon substrate 1, and between the single crystal silicon substrate 1 and the p-type silicon-based layer 41, and the single crystal silicon substrate 1 The first intrinsic silicon-based layer 41 and the second intrinsic silicon-based layer 42 are preferably provided between the n-type silicon-based layer 42 and the n-type silicon-based layer 42. Note that in this specification, the term “intrinsic” layer is not limited to a completely intrinsic layer that does not include a conductive impurity, and a small amount of n-type impurity or the like within a range in which a silicon-based layer can function as an intrinsic layer. It also includes “weak n-type” or “weak p-type” substantially intrinsic layers containing p-type impurities.

  Intrinsic silicon layers 21 and 22 are preferably amorphous silicon thin films, and more preferably hydrogenated amorphous silicon thin films composed of silicon and hydrogen. Since intrinsic hydrogenated amorphous silicon is deposited on the single crystal silicon substrate 1 by CVD, the substrate surface can be effectively passivated while suppressing the diffusion of impurities into the single crystal silicon substrate. Further, by changing the hydrogen concentration in the intrinsic silicon-based layers 21 and 22 in the film thickness direction, an energy gap profile effective for carrier recovery can be formed.

  The film thickness of the intrinsic silicon-based layers 21 and 22 is preferably in the range of 2 nm to 8 nm. By setting the thickness of the intrinsic silicon-based layer to 2 nm or more, the effect as a passivation layer can be further expected. In addition, when the film thickness is 8 nm or less, it is possible to further suppress deterioration in conversion characteristics that may occur due to high resistance.

  Examples of the material of the p-type silicon-based layer 41 include amorphous silicon, oxidized amorphous silicon, and amorphous silicon carbide. Since silicon oxide or silicon carbide is a wide-gap low refractive index material, it has the advantage that loss due to reflection and absorption of incident light can be reduced, while contact with the intrinsic silicon-based layer 21 and the transparent electrode layer 61. May be low. As described in detail later, in the present invention, a transparent electrode layer 61 having a low carrier density is formed on the p-type silicon-based layer 41. From the viewpoint of increasing the contact with such a low carrier density transparent electrode layer and improving the conversion efficiency, the material of the p-type silicon-based layer 41 is particularly preferably amorphous silicon.

  The film thickness of the p-type silicon-based layer 41 is preferably in the range of 5 nm to 50 nm. In the heterojunction solar cell, it is particularly preferable to reduce the film thickness of the conductive layer disposed on the light incident side. For example, when the p layer side (first transparent electrode layer 61 side) is a light incident surface, the thickness of the p-type silicon-based layer 41 is more preferably 15 nm or less, further preferably 10 nm or less, and particularly preferably 8 nm or less. .

  The n-type silicon-based layer 42 may be composed of a single layer of an n-type amorphous silicon-based thin film or an n-type microcrystalline silicon-based thin film, or may be composed of a plurality of thin films. In particular, the n-type silicon-based layer 42 is preferably composed of two layers, an n-type amorphous silicon-based thin film 421 and an n-type microcrystalline silicon-based thin film 422, as shown in FIG. Since n-type microcrystalline silicon can form a good ohmic junction at the interface with the transparent electrode layer formed thereon, it can contribute to the improvement of the conversion characteristics (particularly the curve factor) of the solar cell. On the other hand, in order to form a microcrystalline silicon-based thin film, it is generally necessary to supply a high power and generate a high-density hydrogen plasma. In contrast, after an n-type amorphous silicon-based thin film 421 is formed on the intrinsic silicon-based layer 22 with a film thickness of about 5 nm to 20 nm, an n-type microcrystalline silicon-based thin film 422 is formed thereon. In this case, the power required for forming the n-type microcrystalline silicon layer can be reduced. Therefore, when the n-type silicon-based layer 42 is composed of two layers of an n-type amorphous silicon-based thin film 421 and an n-type microcrystalline silicon-based thin film 422, the n-type silicon-based layer 42 and the transparent electrode layer 62 are The contact property is enhanced, and the diffusion of doped impurities into the intrinsic silicon-based layer 22 and the silicon substrate 1 and film-forming damage are reduced.

  As a material of the n-type amorphous silicon-based thin film 421, amorphous silicon or amorphous silicon nitride is preferable because good bonding characteristics with an adjacent layer can be easily obtained. Examples of the material of the n-type microcrystalline silicon thin film 422 include microcrystalline silicon, microcrystalline silicon carbide, and microcrystalline silicon oxide. From the viewpoint of suppressing generation of defects inside the n layer, an n-type microcrystalline silicon thin film to which impurities other than doped impurities are not positively added is preferably used. On the other hand, by using n-type microcrystalline silicon carbide or n-type microcrystalline silicon oxide as the n-type microcrystalline silicon-based thin film 422, optical merits due to a wide gap and low refraction can be obtained.

  The film thickness of the n-type silicon-based layer 42 is preferably in the range of 5 nm to 50 nm. When the n-type silicon-based layer 42 is composed of two layers of an n-type amorphous silicon-based thin film 421 and an n-type microcrystalline silicon-based thin film 422, the thickness of the n-type amorphous silicon-based thin film 421 is 5 nm or more. Preferably, 10 nm or more is more preferable. By setting the film thickness of the n-type amorphous silicon-based thin film 421 in the above range, the power density when the n-type microcrystalline silicon-based thin film 422 is formed thereon can be kept low. The film thickness of the n-type microcrystalline silicon thin film 422 is preferably 5 nm or more, and more preferably 10 nm or more. By setting the film thickness of the n-type microcrystalline silicon thin film 422 within the above range, the contact property with the transparent electrode layer 62 formed thereon can be improved. On the other hand, the thickness of the n-type amorphous silicon-based thin film 421 is preferably 20 nm or less, and more preferably 15 nm or less, from the viewpoint of suppressing light absorption loss due to doped impurities in the n-type silicon-based layer. The thickness of the n-type microcrystalline silicon thin film 422 is preferably 30 nm or less, and more preferably 20 nm or less.

As a method for forming a silicon-based layer on the single crystal silicon substrate 1, a plasma CVD method is preferable. As conditions for forming a silicon-based layer by plasma CVD, for example, a substrate temperature of 100 to 300 ° C., a pressure of 20 to 2600 Pa, and a high frequency power density of 0.003 to 0.5 W / cm ² are preferably used. As the source gas used for forming the silicon-based layer, a silicon-containing gas such as SiH ₄ or Si ₂ H ₆ or a mixture of these gases and H ₂ is preferably used. As the dopant gas for forming the p-type or n-type silicon-based layer, for example, B ₂ H ₆ or PH ₃ is preferably used. In this case, since the addition amount of impurities such as P and B may be small, a mixed gas diluted with SiH ₄ or H ₂ or the like in advance can also be used. In addition, by adding a gas containing a different element such as CH ₄ , CO ₂ , NH ₃ , GeH ₄ to the above gas to form a silicon alloy such as silicon carbide, silicon nitride, silicon germanium, etc., the energy gap is increased. It can also be changed.

  A first transparent electrode layer 61 and a second transparent electrode layer 62 are formed on the p-type silicon-based layer 41 and the n-type silicon-based layer 42, respectively. The film thickness of the first and second transparent electrode layers 61 and 62 is preferably 40 nm to 120 nm, more preferably 50 nm to 110 nm, and still more preferably 60 nm to 110 nm, from the viewpoints of transparency and conductivity. The transparent electrode layers 61 and 62 only need to have conductivity necessary for transporting carriers to the collector electrode. On the other hand, by setting the film thickness within the above range, it is possible to further suppress the decrease in transmittance due to light absorption of the transparent electrode layer itself and the accompanying decrease in conversion efficiency (particularly, short-circuit current density) of the solar cell.

  The first transparent electrode layer 61 and the second transparent electrode layer 62 are preferably in direct contact with the p-type silicon-based layer 41 and the n-type silicon-based layer 42, respectively. As will be described later, when the transparent electrode layers 61 and 62 have a predetermined surface free energy, the conductive silicon-based thin films 41 and 42 and the transparent electrode layers 61 and 62 are in contact with each other, so that good interface bonding is achieved. Can be formed.

  As a material of the transparent electrode layers 61 and 62, indium tin oxide (ITO) is particularly preferably used. ITO has a high electrical conductivity and transparency, and is a preferable material from the viewpoint of improving the conversion efficiency by reducing the light absorption loss and the resistance loss by the transparent electrode layer.

  The content of tin oxide in the ITO transparent electrode layers 61 and 62 is preferably 7% by weight to 12% by weight and more preferably 8% by weight to 11% by weight with respect to the total of indium oxide and tin oxide. By setting the content of tin oxide to 7% by weight or more, there are advantages that high conductivity is imparted to the transparent electrode layer and an amorphous film is easily obtained. Moreover, by setting the tin oxide content to 12% by weight or less, an ITO transparent electrode layer having a high transmittance can be easily obtained. Furthermore, in this invention, ITO which has the desired surface free energy can be formed by making content of a tin oxide into the said range.

The carrier density of the transparent electrode layers 61 and 62 is preferably in the range of 1 × 10 ²⁰ cm ⁻³ to 4 × 10 ²⁰ cm ⁻³ . By setting the carrier density to 4 × 10 ²⁰ cm ⁻³ or less, a transparent electrode layer having high transparency in a wide wavelength range can be obtained, and the short-circuit current density can be improved by reducing light absorption loss. Further, by setting the carrier density to 1 × 10 ²⁰ cm ⁻³ or more, the resistance of the transparent electrode layer can be reduced, and the conversion efficiency (particularly the curve factor) can be improved. The carrier density of the transparent electrode layers 61 and 62 is more preferably in the range of 1.5 × 10 ²⁰ cm ^{−3 to} 3.5 × 10 ²⁰ cm ⁻³ , and 2 × 10 ²⁰ cm ^{−3 to} 3 × 10 ²⁰ cm ^−. A range of ³ is more preferred.

The transparent electrode layers 61 and 62 preferably have a carrier density in the above range over the entire thickness direction. If there is a region where the carrier density is locally high or low in the film thickness direction, the light absorption loss or resistance loss in that region may become a bottleneck, and the conversion characteristics may deteriorate. Therefore, the thickness direction of the distribution of the carrier density of the transparent electrode layer 61, 62, 3 × preferably ¹⁰ 20 ^{cm -3} or less, more preferably 2 × ¹⁰ 20 ^{cm -3} or less, 1 × ¹⁰ 20 ^{cm -3} or less Is more preferably 7 × 10 ¹⁹ cm ⁻³ or less, and most preferably 5 × 10 ¹⁹ cm ⁻³ or less. The carrier density distribution of the transparent electrode layer can be obtained by fitting a dielectric function in the near-infrared region obtained by optical measurement such as spectroscopic ellipsometry using the Drude model. That is, the fitting by the Drude model can obtain the carrier relaxation time and the film thickness direction profile of the resistivity distribution, and the carrier density can be calculated from the result. Regarding the distribution of the carrier density in the film thickness direction, when the profile in the film thickness direction is averaged for each 5 nm film thickness region, the difference between the maximum value and the minimum value in the film thickness direction of the average value is within the above range. Is preferred.

Hall mobility of the transparent electrode layers 61 and 62 is preferably at least 50 cm ² / Vs, more preferably at least 55cm ² / Vs, more preferably at least 60cm ² / Vs. When the hole mobility is in the above range, the resistance of the transparent electrode layer can be reduced even if the carrier density is 4 × 10 ²⁰ cm ⁻³ or less.

The surface free energy of the transparent electrode layers 61 and 62 is preferably 80 mN / m or more. If the surface free energy is 80 mN / m or more, the wettability with respect to the conductive paste or plating solution is improved when the collecting electrodes 71 and 72 are formed on the surfaces of the transparent electrode layers 61 and 62 by a printing method or a plating method. In addition, the adhesion between the transparent electrode layer and the collector electrode can be improved. Moreover, if the surface free energy is 80 mN / m or more, a solar cell excellent in curve factor and open end voltage can be obtained even when the carrier density of the transparent electrode layer is 4 × 10 ²⁰ cm ⁻³ or less. This is presumed to be because electrical contactability between the conductive silicon-based layers 41 and 42 and the transparent electrode layers 61 and 62 is enhanced by increasing the surface free energy. According to the present invention, even when the ITO thin film having a high carrier density as described in Patent Document 2 is not provided, a solar cell with improved curve factor and open-end voltage can be obtained. In addition, since each of the transparent electrode layers 61 and 62 is made of a single ITO thin film, it is not necessary to change film forming conditions such as a gas flow rate during film formation, and productivity is improved. Furthermore, since an ITO layer having a high carrier density is not required, the light absorption loss in the transparent electrode layer is reduced as described above, and the short-circuit current density of the solar cell is increased.

  The upper limit of the surface free energy of the transparent electrode layers 61 and 62 is not particularly limited. When the surface free energy increases, the adhesion between the transparent electrode layer and the collector electrode tends to be improved. On the other hand, when the surface free energy is too large, the fill factor and open circuit voltage of the solar cell may decrease. Therefore, the surface free energy of the transparent electrode layers 61 and 62 is preferably 130 mN / m or less, more preferably 120 mN / m or less, and even more preferably 110 mN / m or less.

  The method for forming the first transparent electrode layer 61 and the second transparent electrode layer 62 is not particularly limited, but the magnetron sputtering method is preferable from the viewpoints of productivity improvement and film thickness control. The substrate temperature during film formation is preferably 100 ° C. or lower, more preferably 70 ° C. or lower, and further preferably 50 ° C. or lower. By lowering the substrate temperature during sputtering film formation, an ITO film having a low carrier density and a large surface free energy can be easily obtained. Further, as the substrate temperature decreases, an amorphous ITO film tends to be easily formed.

  In film formation, a mixed gas of an inert gas such as argon and oxygen is preferably introduced into the film formation chamber. The amount of oxygen introduced is preferably 0.5% by volume to 10% by volume, more preferably 0.8% by volume to 5% by volume, and still more preferably 1% by volume to 4% by volume with respect to the total amount of gas introduced. When the amount of oxygen introduced is increased within the above range, the carrier density of the transparent electrode layer tends to decrease and the surface free energy tends to increase. On the other hand, if the amount of oxygen introduced is too large, the surface free energy tends to decrease.

  When not only the relative oxygen gas flow ratio during film formation but also the total gas introduction amount is changed, the carrier density and surface free energy of the transparent electrode layer tend to change, and the total gas introduction amount becomes small. , Surface free energy tends to be increased. This is presumed to be because the sputtering reaction rate is lowered due to the reduction of the gas introduction amount, and the amount of oxygen taken into the film is reduced. That is, when the amount of oxygen taken into the ITO film decreases, oxygen deficiency in the film increases, and this oxygen deficiency is considered to be a driving force that increases surface free energy (wetting).

  The optimum value of the amount of gas introduced to make the surface free energy of the ITO transparent electrode layer within the above-mentioned range depends on other factors such as the capacity of the film forming apparatus, the substrate temperature, and the power density. Although not possible, it is preferable to reduce the amount of carrier gas introduced relative to the exhaust capacity of the film forming system (film forming chamber). For example, it is preferable to adjust the introduction amount of the inert gas so that the total gas introduction amount into the film forming chamber is not more than 5 times the minimum dischargeable gas flow rate. Here, the “minimum dischargeable gas flow rate” is a value obtained when the film forming chamber and the exhaust means (for example, a vacuum pump) are connected without an exhaust resistance (for example, when the exhaust pressure regulating valve is fully opened). This is the flow rate of the introduced gas required to maintain the minimum pressure for causing sputter discharge in the film system.

The pressure in the film forming chamber when forming the ITO transparent electrode layers 61 and 62 is preferably 0.1 Pa to 0.5 Pa. The oxygen partial ^{pressure, 1 × 10 -3 Pa~2 × 10} -2 Pa is preferred, more preferably ^{2 × 10 -3 Pa~1 × 10 -2} Pa, 4 × 10 -3 Pa~8 × 10 ^-3 Pa is more preferable. Power density is preferably ^{0.2W / cm 2 ~1.2W / cm 2} . In order to obtain an amorphous ITO film, it is preferable that the film is formed at a low power density of 1.0 W / cm ² or less. Further, by reducing the power density at the time of forming the transparent electrode layers 61 and 62, damage to the silicon thin film or single crystal silicon substrate which is the base at the time of film formation is reduced, and the open end voltage and the fill factor of the solar cell are reduced. There is a tendency that the decrease of the is suppressed. Also, an amorphous ITO film tends to be easily obtained by increasing the film forming pressure.

  The first transparent electrode layer 61 and the second transparent electrode layer 62 are more preferably amorphous films. The “amorphous film” refers to a film in which no crystal-derived peak is observed in X-ray diffraction. In the case of an ITO film, the (220) plane, (222) plane, (400) plane, (440) is determined by X-ray diffraction. The film having no diffraction peak on the surface is an amorphous film. Note that even if the crystal grains can be observed by high-resolution observation such as TEM, those having a small crystallite size and no X-ray crystal diffraction peak are included in the amorphous film.

  In general, when the thickness of the single crystal silicon substrate is as small as, for example, 250 μm or less, the solar cell is warped after the transparent electrode layer is formed, and conversion characteristics (particularly, open-circuit voltage) tend to decrease. In addition, when fixing the cell on the processing table by the adsorption method in the process of forming the collecting electrode and measuring the conversion efficiency after forming the transparent electrode layer, if the amount of warpage of the cell is large, an adsorption failure occurs and the cell cracks. In addition, the production efficiency tends to decrease due to problems such as foreign matter sucking into the gap.

On the other hand, if the transparent electrode layer is an amorphous film, cell warpage is suppressed and high conversion efficiency is easily maintained. This is presumed to be related to the fact that the amorphous ITO film has less residual stress than the crystalline ITO film or does not have the residual stress, so that the stress difference between the front and back of the substrate is less likely to occur. Is done. In particular, in a heterojunction solar cell, transparent electrode layers having different film thicknesses may be formed on the light incident side and the back surface side. For example, have the function of anti-reflection layer to increase the light uptake into the solar cell, the film thickness d ₁ of the light incident side transparent electrode layer is often set to about 100 nm. On the other hand, the transparent electrode layer on the back surface side is mainly to increase the extraction efficiency of electricity in terms of the electrical design, such as resistance, often a thickness d ₂ is determined. Thus, when the thickness of the transparent electrode layer differs between the front and back surfaces of the substrate, the substrate is likely to warp if the transparent electrode layer is crystalline, but the first transparent electrode layer 61 and the second transparent electrode layer If both 62 are amorphous ITO films, the warpage of the substrate tends to be suppressed.

  The collector electrodes 71 and 72 are preferably formed on the first transparent electrode layer 61 and the second transparent electrode layer 62, respectively. The collector electrode can be formed by a printing method such as inkjet printing or screen printing, a plating method, or the like. From the viewpoint of productivity, the collector electrode is preferably formed by screen printing. In screen printing, for example, a conductive paste composed of metal particles and a resin binder is printed by screen printing.

  After the collector electrode is formed, the cell may be annealed also for solidifying the conductive paste used for the collector electrode. By annealing, improvement of each interface characteristic such as improvement of the transmittance / resistivity ratio of the transparent electrode layer and reduction of contact resistance and interface state can be obtained.

  The crystalline silicon solar cell of the present invention is preferably modularized for practical use. The modularization of the solar cell is performed by an appropriate method. For example, a bus bar is connected to a collector electrode via an interconnector such as a tab, so that a plurality of solar cells are connected in series or in parallel, and sealed with a sealant and a glass plate to be modularized. Done.

  EXAMPLES Hereinafter, although an Example demonstrates this invention concretely, this invention is not limited to a following example.

[Evaluation method]
(Film thickness)
The film thickness of the transparent electrode was obtained by observing at a magnification of 100,000 times using SEM (Field Emission Scanning Electron Microscope S4800, manufactured by Hitachi High-Technologies Corporation).

(Carrier density)
An ITO film was formed on alkali-free glass (trade name “OA-10”, manufactured by Nippon Electric Glass Co., Ltd.) under the same film forming conditions as those of the transparent electrode layers of the examples and comparative examples. The sample was folded into a 1 cm square, and metal indium was fused to the four corners as electrodes to prepare a sample for hole measurement. The carrier mobility was calculated by measuring the hole mobility by the van der pauw method based on the potential difference when a current of 1 mA was passed in the diagonal direction of the substrate with a magnetic force of 3500 gauss.

(Surface free energy)
Water, diethylene glycol, and methylene iodide were dropped on the transparent electrode layer, and the surface free energy of the transparent electrode layer was calculated by the Owens-Wendt method from the contact angle θ between each droplet and the transparent electrode layer surface. Specifically, the surface free energy γ _s of the solid (transparent electrode layer) is determined by substituting the contact angle θ of each droplet into the following equations (1) and (2) and solving the simultaneous equations. did.

式（１），（２）において、γは表面自由エネルギーであり、Ｌは液体、Ｓは固体を表す記号であり、ｄおよびｈは、それぞれ表面自由エネルギーの分散成分と水素結合成分を表す記号である。

In equations (1) and (2), γ is a surface free energy, L is a liquid, S is a symbol representing a solid, and d and h are symbols representing a surface free energy dispersion component and a hydrogen bond component, respectively. It is.

(Crystallinity of transparent electrode)
The crystallinity of the transparent electrode layer was evaluated by identifying the presence or absence of a peak by an X-ray diffraction method using a sample in which an ITO film was formed on the same alkali-free glass as the hole measurement sample. X-ray diffraction measurement was performed by the 2θ / θ method, and the 2θ measurement range was 20 to 80 °.

(Adhesion of Ag collector electrode)
A silver paste (Dotite FA-333 manufactured by Fujikura Kasei Co., Ltd.) is formed on the p-type side transparent electrode layer of each example and comparative example by screen printing in the range of 30 mm square, and then dried to obtain a silver having a thickness of 30 μm. The layer was formed and the sample for adhesion strength evaluation was produced. With a cutter knife, the silver layer was cut into a 2 mm wide grid, a test tape (550P manufactured by Sumitomo 3M) was attached, and the silver layer was peeled off in a vertical direction.
The case where the adhesion rate of the silver layer to the peeled tape pressure-sensitive adhesive surface was less than 40% was rated as “〇”, the case where it was 40% or more and less than 60% was Δ, the case where it was 60% or more was rated as “X”.

(Photoelectric conversion characteristics)
The light of AM1.5 was irradiated with the light quantity of 100 mW / cm < ² > with the solar simulator, and the open end voltage (Voc), the short circuit current density (Jsc), the fill factor (FF), and the conversion efficiency (Eff) were measured.

[Example 1]

In Example 1, a heterojunction solar cell schematically shown in FIG. 1 was produced.
As the silicon substrate, an n-type single crystal silicon substrate having a thickness of 200 μm and having a pyramidal texture with an exposed (111) plane formed on the surface by anisotropic etching was used.

This single crystal silicon substrate was introduced into a CVD apparatus, and an intrinsic amorphous silicon layer having a thickness of 3 nm was formed on the light incident surface. The film forming conditions were a substrate temperature of 150 ° C., a pressure of 120 Pa, a SiH ₄ / H ₂ flow rate ratio of 3/10, and a power density of 0.011 W / cm ² .

A p-type amorphous silicon layer having a thickness of 4 nm was formed on this intrinsic amorphous silicon layer. The film forming conditions for the p-type amorphous silicon layer were a substrate temperature of 150 ° C., a pressure of 60 Pa, a SiH ₄ / B ₂ H ₆ flow rate ratio of 1/3, and a power density of 0.01 W / cm ² . As the B ₂ H ₆ gas, a gas diluted with H ₂ to a B ₂ H ₆ concentration of 5000 ppm was used.

An intrinsic amorphous silicon layer having a thickness of 6 nm was formed on the back side of the single crystal silicon substrate under the same conditions. An n-type amorphous silicon thin film was formed thereon with a thickness of 10 nm. The deposition conditions for the n-type amorphous silicon thin film were a substrate temperature of 150 ° C., a pressure of 60 Pa, a SiH ₄ / dilution PH ₃ flow rate ratio of 1/2, and a high frequency power density of 0.011 W / cm ² . As the diluted PH ₃ gas, a gas diluted with H ₂ to a PH ₃ concentration of 5000 ppm was used.

On the p-type amorphous silicon layer, ITO was formed as a first transparent electrode layer 61 with a film thickness of 75 nm. ITO having a tin oxide content of 10% by weight is used as a target, and argon and oxygen are introduced as carrier gases at flow rates of 25 sccm and 0.3 sccm, respectively, a substrate temperature of 25 ° C., a pressure of 0.2 Pa, a power density of 0. Film formation was performed under the condition of 7 W / cm ² .

  A second transparent electrode layer made of ITO having a thickness of 80 nm was also formed on the n-type amorphous silicon layer by sputtering under the same film forming conditions as those of the first transparent electrode layer.

  On each of the first transparent electrode layer and the second transparent electrode layer, silver paste (Dotite FA-333 manufactured by Fujikura Kasei Co., Ltd.) was screen-printed as a collecting electrode to form a comb-shaped electrode. The interval between the collector electrodes was 10 mm. After the collector electrode was formed, an annealing treatment was performed at 150 ° C. for 1 hour.

[Examples 2 to 4, Comparative Examples 1 to 7]
In the formation of the first transparent electrode layer and the second transparent electrode layer in Example 1, the film forming conditions (tin oxide content, substrate temperature, pressure, power density, gas introduction amount in the target) are shown in Table 1. Was changed to Other than that was carried out similarly to Example 1, and produced the heterojunction solar cell.

[Example 5]
In Example 5, the first transparent electrode layer and the second transparent electrode layer were formed under the same conditions as in Example 2. However, the intrinsic amorphous silicon layer on the back side of the single crystal silicon substrate and the second transparent electrode layer were formed. The second embodiment was different from the second embodiment in that an n layer composed of an n-type amorphous silicon thin film and an n-type microcrystalline silicon thin film was formed between the transparent electrode layer. In Example 5, an n-type amorphous silicon thin film having a thickness of 10 nm is formed on the intrinsic amorphous silicon layer in the same manner as in Example 2, and a microcrystalline silicon thin film having a thickness of 20 nm is formed thereon. It was. The deposition conditions for the n-type microcrystalline silicon thin film were as follows: the substrate temperature was 170 ° C., the pressure was 800 Pa, the SiH ₄ / PH ₃ / H ₂ flow rate ratio was 1/5/180, and the input power density was 0.08 W / cm ² . .

[Comparative Example 8]
In Comparative Example 8, the first transparent electrode layer has a two-layer configuration. First, ITO having a tin oxide content of 10% by weight is used as a target, and argon and oxygen are introduced as carrier gases at flow rates of 50 sccm and 0.2 sccm, respectively, a substrate temperature of 150 ° C., a pressure of 0.2 Pa, and power An ITO layer A having a thickness of 10 nm was formed under conditions of a density of 0.7 W / cm ² . Thereafter, the oxygen flow rate was changed to 0.8 sccm, and an ITO layer B having a film thickness of 65 nm was formed.

  Table 1 shows the film forming conditions, film characteristics (carrier density, hole mobility, surface free energy, crystallinity, and adhesion to the silver layer), and solar cell conversion characteristics in each Example and Comparative Example. Shown in

  According to the results of the above Examples and Comparative Examples, the solar cells of Examples 1 to 5 whose surface free energy of the transparent electrode layer is 80 mN / m or more are all excellent in adhesion with the silver electrode, and Conversion efficiency higher than each comparative example was shown. In particular, in each example, the fill factor and the open end voltage are improved as compared with the comparative example. This is considered to be due to the improved interface bonding state between the conductive silicon layer and the transparent electrode layer and the adhesion with the silver electrode.

  When Examples 1-4 and Comparative Example 1 are contrasted, the carrier density tends to decrease as the oxygen flow rate increases. This is considered to be due to a decrease in oxygen deficiency in ITO as the oxygen flow rate increases. Further, as the carrier density decreases, the light absorption loss due to the transparent electrode layer decreases, and thus the short-circuit current density tends to increase.

  On the other hand, the surface free energy tended to be different from the carrier density. In the range where the oxygen flow rate is up to 0.7 sccm (Examples 1 to 3), the surface free energy tends to decrease as the oxygen flow rate increases, but when the oxygen flow rate is 1.0 sccm (Example 4), When the surface free energy increased and the oxygen flow rate was further increased to 2.0 sccm (Comparative Example 1), the surface free energy decreased again.

  In Comparative Example 2 in which the amount of argon introduced was increased to 50 sccm, the amount of oxygen introduced was an intermediate value between Example 3 and Example 4, and the flow rate ratio of argon and oxygen was an intermediate value between Example 1 and Example 2. However, as compared with Examples 1 to 4, the carrier density increased and the surface free energy decreased. It is presumed that this is related to the fact that oxygen is easily taken into the ITO film and the oxygen vacancies are reduced as the amount of supply gas is increased.

  In Comparative Example 4 in which an ITO transparent electrode layer having a tin oxide content of 5% by weight was formed, the other film forming conditions were the same as in Example 3, but the film was crystallized. In comparison, the carrier density increased and the surface free energy decreased. On the other hand, in Comparative Example 5, an amorphous ITO film was obtained even at a high temperature of 150 ° C. by reducing the film forming power density. However, the surface free energy was small, and the open end voltage and the fill factor of the solar cell were low. Decrease was observed.

  Further, in Comparative Example 6 in which the transparent electrode layer was formed under the same conditions as in Example 2 except that film formation was performed at 150 ° C., an increase in carrier density and a decrease in surface free energy occurred, Conversion characteristics were degraded. In Comparative Example 7, by increasing the amount of introduced gas, the carrier density is reduced and the short-circuit current density is improved as compared with Comparative Example 6, but the open-circuit voltage and the fill factor are low, and each example has The conversion efficiency is inferior.

  From the above results, it is possible to adjust the surface free energy within a predetermined range by adjusting the tin oxide content in the transparent electrode layer and the film forming conditions (particularly the supply gas amount), and the curve factor and open-end voltage are improved. It can be seen that the obtained heterojunction solar cell is obtained. By reducing the amount of gas supplied and forming the film at a low temperature such as room temperature, the effect of reducing the kinetic energy of the sputtered particles and preventing the oxidation of the surface of the film is obtained. It is presumed that the interface state with the layer is improved and the adhesion to the collector electrode is enhanced, and these factors also contribute to the improvement of the fill factor and the open-circuit voltage.

In Comparative Example 8, the fill factor is improved and the conversion efficiency is improved as compared with Comparative Examples 1-7. This is because in Comparative Example 8, in forming the transparent electrode layer, an ITO film A having a low oxygen flow rate and a high carrier density was formed, and an ITO film B having a high oxygen flow rate and a low carrier density was formed thereon. This is considered to be due to the improved interface bonding between the transparent electrode layer and the conductive silicon layer, as compared with Comparative Example 7 in which only one ITO film having a high oxygen flow rate and a low carrier density was formed. On the other hand, in Comparative Example 8, although the carrier density of the entire film was less than 4 × 10 ²⁰ cm ⁻³ , the short-circuit current density was reduced as compared with Examples 1 to 4. This is presumably because the light absorption by the ITO film A having a high carrier density is large and the light absorption loss by the transparent electrode layer is increased. Further, in Comparative Example 8, since the surface free energy of the transparent electrode layer was small, the open-circuit voltage and the fill factor were reduced as compared with Examples 1 to 3.

  From this result, although the transparent electrode layer composed of a two-layer structure of an ITO film with a high carrier density and an ITO film with a low carrier density is useful in improving the interface bonding with the conductive silicon layer, the light in the transparent electrode layer Considering overall reduction in absorption loss, adhesion to the collector electrode, and the like, the transparent electrode layer made of a single ITO film having a predetermined surface free energy is excellent in improving the conversion efficiency as in the embodiment. It can be said.

  On the other hand, in Example 5 in which an n layer composed of two layers of an n-type amorphous silicon thin film and an n-type microcrystalline silicon thin film was formed from the intrinsic amorphous silicon layer side (silicon substrate side), the transparent electrode layer was Compared to Example 2 formed under the same conditions, the fill factor was improved and the conversion efficiency was also improved. In Example 5, as compared with Example 2, a decrease in short-circuit current density due to an increase in the total film thickness of n layers and a decrease in open-circuit voltage estimated to be caused by defects in the n-type microcrystalline silicon thin film are observed. However, these reduction amounts are slight, and it is considered that the conversion efficiency is improved because the improvement of the fill factor due to the improvement of the interfacial junction more than compensates for the decrease of the short circuit current density and the open circuit voltage.

1: Single crystal silicon substrate 21, 22: Intrinsic silicon thin film 41: p-type silicon thin film 42: n-type silicon thin film 421: n-type amorphous silicon thin film 422: n-type microcrystalline silicon thin film 61, 62 : Transparent electrode layer 71, 72: Collector electrode

Claims (7)

A p-type silicon-based layer and a first transparent electrode layer in contact with the p-type silicon-based layer are provided on one surface of the conductive single crystal silicon substrate, and an n-type is formed on the other surface of the conductive single crystal silicon substrate. A method for producing a crystalline silicon-based solar cell having a silicon-based layer and a second transparent electrode layer in contact with the n-type silicon-based layer,
The first transparent electrode layer and the second transparent electrode layer are both
It is formed by a magnetron sputtering method with a substrate temperature of 100 ° C. or lower,
The carrier density is 1 × 10 ²⁰ cm ⁻³ to 4 × 10 ²⁰ cm ^−3, which is made of an indium tin composite oxide having a tin oxide content of 7 wt% to 12 wt% with respect to the total of indium oxide and tin oxide. The surface free energy is 80 mN / m or more,
The oxygen partial pressure in the film ^- forming chamber in the sputter deposition of the first transparent electrode layer and the sputter deposition of the second transparent electrode layer is both 1 × 10 ⁻³ Pa to ² × 10 ⁻² Pa. A method for producing a crystalline silicon solar cell.   2. The method for producing a crystalline silicon solar cell according to claim 1, wherein water is not introduced into the deposition chamber in the sputtering deposition of the first transparent electrode layer and the sputtering deposition of the second transparent electrode layer.   2. The production of a crystalline silicon solar cell according to claim 1, wherein only argon and oxygen are introduced into the film forming chamber in the sputter film formation of the first transparent electrode layer and the sputter film formation of the second transparent electrode layer. Method.   The crystalline silicon according to any one of claims 1 to 3, wherein the first transparent electrode layer and the second transparent electrode layer have a surface free energy immediately after sputtering film formation of 80 mN / m or more. Of the solar cell solar cell.   The crystalline silicon solar cell according to any one of claims 1 to 4, wherein each of the first transparent electrode layer and the second transparent electrode layer is made of an amorphous indium tin composite oxide. Production method. 6. The crystalline silicon solar cell according to claim 1, wherein the first transparent electrode layer and the second transparent electrode layer both have a hole mobility of 50 cm ² / Vs or more. Manufacturing method.   The manufacturing method of the crystalline silicon solar cell of any one of Claims 1-6 which forms a collector electrode on each of said 1st transparent electrode layer and said 2nd transparent electrode layer.

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