JP2015185808A - Photoelectric conversion device and manufacturing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a photoelectric conversion device having a high absorption rate.SOLUTION: When performing anisotropic etching of an n-type single crystal silicon substrate 1 by using an alkaline solution, texture etching is performed while deflecting the n-type single crystal silicon substrate 1 by applying a stress thereto. Consequently, a texture is formed so that an angle θ of a quadrangular pyramidal surface, formed due to distortion of crystal orientation, is larger than 54.75 degrees, that is the original angle of the 100 plane and 111 plane, for the substrate surface.

Description

  The present invention relates to a photoelectric conversion device and a method for manufacturing the same, and more particularly to a crystalline silicon solar cell.

  In a solar power generation device, in order to absorb sunlight more efficiently, it is essential to absorb the light irradiated from the light receiving surface inside the substrate as much as possible without loss. A crystalline silicon solar cell using a crystalline silicon substrate has high photoelectric conversion efficiency, and has been widely put into practical use as a photovoltaic power generation system.

  In photovoltaic devices using these crystalline silicon substrates, textured pyramid-shaped irregularities are formed on the light receiving surface side of the crystalline silicon substrate in order to reduce reflection loss due to the light confinement structure. As a method for forming irregularities having a texture structure on the surface of the crystalline silicon substrate, a method of anisotropically etching the surface of the crystalline silicon substrate using an alkaline solution is known.

  For example, as described in Patent Document 1, there is one in which a crystalline silicon substrate having a (100) plane on the surface has a texture structure on the light incident surface, side surface, and back surface peripheral portion by anisotropic etching.

  As a method of forming a texture structure on the surface of a crystalline silicon substrate, an anisotropic etching method using an alkaline solution and utilizing a difference in etching rate between the (100) plane and (111) plane of crystalline silicon is known. ing. When a single crystal silicon substrate having a (100) plane is etched with an alkaline solution, a quadrangular pyramid texture consisting of a (111) plane with a very slow etching rate is formed on the substrate surface.

  In order to further improve the light absorption efficiency by the above method, the angle θ formed by the substrate surface ((100) plane) and the quadrangular pyramid texture plane ((111) plane) may be further increased. However, when the texture is formed without mask using anisotropic etching, this angle (θ: 54.75 degrees) is determined by the crystal orientations of the (100) plane and the (111) plane, It is difficult to change.

  Further, Patent Document 2 attempts to increase the angle formed by the substrate surface and the texture surface ((111) surface) by cutting the substrate in a direction shifted from the (100) surface. However, with this method, one surface can be made larger than 54.75 degrees, but the angle of the opposite surface is smaller than 54.75 degrees, which does not lead to an improvement in light absorption efficiency. . In addition, there is a problem in that the shape of the bottom surface of the quadrangular pyramid is not square, and there are more portions where the (100) plane appears on the substrate surface, resulting in increased light reflection.

JP 2006-286820 A JP 2011-181620 A

  In a crystalline silicon solar cell, texture formation with an alkaline solution determines the shape of a quadrangular pyramid formed by an angle (54.75 degrees) formed by a (100) crystal plane and a (111) crystal plane. In order to further increase the light absorption efficiency, the angles of all the faces of the quadrangular pyramid need to be larger than 54.75 degrees.

  However, in the method as in Patent Document 2, it is possible to make the angle larger than 54.755 degrees on one specific surface, but smaller than 54.75 degrees on the surface opposite to the surface. The light reflection increases.

  In addition, the angle formed by the left and right surfaces does not change from 54.75 degrees, so it does not contribute to light reflectance reduction.

  This invention is made | formed in view of the above, Comprising: It aims at obtaining the photoelectric conversion apparatus with a high light absorption rate.

  In order to solve the above-described problems and achieve the object, the present invention applies a stress to the crystalline silicon substrate to deflect it when performing anisotropic etching using an alkaline solution on the crystalline silicon substrate. In this state, the angle θ of the textured quadrangular pyramid surface is formed to be larger than 54.75 degrees that is an angle formed by the original 100 surface 111 surface with respect to the substrate surface. be able to.

  According to the present invention, when anisotropic etching of crystalline silicon with an alkaline solution is performed, due to distortion of crystal orientation, at least one surface of the textured pyramid formed on the substrate surface with respect to the substrate surface, The etching angle is increased by the amount of deflection of the substrate surface. Therefore, by having an angle larger than 54.75 degrees, the surface reflectance can be reduced and the light absorption rate to the crystalline silicon substrate is increased. As a result, the short-circuit current value in the cell characteristics is increased, and the effect that the cell conversion efficiency can be improved is achieved.

1A and 1B are diagrams showing a solar cell according to Embodiment 1, in which FIG. 1A is a plan view and FIG. 1B is a cross-sectional view taken along line AA in FIG. FIGS. 2A to 2C are explanatory views showing the texture structure of the substrate surface of the solar cell of the first embodiment, where FIG. 2A is an enlarged view of one texture, and FIG. 2B is the texture arrangement state. FIG. 4C is a perspective view of the substrate. 3A to 3E are process cross-sectional views illustrating the texture forming method and the heterojunction solar cell manufacturing method of the first embodiment. FIG. 4 is a flowchart showing a texture forming method in the method for manufacturing the solar cell of the first embodiment. FIG. 5 is a table showing the evaluation results of the solar cell of the first embodiment. FIG. 6 is a diagram showing a substrate having a texture used for forming the solar cell of the second embodiment. 7A to 7E are process cross-sectional views illustrating the texture forming process of the solar cell substrate of the second embodiment. FIG. 8 is a flowchart showing a texture forming step in the method for manufacturing the solar cell of the second embodiment. FIG. 9 is an explanatory diagram illustrating the texture forming method according to the third embodiment. FIG. 10 is a cross-sectional view showing the solar cell of the fourth embodiment. FIGS. 11A to 11E are process cross-sectional views illustrating the texture forming method in the method for manufacturing the solar cell of the fourth embodiment. FIG. 12 is a flowchart showing a texture forming method in the solar cell manufacturing method of the fourth embodiment.

  DESCRIPTION OF EMBODIMENTS Hereinafter, a photovoltaic cell according to an embodiment of a photoelectric conversion device and a manufacturing method thereof and a manufacturing method thereof according to the present invention will be described in detail with reference to the drawings. In addition, this invention is not limited by this embodiment, In the range which does not deviate from the summary, it can change suitably. In the drawings shown below, the scale of each layer or each member may be different from the actual for easy understanding, and the same applies to the drawings. Further, even a plan view may be hatched to make the drawing easy to see.

Embodiment 1 FIG.
FIG. 1 is a diagram schematically showing a first embodiment of a solar cell according to the present invention, in which (a) is a plan view and (b) is an AA cross-sectional view of (a). FIGS. 2A to 2C are explanatory views showing the texture structure, FIG. 2A is an enlarged view of one texture, FIG. 2B is a view showing an arrangement state of textures, and FIG. FIG. 3A to 3E are process cross-sectional views illustrating the method for manufacturing the solar cell, and FIG. 4 is a flowchart. When viewed macroscopically, the solar cell of the first embodiment is a typical solar cell, but the angle θ formed with the surface 1A ₀ of the n-type single crystal silicon substrate 1 as the crystalline silicon substrate used here. Has a texture 1T made of a quadrangular pyramid having a side surface S of 54.75 degrees or more. Here, by performing anisotropic etching in a state where stress is applied, the angle θ formed by the inclined surface (side surface S) of the texture 1T and the substrate surface 1A ₀ becomes 54.75 degrees or more.

  First, as shown in FIG. 3A, an n-type single crystal silicon substrate is used as an example of a crystalline silicon substrate. The crystalline silicon substrate includes a single crystal silicon substrate and a polycrystalline silicon substrate. In particular, a single crystal silicon substrate having a (100) plane as a surface is preferable. In this embodiment, the case where an n-type silicon substrate is used will be described; however, a silicon substrate having p-type conductivity may be used. The n-type single crystal silicon substrate 1 has a light receiving surface 1A as a first main surface and a back surface 1B as a second main surface opposite thereto.

Moreover, it is preferable to use the n-type single crystal silicon substrate 1 from which, for example, slice damage caused by slicing a silicon ingot is removed. Here, the removal of the slice damage (step S101) can be performed, for example, by etching with a mixed acid of hydrogen fluoride aqueous solution (HF) and nitric acid (HNO ₃ ) or an alkaline aqueous solution such as NaOH.

  Although the shape and size of the n-type single crystal silicon substrate 1 are not particularly limited, the thickness is preferably 60 μm to 400 μm, and the surface shape is preferably a square shape with a side length of 90 mm to 160 mm, for example.

  Next, a process of forming a quadrangular pyramid texture shape having a side surface with an angle θ of 54.75 degrees or more formed with one surface of the n-type single crystal silicon substrate will be described.

  First, a film having compressive stress and high alkali resistance is formed as a protective film 2 on the back surface side of the n-type silicon substrate 1 (FIG. 3B: (Step S102)). Here, a laminated film of a silicon oxide film 2 a and a silicon nitride film 2 b is used as the protective film 2. As a film having high alkali resistance, a silicon oxide film, a silicon nitride film, or the like can be used in addition to a stacked film of a silicon oxide film and a silicon nitride film. On the other hand, a conductive oxide containing indium oxide as a main component may be used as a protective film. By forming a film having a compressive stress on the back surface 1B, which is the surface opposite to the light receiving surface 1A, the silicon substrate is bent into a spherical shape with the light receiving surface 1A concave, and is subjected to stress. Become.

  The protective film 2 can be formed by a method such as a thermal oxidation method, a plasma CVD (Chemical Vapor Deposition) method, or a sputtering method. In addition, a film having a compressive stress can be formed by adjusting the CVD conditions and the sputtering conditions. Note that the thickness of the film is preferably 80 nm to 800 nm, which can prevent the chemical solution from penetrating into the film and etching the silicon interface by wet etching.

  Next, a process of forming a texture structure on the surface of the n-type single crystal silicon substrate 1 is performed (step S103). A wet etching process is performed by immersing the n-type single crystal silicon substrate 1 in an etching bath. After the wet etching process, a texture formed by a micropyramid having a base length of 1 to 30 μm is randomly formed on the surface of the n-type single crystal silicon substrate 1. When performing anisotropic etching with an alkaline solution, as shown in FIG. 3 (b), the substrate is bent and placed in the solution so that the side to be the back surface 1B of the crystalline silicon substrate becomes convex. Etching is performed in a state where stress is applied.

  The etching solution used in the wet etching process is a solution in which a strong alkali reagent such as NaOH, KOH, or tetramethylammonium hydroxide (TMAH) is dissolved, an alcohol-based additive such as IPA, a surfactant, or a silica such as sodium orthosilicate. An acid salt compound is added. The etching temperature is preferably 40 ° C. to 100 ° C., and the etching time is preferably 10 min to 60 min. As a result, as shown in FIG. 3C, a surface having an angle of the side surface of the quadrangular pyramid with respect to the substrate surface of 54.75 degrees or more with respect to the substrate surface is formed.

FIGS. 2A to 2C show an enlarged view and a perspective view of a main part of the surface of the substrate having the texture 1T formed as described above. (A) is an enlarged view of one texture, (b) is an arrangement state of the texture, and (c) is a perspective view of the substrate. Since the light receiving surface 1A side is formed in a concave shape due to the stress caused by the protective film 2, the remaining 100 surface is also inclined by anisotropic etching. Accordingly, a surface having an angle of 54.75 degrees or more with respect to the substrate surface 1A ₀ is formed with respect to the side surface S of the obtained quadrangular pyramid.

Next, a process of etching the protective film 2 is performed (FIG. 3D: step S104). In order to etch the protective film, the n-type single crystal silicon substrate 1 having a texture formed on the light receiving surface side is immersed in an etching bath containing HF or ammonium fluoride (NH ₄ F). Note that the higher the concentration of HF, the more the protective film is etched. For example, although depending on the acid resistance of the protective film, an HF concentration of 2% or more is preferable.

  Next, in order to clean the silicon interface on the back surface after etching the protective film, the following first and second steps are performed. In the first step, an oxide film is formed on the surface of the n-type single crystal silicon substrate 1 on the light-receiving surface 1A side by immersing in a cleaning solution containing concentrated sulfuric acid and hydrogen peroxide solution, and then n-type single crystal in a hydrofluoric acid solution. The oxide film on the crystalline silicon substrate 1 is removed. In the second step, an oxide film is formed on the surface of the n-type single crystal silicon substrate 1 by dipping in a cleaning solution containing hydrochloric acid and hydrogen peroxide, and then oxidized on the n-type single crystal silicon substrate 1 in a hydrofluoric acid solution. Remove the membrane. The first step and the second step are repeated until organic contamination, metal contamination, and contamination by particles on the surface of the n-type single crystal silicon substrate 1 are sufficiently reduced. Further, cleaning with functional water such as cleaning with ozone water or cleaning with carbonated water may be used (FIG. 3D). In the state where the protective film is removed in this way, the stress on the substrate surface almost disappears and returns to the surface state when the substrate is flat without bending. Although the figure shows a bent state, the protective film is removed, and after a certain period of time, the original state is restored, and only the surface texture becomes a gentle quadrangular pyramid with a large side surface inclination angle θ.

  The above process is utilized as a texture formation process. By this processing, as shown in FIG. 3D, it is possible to form a texture shape in which the angles formed by the substrate surface on all side faces of the quadrangular pyramid are larger than 54.75 degrees.

Next, an intrinsic silicon-based thin film is formed on the light receiving surface side of the n-type single crystal silicon substrate 1 (step S105). For the formation of the thin film, an intrinsic amorphous silicon layer (a-Si: H (i)) 3i formed of silicon and hydrogen is formed by plasma CVD. SiH ₄ gas and H ₂ gas are used to form the silicon thin film, but the band gap due to alloying is changed by mixing gases such as CH ₄ , CO ₂ , NH ₃ , GeH _4. A film may be formed. In addition, silicon thin films having different physical properties such as conductivity, band gap, and crystallization rate may be formed as a single layer or stacked layers.

Next, a p-type silicon thin film is formed on the light-receiving surface 1A side by plasma CVD using B ₂ H ₆ or the like as a doping gas (step S106). In particular, a p-type amorphous silicon layer (a-Si (p)) 4 is used for the p-type silicon thin film. The film thickness is 10 to 40 nm.

Next, an intrinsic silicon thin film is formed on the back side of the n-type single crystal silicon substrate 1 (step S107). In forming the thin film, an intrinsic amorphous silicon layer (a-Si: H (i)) 6i formed of silicon and hydrogen is formed by plasma CVD. SiH ₄ gas and H ₂ gas are used to form the silicon thin film, but the band gap due to alloying is changed by mixing gases such as CH ₄ , CO ₂ , NH ₃ , GeH _4. A film may be formed. In addition, silicon thin films having different physical properties such as conductivity, band gap, and crystallization rate may be formed as a single layer or stacked layers. The film thickness is 1 to 10 nm.

Next, an n-type silicon thin film is formed on the back surface 1B side by plasma CVD using PH ₃ or the like as a doping gas (step S108). In particular, an n-type amorphous silicon layer (a-Si (n)) 7 is used for the n-type silicon thin film. On the back surface 1B side, by forming a highly doped layer, a back surface field effect (Back Surface Field) can be obtained, and surface recombination of carriers can be suppressed by the field effect. In the p-type silicon substrate, the p layer is formed on the light receiving surface side.

Next, the translucent conductive film 5 constituting the antireflection film is formed on the light receiving surface side where the texture is formed (FIG. 2E: step S109). For example, an indium oxide film is used as the light-transmitting conductive film as the antireflection film. As a material for the light-transmitting conductive film other than indium oxide, indium oxide (ITO) doped with 5% to 10% tin, zinc oxide, tin oxide, or the like can be used. Examples of the forming method include sputtering film formation, ion plating vapor deposition, and electron beam vapor deposition. The process gas used at the time of film formation is appropriately added with O ₂ , H ₂ , water vapor, N _2, etc., mainly Ar gas. It is preferable to use a material that absorbs as little light as possible in the entire wavelength region where light is absorbed in silicon.

  On the other hand, a silicon oxide film or a silicon nitride film may be used as the antireflection film. The silicon oxide film and silicon nitride film are preferably dense films having high resistance to HF and alkali. The silicon nitride film can be formed by sputtering or plasma CVD. However, it is necessary to form a film at a temperature of 200 to 300 ° C. or less which does not cause deterioration of characteristics due to the influence of hydrogen desorption from the already formed n-type silicon thin film and p-type silicon thin film. The film thickness is preferably 80 to 150 nm. Furthermore, the refractive index at 700 nm is preferably 1.6 to 2.2. If the film has both a passivation effect and an antireflection effect, such as a silicon oxide film and a silicon nitride film, an intrinsic amorphous silicon thin film and an n-type silicon thin film are formed on the light receiving surface side. There may be no film.

  Next, the current collecting metal electrodes 9 and 8 are formed on the light-receiving surface 1A and the back surface 1B by a technique such as printing and baking or plating (step S110) to form a solar battery cell.

  Next, the fabricated heterojunction solar cell was actually operated, and power generation characteristics were measured and evaluated. The heterojunction solar cell manufactured in Embodiment 1 is referred to as Example 1. For comparison, a heterojunction solar cell of Comparative Example 1 was produced. FIG. 5 is a table showing the relationship among reflectance, conversion efficiency, open circuit voltage, short circuit current, and fill factor. The heterojunction solar cell having the structure shown in Comparative Example 1 was prepared after supporting so that the silicon substrate does not bend during alkali etching during texture formation. Processes other than those described above are fabricated in the same manner as in Example 1.

  As can be seen from the table shown in FIG. 5, by performing anisotropic etching by bending the silicon substrate during texture formation, the angle formed by the side surface of the quadrangular pyramid formed on the surface and the substrate surface can be increased. As a result, it was found that the surface reflectance can be reduced, and as a result, the short-circuit current can be increased and the conversion efficiency can be improved.

  In the heterojunction solar cell of this embodiment, a pn junction is formed by depositing a silicon thin film on a silicon substrate on which a texture structure is formed. However, a pn junction is formed on the silicon substrate on which the texture structure is formed by a thermal diffusion method. May be. For example, an emitter type solar cell in which an impurity having a conductivity opposite to that of a substrate is doped on a crystalline silicon substrate by a thermal diffusion method may be used. Alternatively, the substrate may be p-type and an impurity-added layer may be n-type.

Embodiment 2. FIG.
Next, another embodiment of the texture 1T forming method will be described. FIG. 6 is a diagram showing an n-type single crystal silicon substrate 1 as a solar cell substrate according to the second embodiment of the present invention. Also in this embodiment, like the n-type single crystal silicon substrate 1 described in the first embodiment, the side surface of the quadrangular pyramid formed on the surface is formed by bending the silicon substrate and performing anisotropic etching during texture formation. Although the point of increasing the angle θ formed with the surface is the same, in the present embodiment, after the formation of the texture 1T, the surface is further oxidized and the oxide film is removed to remove the texture having a good surface state. A substrate is obtained. Other steps are the same as those of the solar cell manufacturing method of the first embodiment. FIGS. 7A to 7E show a texture forming step in the manufacturing process of the solar cell, and FIG. 8 shows a flowchart of the manufacturing process of the solar cell.

  The process up to the texture etching step (step S230) is the same as that of the first embodiment, and n is removed by removing the damage (FIG. 7A: step S201) and forming the protective film 2 (FIG. 7B: step S202). Stress is applied to the surface of the type single crystal silicon substrate 1 so that the light receiving surface 1A side forms a recess.

  In this state, a step of forming a texture structure on the surface of the n-type single crystal silicon substrate 1 (step S230) is performed. A wet etching process is performed by immersing the n-type single crystal silicon substrate 1 in an etching bath. After the wet etching process, a texture formed by a micropyramid having a base length of 1 to 30 μm is randomly formed on the surface of the n-type single crystal silicon substrate 1. When performing anisotropic etching with an alkaline solution, as shown in FIG. 7B, the substrate is bent and placed in the solution so that the light receiving surface side of the crystalline silicon substrate becomes concave. Etching is performed in a state where stress is applied.

  The etchant used in the wet etching process is similar to the etchant used in the first embodiment, in which a strong alkali reagent such as NaOH, KOH, tetramethylammonium hydroxide (TMAH) is dissolved, and an alcohol-based additive such as IPA is added. A silicate compound such as an agent, a surfactant or sodium orthosilicate is added. The etching temperature is preferably 40 ° C. to 100 ° C., and the etching time is preferably 10 min to 60 min. The silicon surface after the anisotropic etching may have fine etch pits 1P or lattice defects (FIG. 7C).

  When these etch pits 1p or lattice defects are etched in a state where stress is applied, a shift occurs from the normal silicon lattice bonding state, and this shift causes etch pits and lattice defects that are abnormal in etching. Is likely to occur. In order to remove these, the following steps are performed. In the first step, the oxide film 11 is formed on the surface of the quadrangular pyramid by immersing it in a cleaning solution containing concentrated sulfuric acid and hydrogen peroxide (FIG. 7 (d): Step S231), and then the quadrangular pyramid is formed in a hydrofluoric acid solution. The oxide film is removed (step S232). In particular, since oxidation from the surface proceeds isotropically, if an acute angle portion is formed due to etch pits or lattice defects, the oxidation rate of that portion is increased. Therefore, the final surface shape becomes linear, and a smooth surface can be obtained. As a result, a texture shape having no surface defects and a side angle of the quadrangular pyramid of 54.75 degrees or more can be obtained (FIG. 7 (e)). Here, concentrated sulfuric acid and hydrogen peroxide solution are used as a method for forming the oxide film, but the oxide film may be formed by a method such as hydrochloric acid and hydrogen peroxide solution or thermal oxidation. Alternatively, a method of lightly etching the silicon substrate surface with a mixed solution of nitric acid and hydrofluoric acid without forming an oxide film may be used.

  In this manner, after obtaining the n-type single crystal silicon substrate 1 having the texture 1T having a good surface state, the step of etching the protective film (step S204) is performed. The rest is the same as in the first embodiment.

  Next, the silicon interface on the back surface where the protective film is etched is cleaned. Thereafter, similar to the method of the first embodiment, the light-receiving surface side amorphous silicon i layer is formed (step S205), the light-receiving surface side amorphous silicon p layer is formed (step S206), and the back surface side amorphous silicon i layer is formed. The heterojunction solar cell is configured through (Step S207), back side amorphous silicon n layer formation (Step S208), light receiving side translucent conductive film formation (Step S209), and metal electrode formation (Step S210). To do.

  According to this method, many anisotropic etching pits or fine steps are generated on the (111) plane by performing anisotropic etching under stress. Thereafter, a surface having an angle greater than 54.75 degrees with respect to the substrate surface can be formed by thermal oxidation + oxide film removal or isotropic etching with hydrofluoric acid. Thereby, the angle | corner which the side surface of a quadrangular pyramid forms becomes large, and a surface reflectance falls. As a result, the photocurrent increases and the conversion efficiency can be improved.

  As described above, according to this embodiment, etch pits and lattice defects on the texture surface are reduced, the angle formed between the texture side surface and the substrate surface is increased, and a surface having excellent reflectivity is obtained. Can do. Therefore, the photoelectric conversion efficiency of the solar cell can be improved.

Embodiment 3 FIG.
Next, the manufacturing method of the heterojunction solar cell of Embodiment 3 is demonstrated. In the present embodiment, when the texture structure is formed on the surface of the n-type single crystal silicon substrate 1, only the method for imparting the deflection to the substrate is different, and the other steps are the same as those in the first and second embodiments. In this embodiment, when anisotropic etching with an alkaline solution is performed, as shown in FIG. 9, an arrow 22 is provided by a cylindrical jig 21 for imparting deflection to the substrate holder 20 and the substrate placed at the center of the substrate. As shown in FIG. 2, the substrate is bent and placed in a solution so that the side corresponding to the light receiving surface 1A of the single crystal silicon substrate 1 becomes concave, and etching is performed in a state where stress is applied to the substrate. The rest is the same as in the first and second embodiments.

  By this processing, a texture shape in which the angle formed between the side surface of the quadrangular pyramid and the substrate surface is larger than 54.75 degrees can be formed as in the case shown in FIG. According to this method, a process for forming or peeling off the protective film is unnecessary, so that workability is good and surface contamination and roughness due to wet etching can be avoided. However, in the method of the third embodiment, unlike the examples of the first and second embodiments, the direction of stress applied to the n-type silicon crystal substrate 1 is the left-right direction, so that not all four surfaces of the formed quadrangular pyramid are. The angle between the two surfaces and the substrate surface is larger than 54.75 degrees, and the other two surfaces remain at an angle of 54.75 degrees. However, when two of the four surfaces have an angle larger than 54.75 degrees, the effect of reducing the reflectance can be exhibited.

Hereinafter, a heterojunction solar cell is formed by the same method as that of the first embodiment.
Also in the back junction solar cell of the present embodiment, a pn junction is formed by depositing a silicon thin film on an n-type single crystal silicon substrate having a texture structure, but a thermal diffusion method is applied to the silicon substrate having a texture structure. A pn junction may be formed. For example, an emitter type solar cell in which an impurity having a conductivity opposite to that of a substrate is doped on a crystalline silicon substrate by a thermal diffusion method may be used. The substrate may be p-type and an impurity added layer may be n-type.

Embodiment 4 FIG.
Next, the manufacturing method of the heterojunction solar cell of Embodiment 4 is demonstrated. In the methods described in the first and third embodiments, a protective film is formed when the substrate is bent, and the protective film is removed after the texture etching is completed. In the present embodiment, the protective film is removed. An example in which the film is left as it is without being removed and used as a passivation film will be described. FIG. 10 is a cross-sectional view schematically showing Embodiment 1 of the solar cell according to the present invention. FIGS. 11A to 11E are process cross-sectional views illustrating the method for manufacturing the solar cell, and FIG. 12 is a flowchart. The solar cell of the fourth embodiment has a texture 1T on the back side opposite to the light receiving surface, and this texture 1T contributes to diffusion and reflection to the photoelectric conversion layer together with the metal electrode 18 on the back side. To do. A texture 1T made of a quadrangular pyramid having a side surface with an angle θ of 54.75 degrees or more formed on the back surface 1B side of the n-type single crystal silicon substrate 1 as the crystalline silicon substrate used here. It is characterized by. Here, anisotropic etching is performed in a state where stress is applied to the light receiving surface 1A side by the protective film 12 made of a silicon nitride film acting as an antireflection film or a passivation film and the back surface side 1B is bent so as to be a concave surface. By doing so, the angle θ formed between the texture 1T inclined surface (side surface) and the substrate surface is set to be 54.75 degrees or more.

  In addition, an intrinsic amorphous silicon layer 3i, a p-type amorphous silicon layer 4, and a translucent conductive film 5 are sequentially formed only on the light-receiving surface 1A side of the n-type single crystal silicon substrate 1 without texture. A protective film 12 made of a silicon nitride film having a thickness of 10 to 20 nm is formed thereon. On the other hand, on the back surface 1B side, a two-layer film of an aluminum layer and a silver layer constituting the back surface side metal electrode 18 is formed, and a BSF layer 17 is formed by diffusion from the aluminum layer. On the front surface side, a metal electrode 9 on the light receiving surface side is formed by a printing pattern.

  The formation of the texture structure on the surface of the n-type single crystal silicon substrate 1 is essentially the same as in the first embodiment, except that the surface that gives deflection to the substrate and the film formation order are different.

  First, as shown in FIG. 11A, an n-type single crystal silicon substrate 1 is used as an example of a crystalline silicon substrate. The crystalline silicon substrate includes a single crystal silicon substrate and a polycrystalline silicon substrate. In particular, a single crystal silicon substrate having a (100) plane as a surface is preferable. In this embodiment, the case where an n-type silicon substrate is used will be described; however, a silicon substrate having p-type conductivity may be used.

Moreover, it is preferable to use the n-type single crystal silicon substrate 1 from which, for example, slice damage caused by slicing a silicon ingot is removed. Here, the removal of the slice damage (step S301) can be performed, for example, by etching with a mixed acid of hydrogen fluoride aqueous solution (HF) and nitric acid (HNO ₃ ) or an alkaline aqueous solution such as NaOH.

Next, an intrinsic silicon-based thin film is formed on the light-receiving surface side of the n-type single crystal silicon substrate 1 (step S305). For the formation of the thin film, an intrinsic amorphous silicon layer (a-Si: H (i)) 3i formed of silicon and hydrogen is formed by plasma CVD. SiH ₄ gas and H ₂ gas are used to form the silicon thin film, but the band gap due to alloying is changed by mixing gases such as CH ₄ , CO ₂ , NH ₃ , GeH _4. A film may be formed. In addition, silicon thin films having different physical properties such as conductivity, band gap, and crystallization rate may be formed as a single layer or stacked layers.

Next, a p-type silicon thin film is formed on the light receiving surface 1A side by plasma CVD using B ₂ H ₆ or the like as a doping gas (step S306). In particular, a p-type amorphous silicon layer (a-Si (p)) 4 is used for the p-type silicon thin film. The film thickness is 10 to 40 nm. After that, a translucent conductive film 5 is formed (FIG. 11B: step S309).

  Next, a process of forming a quadrangular pyramid texture shape having a side surface with an angle θ of 54.75 degrees or more formed with one surface of the n-type single crystal silicon substrate will be described.

  First, on the light-receiving surface 1A side of the n-type silicon substrate 1 on which the intrinsic amorphous silicon layer 3i, the p-type amorphous silicon layer 4 and the translucent conductive film 5 are formed, a film having compressive stress and high alkali resistance. As a protective film 12, a silicon nitride film is formed (FIG. 11C: (Step S302S)). Here, a silicon nitride film is used as the protective film 12. As a film having high alkali resistance, a stacked film of a silicon oxide film and a silicon nitride film, a silicon oxide film, or the like can be used. On the other hand, a conductive oxide containing indium oxide as a main component may be used as a protective film. By forming a film having compressive stress on the light-receiving surface 1A, the silicon substrate is bent into a spherical shape with the back surface 1B, which is the surface opposite to the light-receiving surface 1A, recessed, and is subjected to stress. Become.

  The protective film 2 can be formed by a method such as a plasma CVD (Chemical Vapor Deposition) method, a thermal oxidation method, or a sputtering method as in the first embodiment. In addition, a film having a compressive stress can be formed by adjusting the CVD conditions and the sputtering conditions. Note that the thickness of the film is preferably 80 nm to 800 nm, which can prevent the chemical solution from penetrating into the film and etching the silicon interface by wet etching.

  Next, a process of forming a texture structure on the back surface 1B of the n-type single crystal silicon substrate 1 is performed (step S303S). A wet etching process is performed by immersing the n-type single crystal silicon substrate 1 in an etching bath. After the wet etching process, a texture formed by a micropyramid having a base length of 1 to 30 μm is randomly formed on the back surface 1B of the n-type single crystal silicon substrate 1. When performing anisotropic etching with an alkaline solution, as shown in FIG. 11D, the substrate is bent and placed in the solution so that the side that becomes the light receiving surface 1A of the crystalline silicon substrate has a convex shape. Etching is performed with stress applied to the substrate.

  The etching solution used in the wet etching process is a solution in which a strong alkali reagent such as NaOH, KOH, or tetramethylammonium hydroxide (TMAH) is dissolved, an alcohol-based additive such as IPA, a surfactant, or a silica such as sodium orthosilicate. An acid salt compound is added. The etching temperature is preferably 40 ° C. to 100 ° C., and the etching time is preferably 10 min to 60 min. As a result, as shown in FIG. 11D, a surface having an angle of the side surface of the quadrangular pyramid with respect to the substrate surface of 54.75 degrees or more with respect to the substrate surface is formed.

  Next, a metal electrode 18 composed of an aluminum layer and a silver layer is printed on the back side of the n-type single crystal silicon substrate 1 (FIG. 11E: step S307S).

  Next, the metal electrode 9 for current collection is printed on the light receiving surface 1A (step 310S) and baked (step 311) to obtain a solar battery cell.

  By this firing treatment, the BSF layer 17 is formed on the back surface 1B side of the n-type single crystal silicon substrate 1, and the solar cell shown in FIG. 10 is formed.

  According to the solar cell of this embodiment, the angle of the side surface of the quadrangular pyramid is 54.75 degrees or more, and the back surface shape having excellent scattering properties improves the photoelectric conversion efficiency.

  Further, according to the method for manufacturing the solar cell of the present embodiment, the protective film 12 for forming the deflection on the substrate surface is left without being used as it is as a passivation film, so that the number of steps can be reduced. It becomes.

  In the above embodiment, the light receiving surface side is formed without a texture. However, after removing the damage, a texture is formed in the same manner as in the first embodiment, and this embodiment is performed only when the back surface texture is formed. The method may be used. This makes it possible to obtain a solar cell having texture on both sides.

  Also, a protective film is formed on the back side to form a bend on the light receiving surface side, a texture is formed, the protective film is removed, a protective film is further formed on the light receiving surface side, and a bend is formed on the back side. By forming the texture, it is possible to form a texture having a side face of the quadrangular pyramid of 54.75 degrees or more on both sides.

  In any of the first, second, third, and fourth embodiments, the crystalline silicon substrate can be applied to a crystalline silicon substrate such as a single crystal silicon substrate or a polycrystalline silicon substrate.

  Moreover, although the said embodiment demonstrated the solar cell, it is applicable to various photoelectric conversion devices including light receiving elements, such as an image sensor, without being limited to a solar cell.

  Although several embodiments of the present invention have been described, these embodiments are presented as examples and are not intended to limit the scope of the invention. These novel embodiments can be implemented in various other forms, and various omissions, replacements, and changes can be made without departing from the scope of the invention. These embodiments and modifications thereof are included in the scope and gist of the invention, and are included in the invention described in the claims and the equivalents thereof.

  1 n-type single crystal silicon substrate, 1A light-receiving surface, 1B back surface, 1T texture, 2 protective film, 2a silicon oxide film, 2b silicon nitride film, 3i intrinsic amorphous silicon layer, 4 p-type amorphous silicon layer, 5 Antireflection film, 6i intrinsic amorphous silicon layer, 7 n-type amorphous silicon layer, 8 metal electrode, 9 metal electrode, 12 protective film, 17 BSF layer, 18 metal electrode.

Claims (8)

A first conductivity type crystalline silicon substrate having first and second main surfaces;
A second conductive type silicon thin film formed on the first main surface of the crystalline silicon substrate; and an electrode;
A photoelectric conversion device having a quadrangular pyramid-like texture having a side surface whose angle with the surface is larger than 54.75 degrees on the first or second main surface.   The photoelectric conversion device according to claim 1, wherein the side surface is a (111) surface.   The photoelectric conversion device according to claim 1, wherein the crystalline silicon substrate is a single crystal silicon substrate. For the first conductivity type crystalline silicon substrate having the first and second main surfaces,
In a state where stress is applied so that the first main surface is bent concavely, anisotropic etching is performed on the first main surface,
A step of providing the first main surface with a quadrangular pyramid texture having a side surface with an angle formed by the surface larger than 54.75 degrees;
And a step of forming a photoelectric conversion layer on the first or second main surface. The step of forming the texture includes:
The manufacturing method of the photoelectric conversion apparatus of Claim 4 including the process of forming a protective film in the said 2nd main surface, and performing anisotropic etching in the state which made the said 1st main surface bend in a concave shape.   The method for manufacturing a photoelectric conversion device according to claim 5, wherein the step of forming the photoelectric conversion layer is performed after removing the protective film.   The method for producing a photoelectric conversion device according to claim 5, wherein the step of forming the photoelectric conversion layer is performed prior to the step of forming the protective film, and the protective film is left as a protective film of the photoelectric conversion device. . The step of forming the texture includes:
The method for manufacturing a photoelectric conversion device according to claim 4, comprising a step of performing anisotropic etching in a state in which a rod-shaped jig is mounted on the first main surface and the first main surface is bent in a concave shape.

Images