JP2013232581A - Method for manufacturing photovoltaic device and photovoltaic device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a photovoltaic device which never causes substrate strength deterioration resulting from a damage by laser processing and lifetime characteristic deterioration.SOLUTION: A method for manufacturing a photovoltaic device includes the steps of: forming a laser irradiation part 2 in which affected layers are arranged in an island shape by directly irradiating a single-crystal silicon substrate 1 with a laser beam; forming a texture of an inverted pyramid shape by selectively etching the laser irradiation part 2 by anisotropic etching; and forming a photoelectric conversion part so that a surface on which the texture of the inverted pyramid shape is formed serves as a light-receiving surface and forming a photovoltaic device by forming electrodes.

Description

  The present invention relates to a photovoltaic device manufacturing method and a photovoltaic device.

  In order to improve the performance of photovoltaic devices such as solar cells, it is important to efficiently incorporate sunlight into the photovoltaic device. Therefore, in the conventional photovoltaic device, texture processing is performed on the substrate surface on the light incident side, and light once reflected on the surface is incident again on the surface, so that more sunlight is taken into the substrate and photoelectric The conversion efficiency is improved. Here, the texture processing refers to processing for intentionally forming a fine uneven structure (texture structure) having a size of about several tens of nanometers to several tens of micrometers on the substrate surface.

  As a method of forming fine irregularities on the surface of a substrate in a photovoltaic device, a step of forming a groove by irradiating a laser beam on the surface of the silicon substrate, and a silicon substrate by selectively etching the groove by chemical etching There has been proposed a method including a step of forming irregularities on the surface (see, for example, Patent Document 1).

JP 2003-258285 A

  In the method of Patent Document 1, when the texture structure is formed, in the step of forming the groove by irradiating the surface of the silicon substrate with laser light, high-energy laser irradiation is performed for a long time to form the groove. Therefore, there is a problem that laser damage is likely to occur. In this way, laser processing such as continuously forming grooves on the substrate surface to form textures causes laser damage to the processed surface centering on the processed part when processing the semiconductor substrate. There is a problem that the strength of the substrate is lowered and the lifetime characteristics are likely to deteriorate.

  The present invention has been made in view of the above, and an object of the present invention is to obtain a photovoltaic device that does not cause a reduction in substrate strength or deterioration in lifetime characteristics due to damage caused by laser processing.

  In order to solve the above-described problems and achieve the object, a laser irradiation process for forming a laser irradiation portion in which work-affected layers are arranged in an island shape by direct laser irradiation on a semiconductor substrate is different from the laser irradiation portion. Etching process selectively etching by isotropic etching to form an inverted pyramid-like texture, and forming a photoelectric conversion portion so that the surface on which the inverted pyramid-like texture is formed serves as a light receiving surface, and an electrode And forming a photovoltaic device.

  According to the present invention, a semiconductor substrate is directly irradiated with a laser to form island-arranged laser irradiation portions, and then the laser irradiation portions are selectively etched by anisotropic etching. Form a pyramid texture. According to this method, after forming a laser irradiation portion formed in an island shape, that is, in a spot shape, the laser damaged portion is removed by performing anisotropic etching. For this reason, it can form, without irradiating a laser continuously over a long time, and can prevent the intensity | strength fall of a board | substrate and lifetime characteristic deterioration. In addition, by selectively etching away the laser damage portion, the (111) plane remains, and an effect of forming an inverted pyramid-shaped texture surrounded by the (111) plane is achieved.

1-1 is a diagram showing a texture forming step in the method for manufacturing a photovoltaic device according to the first embodiment of the present invention, and is a partially broken perspective view of a single crystal silicon substrate on which a laser pattern is formed in a laser irradiation step It is. 1-2 is a diagram showing a texture forming step in the method for manufacturing a photovoltaic device according to the first embodiment of the present invention, and is a partially broken perspective view of a single crystal silicon substrate on which a texture is formed. FIG. 1-3 is a cross-sectional view showing a photovoltaic device formed using a single crystal silicon substrate formed in the texture forming step in the method for manufacturing a photovoltaic device according to Embodiment 1 of the present invention. FIG. FIG. 2 is a diagram showing a configuration of a laser processing apparatus used in a laser irradiation process in the method for manufacturing a photovoltaic device according to the first embodiment of the present invention. FIG. 3 is a diagram schematically showing an inverted pyramid-like texture structure in anisotropic etching according to Embodiment 1 of the present invention. FIGS. 4-1 is process sectional drawing which shows the texture formation process by Embodiment 1 of this invention. FIGS. FIGS. 4-2 is process sectional drawing which shows the texture formation process by Embodiment 1 of this invention. FIGS. FIG. 5 is a diagram showing an inverted pyramid-like texture structure obtained by using a plurality of laser patterns according to the second embodiment of the present invention. FIG. 6A is a top view showing a texture structure in which the bottom of the texture shape according to the third embodiment of the present invention is a plane. FIG. 6-2 is a cross-sectional view showing a texture structure in which the bottom of the texture shape according to the third embodiment of the present invention is a plane. FIG. 7-1 is an explanatory diagram showing the relationship between the pattern of the laser irradiation portion and the inverted pyramid-shaped etching portion when a laser pattern having an aspect ratio of 1 or more is used in the texture forming method of the fourth embodiment. FIG. 7A is a top view. FIG. 7-2 is an explanatory diagram showing the relationship between the pattern of the laser irradiation portion and the inverted pyramid-shaped etching portion when a laser pattern having an aspect ratio of 1 or more is used in the texture forming method of the fourth embodiment. FIG. 7-2 is a cross-sectional view taken along line AA of FIG. 8-1 is a figure which shows the formation process of the texture of the inverted pyramid shape of Embodiment 4 of this invention. 8-2 is a figure which shows the formation process of the texture of the inverted pyramid shape of Embodiment 4 of this invention. FIG. 9 is an explanatory diagram showing a modified example of the process of forming an inverted pyramid-shaped texture according to the fourth embodiment of the present invention. FIG. 10 is a diagram showing a laser pattern when the aspect ratio is less than 1 in the fourth embodiment of the present invention.

  Embodiments of a method for manufacturing a photovoltaic device according to the present invention will be described below in detail with reference to the drawings. Note that the present invention is not limited to the embodiments. Moreover, sectional views of the photovoltaic devices used in the following embodiments are schematic, and the relationship between the thickness and width of the layers, the ratio of the thicknesses of the layers, and the like are different from the actual ones.

Embodiment 1 FIG.
FIGS. 1-1 to 1-2 are schematic views showing the texture forming step of the first embodiment of the method for manufacturing a photovoltaic device according to the present invention. 1-3 is a cross-sectional view showing a photovoltaic device formed using a single crystal silicon substrate formed in the texture forming step in the method for manufacturing a photovoltaic device according to Embodiment 1 of the present invention. FIG.

  The texture structure of the photovoltaic device according to the first embodiment is such that an inverted pyramid-shaped uneven structure is provided on the front surface or the back surface of a single crystal silicon substrate as a semiconductor substrate. Among texture structures, the inverted pyramid-shaped texture structure has the advantages of high light confinement efficiency and a relatively small occupied area. By using an inverted pyramid-like texture structure, incident sunlight is absorbed while being reflected multiple times by the concavo-convex structure on the substrate surface, so that sunlight is confined inside the substrate and light is efficiently emitted by a substrate with a limited thickness. -Effective for electrical conversion.

  This inverted pyramid-shaped texture structure T includes a laser irradiation process for forming a laser irradiation portion 2 in which a work-affected layer is formed in an island shape, that is, a spot shape by direct laser irradiation on a single crystal silicon substrate 1, and anisotropic etching. This is obtained by an etching process in which the laser irradiation part 2 is selectively etched by (wet etching) and an inverted pyramid-like texture exposed from the etching part 3 is formed. This single crystal silicon substrate 1 has a large number of inverted pyramid-shaped recesses 4 on the surface, and each surface constituting the inverted pyramid forms a (111) plane. A photoelectric conversion part is formed and an electrode is formed to form a photovoltaic device (solar cell) so that the surface on which the inverted pyramid-like texture structure T formed in this way is formed becomes a light receiving surface. . In the texture structure T of the present embodiment, the inverted pyramid-shaped concave portions 4 are repeatedly formed so as to take a close-packed arrangement structure, and the adjacent lattices are also linear, resulting in a structure with a large surface area. ing. The texture structure constituting the inverted pyramid-shaped concave portion 4 has the same surface area as the texture structure constituting the pyramid-shaped convex portion, but is less distorted and less in strength due to a smaller etching amount. There are advantages such as certain points.

  A laser damage pattern, that is, a work-affected layer is formed on the substrate itself by direct laser processing on the single crystal silicon substrate 1, and etching is selectively performed from the work-affected layer, thereby suppressing residual laser damage. An inverted pyramid-like texture structure T regularly formed on the front surface or the back surface is formed.

  FIG. 1-1 is a partially broken perspective view of a single crystal silicon substrate on which a laser irradiation portion 2 composed of a laser pattern (2p) is formed in a laser irradiation step. A laser irradiation portion 2 is formed in a regular arrangement by a laser irradiation process on the single crystal silicon substrate 1 formed so that the surface becomes a (100) plane, thereby forming a work-affected layer (laser damage). Here, the laser irradiating portions 2 made of a lattice-shaped laser pattern (2p) are regularly arranged so that the lattice point is the center of the laser beam. The work-affected layer refers to a surface layer that has been changed in material properties by processing. The cause is due to mechanical energy, thermal energy, or a combination of both. However, disorder and increase of lattice defects on the surface due to plastic deformation, deformation, refinement of crystal grains, surface flow, phase It refers to a region where any of transformation, structure change, and thermal cracking has occurred. In such a work-affected layer, residual strain is generated, so that residual stress is generated and energy is stored in the form of potential energy. For this reason, the work-affected layer portion has a higher energy level and is likely to undergo a chemical change compared to the portion not subjected to laser irradiation.

  FIG. 1-2 is a partially broken perspective view of a single crystal silicon substrate after forming an inverted pyramid texture by an etching process. Etching proceeds from the laser irradiation portion 2 on the surface of the single crystal silicon substrate 1 by the etching process, and the (111) plane remains. As a result, the etching progresses from the work-affected layer that is the laser damage portion of the laser irradiation portion 2, and the etching portion 3 is removed so that each lattice point of the lattice-like laser pattern becomes the center of the inverse pyramid, and the inverse pyramid is removed. A texture structure T having a concave portion 4 is formed.

  1-3 is a cross-sectional view showing a photovoltaic device formed using the single crystal silicon substrate 1 formed in the texture forming step in the method for manufacturing a photovoltaic device according to Embodiment 1 of the present invention. is there. The first conductivity type substrate 100 made of the single crystal silicon substrate 1 having the texture structure T obtained by the method of the first embodiment is subjected to a generally used process, and then the second conductivity type layer 102. Then, the insulating layer (passivation film) 104, the light receiving surface side electrode 106, the back surface side electrode 105, and the first conductivity type layer 103 are formed, and the solar battery cell (photovoltaic device) shown in FIG. .

  Next, laser irradiation to the single crystal silicon substrate 1 for forming the texture structure T in the manufacturing method of the photovoltaic device of the present embodiment will be described below. FIG. 2 is a diagram showing a configuration of a laser processing apparatus used in a laser irradiation process in the method for manufacturing a photovoltaic device according to the first embodiment.

  This laser processing apparatus is an apparatus for irradiating a semiconductor substrate to be processed such as the single crystal silicon substrate 1 with a laser beam L. This laser processing apparatus reflects a laser oscillator 11 that outputs a laser beam L, a beam adjustment optical system 13 that adjusts and expands the beam shape of a perfect circular beam 12 generated by the laser oscillator 11, and a laser beam L. A light guide mirror 14 that leads to the optical path, a laser beam splitting unit 15 that splits the laser beam L into a plurality of laser beams L, a condensing lens 16 that condenses the laser beam split by the laser beam splitting unit 15, And a control unit 18 that controls the relative position and speed between the stage 17 and the laser beam splitting unit 15 according to the region to be processed. Here, it is assumed that the distance between the lenses constituting the beam adjusting optical system 13 is fixed. Here, dx and dy indicate the stage scanning direction, which are the x-axis direction and the y-axis direction, respectively.

As the laser oscillator 11, for example, a TEM00 mode Q-switch Nd: YAG (Yttrium Aluminum Garnet) laser that outputs a fundamental wave (wavelength: 1064 nm) is used. By using a laser with high beam quality, the allowable depth of processing can be increased even when the condensing diameter of the laser light L is reduced. For this reason, the laser beam L in the TEM00 mode is used here. In general, the repetition frequency of the Q-switched laser is 10 Hz to several hundred kHz. For example, it is possible to form a work-affected layer pattern on the surface of single crystal silicon at 1 J / cm ² or more within the range of the repetition frequency. However, it is not limited to the said conditions, What is necessary is just irradiation conditions of the grade which can form the process-affected layer of the grade which produces etching selectivity. Although the fundamental wave of the Nd: YAG laser was used as the laser light source, the fundamental wave, the second harmonic, the third harmonic, such as an Nd: YVO ₄ laser, an Nd: YLF (Lithium Yttrium Fluoride) laser, etc. A quadruple wave can also be used.

  The beam adjusting optical system 13 adjusts the beam diameter and shapes the beam shape of the perfect circular beam 12 generated from the laser oscillator 11. In the beam adjusting optical system 13, the optical system for adjusting the beam diameter is formed by a combination of a concave lens and a convex lens. A cylindrical lens, a prism, and a diffractive optical element can be used as an optical system that performs beam shaping such as an elliptical beam.

  The adjusted and shaped laser beam is guided by the light guide mirror 14, and the laser beam splitting unit 15 branches the laser beam into a laser beam branching pattern having a geometric structure. As the laser beam splitting unit 15, a diffractive optical element, a mask having a plurality of openings, or the like is used.

  A semiconductor substrate such as the single crystal silicon substrate 1 may be processed by condensing the laser beam with the condenser lens 16 without branching the laser beam dividing unit 15 without installing the laser beam. However, when considering the uniformity and efficiency of the beam, it is desirable to use a diffractive optical element as the laser beam splitter 15.

  Laser patterning is performed by condensing the laser beam divided by the laser beam dividing unit 15 onto the laser beam incident surface of the single crystal silicon substrate 1 by the condenser lens 16 and scanning the stage 17. When condensing the divided laser beam, it is desirable to use an Fθ lens as the condensing lens 16.

  In the relative scanning of the single crystal silicon substrate 1 with respect to the irradiated laser beam, the same effect can be obtained even when the stage 17 is fixed and the laser beam is scanned by a galvano mirror, a polygon mirror, or the like.

  Further, in addition to the timing of laser oscillation, the control unit 18 scans the laser beam L onto the processing target at a predetermined speed or forms a laser beam to form the laser irradiation unit 2 in the region to be processed, as will be described later. In order to change the irradiation position of the light L, the laser light L or the stage 17 is moved.

  In such a laser processing apparatus, the laser light L output from the laser oscillator 11 is enlarged by the beam adjusting optical system 13, the optical path is changed by the light guide mirror 14, and then incident on the laser beam splitting unit 15. The In the laser beam splitting unit 15, after being split into a plurality of laser beams L by diffraction with a diffractive optical element, the laser beam is condensed by a condenser lens 16, and a plurality of fine particles are formed on the surface of the single crystal silicon substrate 1 disposed on the stage 17. A condensing spot is formed. As a result, the laser irradiation portion 2 is formed on the single crystal silicon substrate 1 at the position where the focused spot is irradiated, and laser damage is formed. Since the laser irradiation unit 2 is sequentially provided over a wide area on the surface of the single crystal silicon substrate 1, this laser processing apparatus irradiates the stage 17 on which the single crystal silicon substrate 1 is installed or the single crystal silicon substrate 1 at the time of laser irradiation. The laser beam L is scanned in a predetermined direction, and beam spots are formed at predetermined intervals. Further, the scanning speed of the laser light L is controlled by the control unit 18 so as to form the laser irradiation unit 2 for each period in which the laser light L is output at a repetition frequency by the laser oscillator 11. Note that the scanning direction of the stage 17 or the laser light L is an x axis (dx), and the direction on the surface of the stage 17 perpendicular to the scanning direction is a y axis (dy). Further, in this specification, in the case of scanning with the laser light L, both the case where the laser light is moved with respect to the stage 17 and the case where the stage 17 is moved with respect to the laser light are included.

  FIG. 3 is an explanatory view schematically showing the laser pattern 2p formed on the object to be processed by the laser beam splitter 15 and the inverted pyramid texture obtained by etching. FIGS. 4-1 to 4-2 are process cross-sectional views illustrating a texture forming process using the method of the first embodiment. 4A corresponds to the AA cross section of FIG. In the laser pattern 2p, 4 rows × 4 = 16 beam spots are arranged in a lattice shape at points corresponding to the apexes of the inverted pyramid. In the laser irradiation process, laser damage is merely formed and often remains without being removed, but the region corresponding to the laser pattern 2p may be melted and removed by the laser. Usually, the laser irradiation part 2 is often selectively removed at the initial stage of the etching process. In the laser irradiation process, it is desirable that the laser irradiation part 2 is left as it is as a process-affected layer without evaporating to the extent that laser damage is formed, and the irradiation conditions are such that the cross-sectional shape as shown in FIG. .

  In FIG. 2, the left-right direction in the paper plane is the x-axis direction that is the scanning direction of the laser processing apparatus, and the direction perpendicular to the x-axis direction in the paper plane is the y-axis direction. Further, the outline of the laser pattern 2p has a perfect circle shape. Hereinafter, the length of the laser pattern 2p in the direction (y-axis direction) perpendicular to the scanning direction (x-axis direction) is referred to as “width”. By scanning the laser pattern 2p in the x-axis direction, the laser irradiation portions 2 composed of the beam spot-shaped laser pattern 2p having the irradiation diameter c are formed on the single crystal silicon substrate 1 at a predetermined interval.

  As a semiconductor substrate for forming a photovoltaic device, a p-type single crystal silicon substrate having a thickness of 200 to 400 μm sliced along the (100) plane is used. Other photovoltaic devices include crystalline silicon solar cells such as an n-type single crystal silicon substrate and an n-type polycrystalline silicon substrate, or polycrystalline crystal solar cells using a compound semiconductor such as SiC. These may be used. However, it goes without saying that it is desirable to use a single crystal substrate in order to form a texture structure having inverted pyramid-shaped recesses aligned along the crystal plane. Variations may occur.

  The purpose of the laser irradiation process is to cause laser damage to the semiconductor substrate by laser irradiation. Therefore, the drawing energy is small, and the laser beam shape can be not only a perfect circle but also an ellipse or a rectangular shape. However, it is necessary to form an arrangement so that an inverted pyramid texture structure can be obtained in the subsequent etching process. Further, the number of beam patterns and the total number of pyramids are not limited to the above, and can be appropriately selected.

  After the laser irradiation process, as shown in FIG. 4A and FIG. 4B, the texture structure T including the inverted pyramid-shaped concave portions 4 surrounded by the texture wall portions 4w is formed by anisotropic etching. The laser irradiation portion 2 corresponding to the laser pattern 2p shown in FIG. 4A is first removed by etching. Etching by anisotropic etching proceeds from this laser irradiation portion 2, and an inverted pyramid shape is obtained as shown in FIG. The texture structure T composed of the concave portions 4 is formed. FIG. 3 shows an inverted pyramid-like texture structure T in the etching according to the first embodiment by a broken line 5. A square having the laser pattern 2p indicated by the broken line 5 as the center of the lattice forms one unit of the texture structure T. For the etching process, wet etching having anisotropy with respect to the (111) plane is used. The perfect circular laser pattern 2p is a laser irradiation part 2 by laser irradiation. At the time of wet etching, since the etching proceeds centering on the laser damaged part of the laser irradiation part 2, if the pattern of the laser irradiation part 2 (laser pattern 2p) is continuous, an inverted pyramid shape cannot be obtained. As shown in FIG. 3, the irradiation diameter c of the laser beam must be shorter than the length a of one side of the inverted pyramid shape in order to obtain the arranged inverted pyramid shape.

  Next, an etching method for forming the inverted pyramid texture structure T will be described. Here, the etching solution has an anisotropy with respect to silicon, such as sodium hydroxide (NaOH), potassium hydroxide (KOH), an aqueous solution of tetramethylammonium hydroxide (TMAH), an alkali such as ethylenediamine pyrocatechol (EDP). Use solution.

  In the present embodiment, an etching solution such as an aqueous solution of potassium hydroxide (KOH) or sodium hydroxide (NaOH) reacts with the laser damage portion, which is the laser irradiation portion 2, and is anisotropic with respect to the single crystal silicon substrate 1. Perform wet etching. Here, the laser damage part in the laser irradiation part 2 refers to a part where a thermal melt layer or microcrack is generated by a laser, or a part where bonds between atoms are weak. The laser damaged portion caused by laser irradiation has a higher etching rate than the non-irradiated portion of the semiconductor material, and etching proceeds preferentially.

  In the anisotropic etching of the single crystal silicon substrate 1, when potassium hydroxide (KOH) or ethylenediamine pyrocatechol (EDP) is used as an etching solution, the etching rate of the (111) plane is higher than the etching rate of other crystal orientations. And extremely slow. Using this, anisotropic etching is performed.

  When the laser irradiation portion 2 on which the work-affected layer is formed is removed by etching on the single crystal silicon substrate 1 having a (100) surface, the single crystal silicon substrate is subjected to anisotropic etching with an alkaline aqueous solution. 1 is anisotropically etched along the (111) plane to obtain an inverted pyramid-shaped recess 4 having a V-shaped cross section formed by four texture wall portions 4w oriented in the (111) plane.

  Etching is terminated when the apex of the inverted pyramid-shaped recess 4 comes out. In this way, the inverted pyramid-shaped recess 4 has the inverted pyramid-shaped recess 4 by terminating the etching immediately before or just after contacting the adjacent inverted pyramid-shaped recess 4 of substantially the same size. A single crystal silicon substrate 1 having a texture structure T can be formed. As described above, the texture structure T composed of the concave portion 4 having the inverted pyramid shape can be formed through the laser irradiation process and the etching process.

  A pn junction is formed on the surface of the single crystal silicon substrate 1 having the texture structure T formed as described above to form a solar cell as a photovoltaic device.

  According to the photovoltaic device formed by the method of the present embodiment, various lasers such as dots, rectangles, and circles are formed on the front surface or the back surface of the single crystal silicon substrate, instead of performing groove processing by a laser. Laser damage is done by drawing the pattern directly in the form of dots. This damaged layer, which is a laser-damaged part, refers to a part where a laser-melted layer or microcrack is generated, or a part where the bonds between atoms are weakened. The rate is high. For this reason, in the etching for texture formation, the laser damaged portion is selectively etched, so that an aligned inverted pyramid texture can be formed on the front surface or the back surface of the single crystal silicon substrate. In addition, compared with laser irradiation for groove processing, according to the laser irradiation process of the present embodiment for forming a spot-like laser pattern, a decrease in the strength of the substrate is greatly suppressed. Further, since laser damage is removed by etching, deterioration of lifetime characteristics can be prevented.

Embodiment 2. FIG.
Next, a second embodiment of the present invention will be described. In the formation process of the texture structure T of the first embodiment, the laser pattern 2p constituting the laser irradiation unit 2 is one for each lattice unit. However, in the present embodiment, a plurality of laser patterns 2p are used. Configure. That is, as for the pattern of the laser irradiation unit 2, a plurality of laser patterns 2p may be aligned with one inverted pyramid shape formed on the single crystal silicon substrate 1 as shown in FIG. However, the overall pattern width b of the laser irradiation unit 2 and the diameter (vertical width) c of the laser pattern 2p of the laser irradiation unit 2 are sufficiently shorter than the diagonal length t = √2a of the inverted pyramid to be formed. There must be.

  When laser damage is given to the single crystal silicon substrate 1, microcracks generated by the laser are fine and have an effect of increasing the etching rate of the semiconductor material with respect to the etching solution. For this reason, it is more preferable that a lot of microcracks occur.

  However, if a crack or crack larger than the inverted pyramid shape formed by etching occurs, the etching solution enters the crack or the cracked portion and etching progresses, making it difficult to form the inverted pyramid shape. For this reason, the laser irradiation conditions are distributed and set, for example, by dividing into a plurality of laser patterns so as not to cause large cracks or cracks, so that the texture structure T in which the inverted pyramid-shaped recesses 4 are regularly arranged with higher accuracy can be obtained. Can be formed.

  The processing conditions for laser processing include not only the irradiation beam diameter but also the average output, pulse repetition frequency, and stage speed. Irradiation is performed using these parameters under conditions that do not cause large cracks or cracks.

  Since the laser pattern is applied to the surface on which the texture structure T is to be formed, the laser pattern may be formed on one or both of the front surface and the back surface of the single crystal silicon substrate 1 as a semiconductor substrate.

Embodiment 3 FIG.
Next, in the third embodiment, a modification of the texture shape will be described. In the first embodiment, the inverted pyramid-shaped recess 4 has the inverted pyramid-shaped recess 4 by terminating the etching immediately before or just after contacting the adjacent inverted pyramid-shaped recess 4 of approximately the same size. Although the texture structure T is formed, in this embodiment, as shown in FIGS. 6A and 6B, the texture structure T having the inverted pyramid trapezoidal recesses 9 is formed.

  When the laser irradiation portion 2 on which the work-affected layer is formed is removed by etching with respect to the single crystal silicon substrate 1 having a (100) surface, the single crystal silicon substrate 1 is subjected to anisotropic etching with an alkaline aqueous solution. Is anisotropically etched along the (111) plane to obtain a concave pyramid-shaped recess 9 having a V-shaped cross section formed by four texture wall portions 9w oriented in the (111) plane.

  Etching is terminated when the apex of the inverted pyramid-shaped recess 9 comes out. In addition to this, the etching is also terminated immediately before or just after the inverted pyramid trapezoidal concave portion 9 is in contact with the adjacent inverted pyramid trapezoidal concave portion 9 having the same size.

  FIGS. 6A and 6B are a plan view and a cross-sectional view of the texture structure T in which the bottom of the texture shape formed using the method of the third embodiment is a plane. FIG. 6B is a cross-sectional view taken along line AA in FIG. If the etching is continued even after the apex of the inverted pyramid-shaped concave portion 9 appears, the (100) plane etching proceeds, the apex of the inverted pyramid-shaped concave portion 9 disappears, and the bottom of the plane orientation (100) is generated. Furthermore, if the etching is continued, the size of one side of the pyramid becomes longer, but the bottom of the plane orientation (100) in the texture bottom portion 9b in FIG. This is because the cross-sectional shape shown in FIG. 6B is a trapezoidal inverted pyramid trapezoidal recess 9 and is not an inverted pyramid texture.

  In the present embodiment, the reverse pyramid trapezoidal texture forming method has been described. However, by using the single crystal silicon substrate 1 having the plane orientation (110) and performing anisotropic etching, it is deeply perpendicular to the substrate surface. A groove texture can also be formed. In addition, it can also be used for honeycomb texture formation by isotropic etching using hydrofluoric acid.

  As described above, according to the present embodiment, a laser pattern is formed by direct laser irradiation on a single crystal silicon substrate, and laser irradiation portions arranged in an island shape are formed. An etching step of selectively etching the laser irradiation portion to form an inverted pyramid texture, and forming the photoelectric conversion portion so that the surface on which the inverted pyramid texture is formed becomes a light receiving surface The electrode is formed to form a solar cell. For this reason, laser damage occurs on the single crystal silicon substrate during laser processing for texture formation. Laser damage to a single crystal silicon substrate affects deterioration of characteristics such as strength reduction and lifetime of the substrate.

  Also, in the method of this embodiment, the laser is not directly processed into the groove, but the laser is directly irradiated on the front or back surface of the silicon substrate to draw various patterns such as dots, rectangles, circles, etc. Damage is done. For this reason, the laser damage part refers to the part where the thermal melt layer or microcrack is generated by the laser, or the part where the bond between atoms is weak, and the etch rate is higher than the part without laser damage. . In the etching for texture formation, the laser irradiated portion, that is, the laser damaged portion is selectively etched, so that it is possible to form an aligned inverted pyramid texture on the front surface or the back surface of the single crystal silicon substrate. Further, since laser damage is removed by etching, it is possible to prevent a reduction in substrate strength and deterioration in lifetime characteristics.

  As described above, in the method of the present embodiment, a laser damage pattern is formed by directly irradiating a semiconductor substrate such as a single crystal silicon substrate with a laser, and then the laser damaged portion of the laser irradiated portion is etched. Thus, it is possible to form an inverted pyramid texture regularly arranged on the front surface or the back surface of the substrate that prevents the strength of the substrate from being lowered and the deterioration of the lifetime characteristics.

Embodiment 4 FIG.
Next, a fourth embodiment of the present invention will be described. In the fourth embodiment, the laser damage portion is formed by laser irradiation with the same configuration as in FIG. 2 of the first embodiment, but the aspect ratio of the width and depth of the substrate surface of the laser damage portion is 1 or more. It is characterized by being.

  FIGS. 7A and 7B illustrate the etching of the laser pattern (processed deteriorated layer) 2p of the laser irradiation unit 2 and the inverted pyramid shape when the laser pattern having an aspect ratio of 1 or more is used in the fourth embodiment. FIGS. 7A and 7B are explanatory views showing a relationship with the section 3, FIG. 7-1 is a top view, and FIGS. FIGS. 8-1 and FIGS. 8-2 are figures which show the formation process of the texture of the inverted pyramid shape of Embodiment 4 of this invention.

  When etching a shape having a vertex at the bottom of the texture like an inverted pyramid texture, it is necessary to increase the etching amount in the depth direction of the substrate.

  When the surface of the single crystal silicon substrate 1 has a plane orientation (100), etching is faster than the four plane orientations (111) of the side surfaces of the inverted pyramid shape.

  As shown in FIG. 10, when the work-affected layer (laser damaged portion) in the depth direction of the single-crystal silicon substrate 1 is shallow, that is, the aspect ratio of the width and depth of the work-affected layer on the substrate surface is less than 1. In the case of such a laser pattern 21, as shown in FIGS. 6-1 to 6-2, it is easy to form a texture in which the texture bottom portion 9b has a square surface orientation (100).

  Therefore, in the present embodiment, as shown in FIG. 7B, processing in the substrate depth direction is performed by forming a laser pattern 23 in which the aspect ratio of the width and depth of the substrate surface of the laser irradiation unit 2 is 1 or more. The deteriorated layer is increased so that the etching solution can easily enter the bottom. According to this method, since the removal amount in the substrate depth direction increases, the vertex of the inverted pyramid is likely to appear.

  In order to form the laser irradiation part 2 having a high aspect ratio, it is desirable to irradiate under the condition of high energy density that does not generate cracks and cracks larger than the inverted pyramid shape, although a hot melt layer and microcracks are generated. .

  In addition to the above, the amount of laser damage formed by the laser irradiation unit 2 depends on the laser penetration length into the semiconductor substrate (single crystal silicon substrate), and therefore the laser penetration length into the semiconductor material is the depth of the inverted pyramid texture shape. The following is desirable. In the etching for texture formation, the laser damage part is selectively etched, so when etching a shape having a vertex at the bottom of the texture shape like an inverted pyramid texture, it is necessary to increase the etching amount in the depth direction of the substrate. is there. However, if laser damage enters more than the depth of the formed inverted pyramid texture shape, the laser damage remains. Since the amount of laser damage depends on the penetration depth of the laser into the semiconductor substrate, laser damage is caused in the depth direction of the semiconductor substrate by setting the penetration depth of the laser below the depth of the inverted pyramid texture shape.

  For example, when Nd: YAG which is a solid-state laser is used and an inverted pyramid shape having a side of about several tens of μm is used, it is better to use 1064 nm having a longer penetration depth of silicon than 532 nm and 355 nm under the same processing conditions. It is desirable because the laser damage in the substrate depth direction is large.

  FIGS. 8-1 to FIGS. 8-2 are diagrams showing a process of forming an inverted pyramid-shaped texture according to the fourth embodiment of the present invention. By scanning the laser beam in the x-axis direction, laser irradiation portions (2) constituted by a beam spot-like laser pattern 23 are formed on the single crystal silicon substrate 1 at predetermined intervals (FIG. 8-1). .

  Then, after the laser irradiation step, as shown in FIG. 8B, an inverted pyramid texture structure T composed of inverted pyramid-shaped irregularities is formed by anisotropic etching. Since the conditions such as the etching solution are the same as those in the first embodiment, the description thereof is omitted here. Also in this embodiment, as in Embodiment 2, the laser irradiation portion (2) has the single crystal silicon removed by evaporation, but is not removed in the laser irradiation process, and in the initial stage of the etching process in FIG. It may be removed as shown.

  In the fourth embodiment, the cross-sectional aspect ratio of the laser irradiation part is 1 or more and the shape is a high aspect ratio so that the inverted pyramid shape can be stably obtained. On the other hand, even when a work-affected layer (laser damage layer) having the same surface area is provided, a texture having a bottom surface having a rectangular surface orientation (100) shown in FIGS. 6-1 to 6-2 is formed. It becomes easy to be done. Therefore, as shown in FIG. 9 showing a modification of the pattern of the laser irradiation portion, the etching amount can be reduced by reducing the surface area of the bottom portion of the inverted pyramid shape with respect to the etching solution 30.

  Laser irradiation in the depth direction is formed by forming the laser irradiation portion 2 having a taper angle α1 (α2> α1) at the bottom of the laser irradiation portion 2 having a taper angle α2 in the depth direction with respect to the substrate. The portion 2 is reduced to reduce the etching amount near the top of the inverted pyramid bottom. The taper angle α1 of the laser irradiation part 2 near the apex of the bottom of the inverted pyramid is desirably 55 ° or less.

  When the aspect ratio of the laser irradiation unit 2 is high, the etching solution 30 is difficult to enter the tapered laser irradiation unit 32 removed by etching, or the bubbles 31 generated during the etching are not sufficiently removed, so that the etching is difficult to proceed.

  Etching can be promoted by etching while applying ultrasonic waves to the entire substrate so that the bubbles 31 generated during the etching can be easily discharged to the outside of the texture recess.

  Furthermore, although the wet etching process is used for anisotropic etching in the first to fourth embodiments, dry etching may be used by selecting an etchant that slows the (111) plane etching rate. Good.

  As described above, when the laser damage is patterned on the semiconductor substrate, the width and depth aspect ratio of the substrate surface of the laser damaged portion which is the work-affected layer is 1 or more, and the inverted pyramid has a vertex at the bottom of the texture. A texture can be formed.

  As described above, the method for manufacturing a solar cell according to the present invention is useful for improving the photoelectric conversion efficiency, and is particularly suitable for manufacturing a solar cell using a single crystal silicon substrate.

1 Single crystal silicon substrate 2 Laser irradiation part 2p Laser pattern (worked layer)
3 Etched portion of semiconductor substrate 4 Recessed portion T Texture structure 5 Broken line (reverse pyramid texture array)
a Length of one side of reverse pyramid b Overall width of laser pattern c Laser beam width (irradiation diameter)
DESCRIPTION OF SYMBOLS 9 Concave part 9b Texture bottom part 9w Texture wall part dx, dy Stage scanning direction 11 Laser oscillator 12 Perfect circular beam 13 Beam adjustment optical system 14 Light guide mirror 15 Laser beam splitting part 18 Control part 17 Stage 21 Laser pattern with aspect ratio of less than 1 23 Laser pattern having an aspect ratio of 1 or more 30 Etching solution 31 Bubble 32 Tapered laser irradiation unit 100 removed First conductive type substrate 102 Second conductive type layer 103 First conductive type layer 104 Insulating layer (passivation film)
105 Back side electrode 106 Light receiving side electrode

Claims (6)

A laser irradiation process for forming a laser irradiation portion in which processing-affected layers are arranged in an island shape by direct laser irradiation on a semiconductor substrate;
The laser irradiation portion is selectively etched by anisotropic etching to form an inverted pyramid-like texture, and
Forming a photoelectric conversion portion so that the surface on which the inverted pyramid-like texture is formed becomes a light-receiving surface, and forming a photovoltaic device by forming an electrode. Device manufacturing method. The semiconductor substrate is a single crystal silicon substrate;
The method for manufacturing a photovoltaic device according to claim 1, wherein the etching step is a wet etching step. The laser irradiation step is a step of regularly arranging and forming a lattice-like laser pattern so that the laser irradiation portion has a lattice point at the center of a laser beam.
The method of manufacturing a photovoltaic device according to claim 1, wherein the etching step is a step of forming an etching portion in which each lattice point is the center of an inverted pyramid.   The said laser irradiation process is a manufacturing method of the photovoltaic apparatus of Claim 3 with which the said lattice point is comprised with a several laser pattern.   The laser irradiation step is a step of drawing a laser pattern in which an aspect ratio of a width and a depth on the semiconductor substrate surface of the work-affected layer as the laser irradiation portion is 1 or more. 5. The method for manufacturing a photovoltaic device according to claim 4. A photovoltaic device formed by the method for manufacturing a photovoltaic device according to any one of claims 1 to 5,
A single crystal silicon substrate having a large number of inverted pyramid-shaped recesses on the surface, and each surface constituting the inverted pyramid is a (111) plane;
A photovoltaic device comprising: a photoelectric conversion portion formed such that a surface of the single crystal silicon substrate on which the concave portion is formed becomes a light receiving surface.

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