JP4684306B2 - Method for manufacturing thin film solar cell - Google Patents

Description

  The present invention relates to a thin-film solar cell and a method for manufacturing the same, and more particularly to a thin-film solar cell related to a light confinement technique and a method for manufacturing the same.

  The light confinement technology currently used for thin film solar cells is a method of forming a concavo-convex structure on the surface of a transparent conductive film formed on a transparent insulating substrate in the case of a thin film solar cell that receives light from the transparent insulating substrate side. It is used. It is generally known that the light confinement technology for forming this concavo-convex structure improves the light conversion efficiency of the thin-film solar cell by reducing the light reflectance and the light scattering effect. Specifically, the light incident from the transparent insulating substrate side is scattered at the interface between the concavo-convex transparent conductive film and the photoelectric conversion layer and then enters the photoelectric conversion layer. To do. When light is incident obliquely, the substantial optical path of the light is extended and the light absorption is increased, so that the photoelectric conversion characteristics of the photovoltaic element are improved and the output current is increased.

Conventionally, a SnO ₂ transparent conductive film is well known as a transparent conductive film forming an uneven structure. Generally, the concavo-convex structure formed in the SnO ₂ transparent conductive film is formed by growing crystal grains having a diameter of several tens to several hundreds of nanometers on the film surface by a thermal CVD method. However, in order to form a good concavo-convex structure on the surface of the SnO ₂ film, a high-temperature process of 500 to 600 ° C. is required and a film thickness of about 1 μm is required. It has become one.

Therefore, in recent years, ZnO is becoming popular as a material to replace SnO ₂ from the viewpoint of excellent plasma resistance and abundant resources. However, in the case of ZnO, there is a problem that a film thickness of about 2 μm is required in order to form a good uneven structure on the surface. Therefore, as a method of forming a concavo-convex structure having a good light confinement effect even when the ZnO film is thinned by low-temperature formation, a transparent conductive film is formed on a glass substrate by sputtering and etched with acid. A technique for forming a concavo-convex structure has been reported. This method is expected to reduce the cost of the solar cell device. Further, in Patent Document 1 below, even when the crystalline film is formed on a transparent insulating substrate and the transparent conductive film is formed thereon, the transparent conductive film is thinned and further cooled. It is disclosed that a favorable uneven structure is formed on the surface.

  A method for forming irregularities on the surface of a transparent insulating substrate is also known. For example, Patent Document 2 below discloses a method of forming irregularities by placing powder glass on a flat glass and melting it. Japanese Patent Application Laid-Open No. H10-228561 discloses that irregularities are formed on the surface of a transparent insulating substrate by sandblasting.

Japanese Patent Laid-Open No. 11-189436 JP 62-98677 A JP-A-9-199745

  However, the technology for forming a concavo-convex structure on the surface by etching with the above-mentioned acid can cause pinholes due to locally steep protrusions due to etching variations, thereby generating defects such as short-circuits. There was a problem of lowering the yield and reliability.

  Further, the technology for forming the concavo-convex structure described in the above-mentioned Patent Document 1 is that an amorphous film formed on a transparent insulating substrate is as thin as 20 nm. There was a problem that no material film was obtained, and incident light was absorbed by the crystalline film, resulting in a decrease in light transmittance. In addition, in the method of attaching particles and the mechanical processing method as in Patent Document 2 and Patent Document 3, the unevenness of a large step can be formed as compared with the film thickness of the photoelectric conversion layer such as the amorphous film as described above. Cheap. For this reason, a big level | step difference arises in a photoelectric converting layer, a disconnection etc. are produced, and there exists a problem of reducing the performance of a thin film solar cell.

  Therefore, the present invention has been made to solve such a problem, and even when the transparent conductive film is thinned, a concavo-convex structure having a good light confinement effect is formed, and light absorption loss is reduced and reliability is improved. It aims at obtaining a high-thin film solar cell and its manufacturing method.

  The method for producing a thin-film solar cell in the present invention includes (a) a step of preparing a transparent insulating substrate, (b) a step of forming an uneven structure on the surface of the transparent insulating substrate, and (c) on the transparent insulating substrate. A step of forming a transparent electrode layer, (d) a step of forming a photoelectric conversion layer on the transparent electrode layer, and (e) a step of forming a back electrode layer on the photoelectric conversion layer. (B) (f) forming an amorphous film on the transparent insulating substrate; (g) forming a crystallized crystalline film by irradiating the amorphous film with a laser; (H) etching the crystalline film to remove the crystalline film.

  The thin film solar cell in the present invention has a plurality of element regions in which photoelectric conversion elements are formed and a connection region that interconnects the photoelectric conversion elements between the element regions on a transparent insulating substrate. The surface of the transparent insulating substrate in the element region has an uneven structure, and the surface of the transparent insulating substrate in the connection region does not have an uneven structure.

  According to the thin film solar cell and the manufacturing method thereof of the present invention, since the step of forming the concavo-convex structure in the transparent conductive film is not performed, the formation of the transparent conductive film is optimized without restrictions at the time of film formation or after film formation. be able to. Therefore, a favorable transparent conductive film can be formed even at a low temperature such as sputtering. Moreover, since the shape and depth of the concavo-convex structure can be easily controlled by controlling the laser irradiation conditions and the etching rate, the conversion efficiency of the thin-film solar cell can be improved.

<Embodiment 1>
FIG. 1 is a cross-sectional view showing a configuration of thin-film solar cell 20 according to Embodiment 1 of the present invention. The thin film solar cell 20 includes a transparent insulating substrate 1 having a concavo-convex structure on the surface, a transparent conductive film 4 (transparent electrode layer) formed on the transparent insulating substrate 1 and serving as a first electrode layer, and the transparent conductive film 4. A photoelectric conversion layer 12 to be formed, and a back electrode layer 8 formed on the photoelectric conversion layer 12 and serving as a second electrode layer are provided. The photoelectric conversion layer 12 is composed of at least two layers. In this embodiment, the P-type amorphous silicon film 5, the i-type amorphous silicon film 6, and the N-type amorphous silicon are formed from the bottom in this embodiment. A membrane 7.

FIG. 2 is a cross-sectional view showing a manufacturing process of the thin-film solar cell 20 in the present embodiment. Hereinafter, with reference to FIG. 2, the manufacturing method of the thin film solar cell 20 is demonstrated. First, the transparent insulating substrate 1 is prepared. For example, a glass substrate is used as the transparent insulating substrate 1 (hereinafter referred to as a glass substrate 1). This embodiment mode will be described below using an alkali-free glass substrate. In addition, an inexpensive blue plate glass substrate may be used as the substrate. In this case, in order to prevent the diffusion of alkali components from the substrate, it is preferable to form a SiO ₂ film of about 100 nm by PCVD.

  Next, an amorphous silicon film 2 is formed to a thickness of about 50 to 100 nm on the glass substrate 1 by PCVD (FIG. 2A). As a method for forming the amorphous silicon film 2, a thermal CVD method may be used.

Next, the amorphous silicon film 2 is crystallized by a laser annealing method to form a polycrystalline silicon film 3 (FIG. 2B). Here, the amorphous silicon film 2 formed by the PCVD method has a high hydrogen concentration in the film. Therefore, in order to perform uniform crystallization, unnecessary hydrogen is removed by performing heat treatment of the amorphous silicon film 2 in a vacuum or in an inert gas at 300 ° C. or higher before performing laser annealing. It is desirable. The laser used in this embodiment uses Yω laser 2ω (wavelength: 532 nm). When irradiating with laser light, the polycrystalline silicon film 3 at the last solidified portion becomes thicker than the portion solidified first by volume expansion. Therefore, an uneven structure is generated on the surface of the polycrystalline silicon film 3. Irradiation conditions at this time depend on conditions such as the film thickness and film quality of the amorphous silicon film 2, but can be performed at an energy density of 250 mj / cm ² or more. By this crystallization, an uneven structure having a height of 50 nm or more is formed on the surface of the polycrystalline silicon film 3. In this embodiment, the YAG laser is used as the energy beam. However, other excimer lasers, Ar lasers, etc. may be used, and an electron beam or the like may be used. Further, when laser irradiation is performed, the irradiation region is detected by scanning parallel to the amorphous silicon film 2.

Next, the polycrystalline silicon is etched by RIE dry etching to form a concavo-convex structure on the surface of the glass substrate 1 (FIG. 2 (c)). As etching conditions, CF ₄ : 10 sccm, O ₂ : 5 sccm, He: 15 sccm, pressure: 1.3 Pa, RF: 300 W, 3 min, selectivity 2 of glass substrate 1 (SiO ₂ ) / polycrystalline silicon film 3 (Si) By etching under the above conditions, irregularities of 100 nm or more are formed on the surface of the glass substrate 1. As the etching gas at this time, it is also possible to use a single gas containing CHF ₃ , C ₄ F ₈ , CF ₄ , SF ₆ and an etching gas mixed with O ₂ , He, or the like.

Next, the transparent conductive film 4 to be the first electrode layer is formed on the glass substrate 1 having a concavo-convex structure on the surface. The transparent conductive film 4 is formed by depositing a ZnO film having a thickness of 0.4 μm and containing ₃ wt% of Al ₂ O ₃ dopant by sputtering. In this embodiment, Al-doped ZnO is used as the transparent conductive film 4. However, the present invention is not limited to this, and may be an ITO film, a SnO ₂ film, or a transparent conductive film 4 formed by laminating them, and has a high transmittance. The transparent conductive film 4 having low specific resistance may be used.

  Next, the photoelectric conversion layer 12 is formed on the transparent conductive film 4 by a plasma CVD method. In this embodiment, the photoelectric conversion layer 12 includes a P-type a-SiC film 5 (15 nm), an i-type a-Si film 6 (300 nm), and an N-type a-Si film 7 (20 nm) from the bottom. Sequentially formed.

Next, the back electrode layer 8 to be the second electrode layer is formed on the photoelectric conversion layer 12 by a sputtering method. In this embodiment, an aluminum (Al) film having a thickness of 200 nm is formed. However, Ag having a high reflectance may be used as the metal electrode, and ZnO, ITO, SnO ₂ may be used to prevent metal diffusion into silicon. A transparent conductive film such as the above may be formed. Through the above steps, a thin film solar cell 20 as shown in FIG. 1 is formed.

FIG. 3 is a diagram showing the haze ratio (diffuse transmittance / total light transmittance) × 100 when the laser power is changed. Here, the haze ratio is a numerical value representing the degree of light diffusion. From the figure, it can be seen that a sufficient light scattering effect is obtained at 250 mj / cm ² or more. In this way, since a good texture structure can be obtained by forming a concavo-convex structure on the glass substrate, it can be expected that local characteristic defects will be improved due to protrusions having a steep slope, and solar cell characteristics and reliability will be improved. An improvement can be obtained.

Next, light of AM-1.5 and 100 mW / cm ² was incident on the thin film solar cell 20 formed according to the present embodiment from the substrate side, and measurement was performed at 25 ° C. As a result, good characteristics with an open circuit voltage (Voc) of 0.9 V, a short-circuit current density (Jsc) of 15.5 mA / cm ² , a fill factor (FF) of 0.74, and a conversion efficiency (η) of 10.5% are obtained. Can do. Table 1 is a table compared with the results of Patent Document 1 described above.

  In the present embodiment, amorphous silicon is used for the photoelectric conversion layer 12. However, amorphous silicon-based semiconductors such as amorphous silicon germanium and amorphous silicon carbide, and crystalline materials thereof are used. Silicon type can also be used. By making these into a pin structure, relatively good characteristics can be obtained.

  As described above, according to the thin film solar cell 20 and the manufacturing method thereof of the present invention, the step of forming the uneven structure on the transparent conductive film 4 is not performed. It can be optimized without restrictions. Therefore, a satisfactory transparent conductive film 4 can be formed even at a low temperature such as sputtering. Further, the shape and depth of the concavo-convex structure can be easily controlled by controlling the laser irradiation conditions and the etching rate. Thereby, the uneven | corrugated shape of a fine level | step difference can be formed easily rather than processing the glass substrate 1 by machining etc. and forming an unevenness | corrugation. In particular, even when the photoelectric conversion layer 12 is thin, an uneven shape suitable for the thickness can be formed. For this reason, the reliability of the conversion efficiency of the thin film solar cell 20 can be improved.

<Embodiment 2>
FIG. 4 is a cross-sectional view showing the configuration of thin-film solar cell 20 according to Embodiment 2 of the present invention. The difference from the first embodiment is that a part of the surface of the glass substrate 1 has a concavo-convex structure. The separation groove 9, the connection groove 10, and the separation groove 11 are patterns formed by laser irradiation. Since other configurations are the same as those of the first embodiment, description thereof is omitted.

  FIG. 5 is a cross-sectional view showing a manufacturing process of thin-film solar cell 20 in the present embodiment. In the first embodiment, the method for manufacturing the thin-film solar cell 20 having the uneven structure on the entire surface of the polycrystalline silicon film 3 has been described. In this manufacturing method, since the concavo-convex structure is formed on the entire surface of the glass substrate 1, each laminated film formed on the glass substrate 1 also has the concavo-convex structure. On the other hand, in the present embodiment, only a portion that contributes to photoelectric conversion is irradiated with laser. That is, the object is to form a flat portion by not polycrystallizing a portion that does not contribute to photoelectric conversion, and to improve processing reproducibility and reliability. Hereinafter, with reference to FIG. 5, the manufacturing method of the thin film solar cell 20 in this Embodiment is demonstrated. Note that description of the manufacturing method similar to that of the first embodiment is omitted.

  First, the transparent insulating substrate 1 is prepared. For example, a glass substrate is used as the transparent insulating substrate 1 (hereinafter referred to as a glass substrate 1). Next, an amorphous silicon film 2 having a thickness of 50 nm is formed on the glass substrate 1 by PCVD (FIG. 5A). Here, since the amorphous silicon film 2 formed by the PCVD method has a high hydrogen layer degree in the film, in order to perform uniform crystallization, the amorphous silicon film 2 is subjected to vacuum before laser annealing. Heat treatment is performed at 350 ° C. to remove unnecessary hydrogen.

  Next, the amorphous silicon film 2 is crystallized by a laser annealing method to form a polycrystalline silicon film 3 (FIG. 5B). At this time, the polycrystalline silicon film 3 is selectively crystallized only in the region A (for example, 10 mm) shown in FIG. 6, and the amorphous silicon is not crystallized in the region B of 0.1 mm to 1 mm. Leave membrane 2. The region B is removed when each photoelectric conversion element is divided, and hardly contributes to power generation. The thin film solar cell 20 has a configuration in which unit elements in the region A that receives light and generates power are connected in series by the connection structure in the region B.

Next, the polycrystalline silicon film 3 and the amorphous silicon film 2 are etched by the RIE dry etching method to form a concavo-convex structure only in a portion contributing to photoelectric conversion on the surface of the glass substrate (FIG. 5C). As the etching conditions, CF ₄ : 10 sccm, O ₂ : 5 sccm, He: 15 sccm, pressure: 1.3 Pa, RF: 300 W, 3 min, glass substrate (SiO ₂ ) / polycrystalline silicon (Si Etching under the condition of a selectivity ratio 2) of 100 nm or more is formed on the surface of the glass substrate.

Next, the transparent conductive film 4 to be the first electrode layer is formed on the glass substrate 1 having a concavo-convex structure on the surface. The transparent conductive film 4 is formed by depositing a 0.2 μm thick ITO film and a 0.1 μm thick ZnO film containing ₃ wt% Al ₂ O ₃ dopant by sputtering.

  Next, patterning is performed by irradiating the first transparent conductive film 4 with laser light. Specifically, the separation groove 9 is formed by irradiating a YAG laser fundamental wave (wavelength: 1.06 μm) from the surface of the translucent insulating substrate 1. For example, the scribe width of the separation groove 9 is 40 μm.

  Next, the photoelectric conversion layer 12 is formed on the transparent conductive film 4 by a plasma CVD method. In this embodiment, the photoelectric conversion layer 12 includes a P-type a-SiC film 5 (15 nm), an i-type a-Si film 6 (300 nm), and an N-type a-Si film 7 (20 nm) from the bottom. Sequentially formed.

  The photoelectric conversion layer 12 thus laminated is patterned by laser scribing in the same manner as the transparent conductive film 4. Specifically, the connection groove 10 is formed by irradiating 2ω (wavelength: 532 nm) of a YAG laser.

Next, the back electrode layer 8 to be the second electrode layer is formed on the photoelectric conversion layer 12 by a sputtering method. In this embodiment, an aluminum (Al) film having a thickness of 200 nm is formed. However, Ag having a high reflectance may be used as the metal electrode, and ZnO, ITO, SnO ₂ may be used to prevent metal diffusion into silicon. A transparent conductive film such as the above may be formed. At this time, since it is difficult to directly absorb the laser in the back electrode layer 8 having high reflectivity, the laser light energy is absorbed in the semiconductor layer (photoelectric conversion layer 12), and the metal layer together with the semiconductor layer (photoelectric conversion layer 12). The separation groove 11 is formed by locally blowing off. Through the above steps, a thin film solar cell 20 as shown in FIG. 4 is formed.

  As described above, in the present embodiment, fine irregularities are formed on the surface of the transparent insulating substrate 1 in the region A where light is received and power is generated, and the photoelectric conversion characteristics can be improved. On the other hand, no unevenness is formed in the region B connecting the unit cells that do not contribute to power generation. As a result, in this region B, the transparent conductive film 4, the photoelectric conversion layer 12, and the back electrode layer 8 that are laser-scribed are formed flat, so that non-uniform film thickness does not occur and scribe by laser irradiation is high. Since it can be performed with accuracy, separation with a reduced groove width can be performed. Thereby, since a power generation effective area can be increased, processing reproducibility and characteristics of the thin-film solar cell 20 are improved.

It is sectional drawing which showed the structure of the thin film solar cell in Embodiment 1 of this invention. It is sectional drawing which showed the manufacturing process of the thin film solar cell in Embodiment 1 of this invention. It is the figure which showed the haze rate at the time of changing laser power. It is sectional drawing which showed the structure of the thin film solar cell in Embodiment 2 of this invention. It is sectional drawing which showed the manufacturing process of the thin film solar cell in Embodiment 2 of this invention. It is the top view which showed the element surface of the thin film solar cell in Embodiment 2 of this invention.

Explanation of symbols

  1 transparent insulating substrate, 2 amorphous silicon film, 3 polycrystalline silicon film, 4 transparent conductive film, 5 P-type amorphous silicon film, 6 i-type amorphous silicon film, 7 N-type amorphous silicon film , 8 Back electrode layer, 9, 11 Separation groove, 10 Connection groove, 12 Photoelectric conversion layer, 20 Thin film solar cell.

Claims (7)

(A) preparing a transparent insulating substrate;
(B) forming an uneven structure on the surface of the transparent insulating substrate;
(C) forming a transparent electrode layer on the transparent insulating substrate;
(D) forming a photoelectric conversion layer on the transparent electrode layer;
(E) forming a back electrode layer on the photoelectric conversion layer,
The step (b)
(F) forming an amorphous film on the transparent insulating substrate;
(G) irradiating the amorphous film with a laser to form a crystallized crystalline film;
(H) etching the crystalline film and removing the crystalline film; and a method of manufacturing a thin film solar cell.   The method for manufacturing a thin-film solar cell according to claim 1, wherein the step (g) further includes a step of scanning an irradiation region in parallel with the amorphous film.   The method of manufacturing a thin film solar cell according to claim 2, wherein the step (g) is performed on a part of the amorphous film. The step (h) is RIE etching performed with a gas in which CHF ₃ , C ₄ F ₈ , CF ₄ , SF ₆ simple gas and O ₂ , He are mixed,
The method for manufacturing a thin-film solar cell according to any one of claims 1 to 3, wherein the transparent insulating substrate / crystalline film selection ratio is 1 or more.   5. The method for manufacturing a thin-film solar cell according to claim 1, wherein the step (f) forms an amorphous film having a thickness of 50 nm or more.   6. The method of manufacturing a thin-film solar cell according to claim 1, wherein the step (d) forms two or more layers of amorphous silicon or crystalline silicon. In a method for manufacturing a thin-film solar cell having a plurality of element regions in which photoelectric conversion elements are formed and a connection region for interconnecting the photoelectric conversion elements between the element regions on a transparent insulating substrate,
The step (b), claims wherein the uneven structure is formed on a transparent insulating surface of the substrate in the element region, on the surface of the transparent insulating substrate of said connecting region, characterized in that does not form an uneven structure The manufacturing method of the thin film solar cell in any one of 1 thru | or 6 .

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