Patent Application
20150056743
The present invention relates to a manufacturing method of a solar cell.
Conventionally, bulk type solar cells are typically manufactured by the following method. For example, a p-type silicon substrate is prepared first as a first conductivity type substrate, and a damaged layer on the silicon surface generated when being sliced from a cast ingot is removed by, for example, several wt % to 20 wt % caustic soda or carbonated caustic soda and in a thickness of 10 micrometers to 20 micrometers. Thereafter, anisotropic etching is performed with a solution in which IPA (isopropyl alcohol) is added to a similar low concentration alkaline solution, and a texture is formed so as to expose a silicon (111) surface.
Subsequently, the p-type silicon substrate is processed in a mixed gas atmosphere of, for example, phosphorous oxychloride (POCl3), nitrogen, and oxygen for example at a temperature of 800° C. to 900° C. for several tens of minutes, thereby uniformly forming an n-type layer on the entire surface of the p-type silicon substrate as a second conductivity type impurity layer. By setting the sheet resistance of the n-type layer uniformly formed on the surface of the p-type silicon substrate to approximately 30 Ω/□ to 80 Ω/□, excellent electric property of a solar cell can be acquired. Because the n-type layer is uniformly formed on the surface of the p-type silicon substrate, the front surface and the back surface of the p-type silicon substrate are electrically connected. To interrupt the electrical connection, the facet region of the p-type silicon substrate is removed by dry etching to expose the p-type silicon. As another method to be performed to remove the influence of the n-type layer, there is a method of performing facet separation by a laser. Thereafter, the substrate is immersed in a hydrofluoric acid solution and a glassy (PSG: Phospho-Silicate Glass) layer deposited on the surface during a diffusion process is removed by etching.
Next, as an insulating film (an anti-reflective film) for preventing reflection, an insulating film such as a silicon dioxide film, a silicon nitride film, a titanium oxide film is formed on the surface of the n-type layer on the light-receiving surface side with a uniform thickness. When the silicon dioxide film is to be formed as the anti-reflective film, film formation is performed, for example, by a plasma CVD method, using SiH4 gas and NH3 gas as raw materials, at a temperature of 300° C. or higher under reduced pressure. The refraction index of the anti-reflective film is approximately 2.0 to 2.2, and the optimum film thickness is approximately 70 nanometers to 90 nanometers. It is to be noted that the anti-reflective film formed in this manner is an insulating body and the structure obtained by simply forming a light-receiving surface side electrode on the anti-reflective film does not function as a solar cell.
Subsequently, by using a mask for forming a grid electrode and for forming a bus electrode, silver paste to be the light-receiving surface side electrode is applied to the anti-reflective film in the shape of the grid electrode and the shape of the bus electrode by a screen printing method and is dried.
Back aluminum electrode paste to be a back aluminum electrode and back silver paste to be a back silver bus electrode are applied to the back surface of the substrate, respectively, in the shape of the back aluminum electrode and the shape of the back silver bus electrode by the screen printing method and dried.
The electrode paste applied to the front and back surfaces of the p-type silicon substrate is then baked simultaneously at a temperature of approximately 600° C. to 900° C. for several minutes. With this process, the grid electrode and the bus electrode are formed as the light-receiving surface side electrode on the anti-reflective film, and the back aluminum electrode and the back silver bus electrode are formed as the back surface-side electrode on the back surface of the p-type silicon substrate. On the front surface side of the p-type silicon substrate, the silver material comes in contact with silicon while the anti-reflective film is melting by the glass material contained in the silver paste, and is re-solidified. Accordingly, conduction between the light-receiving surface side electrode and the silicon substrate (the n-type layer) is ensured. This process is referred to as “fire-through method”. Furthermore, the back aluminum electrode paste reacts with the back surface of the silicon substrate, to form a p+ layer (BSF (Back Surface Field)) immediately below the back aluminum electrode.
To improve the photoelectric conversion efficiency in the solar cell manufactured in this manner, it is essential that the texture structure formed on the surface of the silicon substrate can capture sunlight to the silicon substrate efficiently. As the texture structure that can capture sunlight to the silicon substrate efficiently, for example, Non Patent Literature 1 discloses an “inverted” pyramid texture structure as one of the optimum structures. The inverted pyramid texture structure is a texture structure that includes microasperity (texture) in an inverted pyramid shape.
Such an inverted pyramid texture structure is manufactured in the following manner. First, an etching mask is formed on a silicon substrate. Specifically, a silicon nitride (SiN) film is formed by the plasma CVD method, or a silicon dioxide (SiO2) film or the like is formed by thermal oxidation. Openings are then formed in the etching mask corresponding to the size of the microasperity in the inverted pyramid shape to be formed. The silicon substrate is then etched in an alkaline solution. With this process, etching of the surface of the silicon substrate proceeds via the openings, and a slow-reacting (111) surface is exposed, thereby forming the microasperity (texture) in the inverted pyramid shape on the surface of the silicon substrate. The inverted pyramid texture structure is acquired in this manner.
In the process of forming the inverted pyramid texture structure described above, the most complicated and time-consuming process is a process of forming the openings in the etching mask. When a photolithography technique, which is a general forming method of the openings in the etching mask, is used, many processes such as application of a photoresist to the etching mask, baking processing, exposure by using a photomask, development, baking, formation of the openings in the etching mask by etching, and removal of the resist need to be performed. Therefore, the method of using the photolithography technique has a problem in the productivity, because the process is complicated and the processing time becomes long.
Furthermore, in recent years, as another forming method of the openings in the etching mask, processing by using a laser has been studied. According to this method, by irradiating the etching mask with a laser beam, openings can be directly formed in the etching mask. However, in order to increase processing accuracy, the laser diameter of the laser beam needs to be narrowed and laser irradiation needs to be performed accurately several times. Therefore, processing by the laser requires a long processing time, thereby causing a problem in the productivity.
The present invention has been achieved to solve the above problems, and an object of the present invention is to provide a manufacturing method of a solar cell that can manufacture a solar cell having an inverted pyramid texture structure and excellent photoelectric conversion efficiency with good productivity.
In order to solve the above problems and achieve the object, a manufacturing method of a solar cell according to the present invention is a manufacturing method of a solar cell including: a first step of forming an impurity diffusion layer by diffusing an impurity element having a second conductivity type on one surface side of a semiconductor substrate having a first conductivity type; a second step of forming, on the one surface side of the semiconductor substrate, a light-receiving surface side electrode that is electrically connected to the impurity diffusion layer; and a third step of forming a back surface-side electrode on another surface side of the semiconductor substrate, wherein the manufacturing method includes a fourth step of forming an asperity structure having a concave portion in an inverted pyramid shape on a surface of the one surface side of the semiconductor substrate at any point in time before the second step, and the fourth step includes a protection-film forming step of forming a protection film on the one surface side of the semiconductor substrate, a first processing step of forming a plurality of first openings having a shape close to a desired opening shape and a size smaller than a target opening size in the protection film by a method having relatively high processing efficiency, a second processing step of forming second openings in the protection film by expanding the first openings up to the target opening size by a method having relatively high processing accuracy, an etching step of forming the asperity structure having the concave portion in the inverted pyramid shape on the one surface side of the semiconductor substrate by performing anisotropic wet etching on the semiconductor substrate in a region under the second openings via the second openings, and a removing step of removing the protection film.
According to the present invention, an effect is obtained where an inverted pyramid texture structure can be formed with good productivity and highly accurately, and a solar cell having excellent photoelectric conversion efficiency can be manufactured with good productivity.
Exemplary embodiments of a manufacturing method of a solar cell according to the present invention will be explained below in detail with reference to the drawings. The present invention is not limited to the following descriptions and can be modified as appropriate without departing from the scope of the invention. In the drawings explained below, for easier understanding, the scale of each member may be different from those of actual products. The same applies to relations between the drawings. In addition, even in plan views, hatching may be applied to facilitate visualization of the drawings.
In the solar cell 1 according to the first embodiment, an n-type impurity diffusion layer 3 is formed on the light receiving surface side of a semiconductor substrate 2 formed of a p-type monocrystalline silicon by phosphorus diffusion, thereby forming a semiconductor substrate 11 having a pn junction. An anti-reflective film 4 formed of a silicon nitride film (SiN film) is formed on the n-type impurity diffusion layer 3. The semiconductor substrate 2 is not limited to the p-type monocrystalline silicon substrate, and a p-type polycrystalline silicon substrate, an n-type polycrystalline silicon substrate, or an n-type monocrystalline silicon substrate can also be used.
An inverted pyramid texture structure including a microasperity (texture) 2a in the inverted pyramid shape is formed on the surface on the light receiving surface side of the semiconductor substrate 11 (the n-type impurity diffusion layer 3) as a texture structure. The microasperity (texture) 2a in the inverted pyramid shape increases the area in the light receiving surface that absorbs light from the outside, and suppresses the reflectance on the light receiving surface to confine the light in the solar cell 1 efficiently.
The anti-reflective film 4 is formed of a silicon nitride film (SiN film), which is an insulating film. The anti-reflective film 4 is not limited to the silicon nitride film (SiN film) and can be formed of an insulating film such as a silicon oxide film (SiO2 film) or a titanium oxide film (TiO2 film).
A plurality of long and thin front silver grid electrodes 5 are arranged side by side on the light receiving surface side of the semiconductor substrate 11. Front silver bus electrodes 6 electrically conducted with the front silver grid electrodes 5 are provided substantially orthogonal to the front silver grid electrodes 5, and are electrically connected to the n-type impurity diffusion layer 3 on the bottom surface portion. The front silver grid electrodes 5 and the front silver bus electrodes 6 are made of a silver material.
The front silver grid electrodes 5 have a width of, for example approximately 100 micrometers to 200 micrometers, are arranged substantially parallel to each other at intervals of approximately 2 millimeters, and collect electricity generated in the semiconductor substrate 11. The front silver bus electrodes 6 have a width of, for example, approximately 1 millimeter to 3 millimeters and two to three front silver bus electrodes 6 are arranged per one solar cell. The front silver bus electrodes 6 extract electricity collected by the front silver grid electrodes 5 to the outside. A light-receiving surface side electrode 12, which is a first electrode having a comb-like shape, is formed by the front silver grid electrodes 5 and the front silver bus electrodes 6. Because the light-receiving surface side electrode 12 blocks sunlight incident on the semiconductor substrate 11, it is desired to reduce the area of the light-receiving surface side electrode 12 as much as possible in view of improvement of power generation efficiency. Therefore, the light-receiving surface side electrode 12 is generally arranged as the comb-like front silver grid electrodes 5 and the bar-like front silver bus electrodes 6 as shown in
Silver paste is normally used as a material of the light-receiving surface side electrode of the silicon solar cell, and for example, lead boron glass is added thereto. The glass is in the form of frit, and has a composition of, for example, 5 to 30 wt % of lead (Pb), 5 to 10 wt % of boron (B), 5 to 15 wt % of silicon (Si), and 30 to 60 wt % of oxygen (0). Several wt % of zinc (Zn), cadmium (Cd), and the like is also mixed in some cases. The lead boron glass is dissolved by heating at several hundreds of degrees centigrade (for example, 800° C.), and has a property of eroding silicon at that time. Furthermore, generally, in a manufacturing method of a crystalline silicon solar cell, a method of acquiring an electrical contact between the silicon substrate and the silver paste by using a characteristic of the glass frit is used.
Meanwhile, a back aluminum electrode 7 made of an aluminum material is provided all over the back surface of the semiconductor substrate 11 (the surface opposite to the light receiving surface) excluding a part of the outer peripheral region, and back silver electrodes 8 made of a silver material are provided such that they extend substantially in the same direction as the front silver bus electrodes 6. A back surface-side electrode 13, which is a second electrode, is formed by the back aluminum electrode 7 and the back silver electrodes 8. The BSR (Back Surface Reflection) effect of reflecting long-wavelength light passing through the semiconductor substrate 11 and reusing the light for power generation is expected of the back aluminum electrode 7.
Furthermore, a p+ layer (BSF (Back Surface Field)) 9 containing high-concentration impurities is formed on the surface layer on the back surface side (the surface opposite to the light receiving surface) of the semiconductor substrate 11. The p+ layer (BSF) 9 is provided to acquire the BSF effect, and increases the electron concentration of a p-type layer (the semiconductor substrate 2) with an electric field in a band structure so that electrons in the p-type layer (the semiconductor substrate 2) do not disappear.
In the solar cell 1 having such a configuration, when the semiconductor substrate 11 is irradiated with sunlight from the light receiving surface side of the solar cell 1, holes and electrons are generated. The generated electrons move toward the n-type impurity diffusion layer 3 and the generated holes move toward the semiconductor substrate 2 by the electric field at the pn junction part (the junction plane between the semiconductor substrate 2 formed of the p-type monocrystalline silicon and the n-type impurity diffusion layer 3). Therefore, there are excess electrons in the n-type impurity diffusion layer 3 and there are excess holes in the semiconductor substrate 2. As a result, photovoltaic power is generated. The photovoltaic power is generated in a direction in which the pn junction is forward biased; therefore, the light-receiving surface side electrode 12 connected to the n-type impurity diffusion layer 3 becomes a negative electrode and the back aluminum electrode 7 connected to the p+ layer 9 becomes a positive electrode. Accordingly, electric current flows in an external circuit (not shown).
Next, a manufacturing method of the solar cell 1 according to the first embodiment is explained next with reference to
First, a p-type monocrystalline silicon substrate having a thickness of, for example several hundreds of micrometers, is prepared as the semiconductor substrate 2 (
Subsequent to the removal of the damaged area, anisotropic etching is performed on the p-type monocrystalline silicon substrate with a solution in which IPA (isopropyl alcohol) is added to a similar low concentration alkaline solution, and the inverted pyramid texture structure formed of the microasperity (texture) 2a in the inverted pyramid shape is formed on the surface on the light receiving surface side of the p-type monocrystalline silicon substrate so as to expose the silicon (111) surface (
A case where the inverted pyramid texture structure is formed on the surface on the light receiving surface side of the p-type monocrystalline silicon substrate is shown here. However, the inverted pyramid texture structure can be formed on both surfaces of the p-type monocrystalline silicon substrate. When the inverted pyramid texture structure is formed also on the back surface of the p-type monocrystalline silicon substrate, light reflected by the back surface-side electrode 13 and returned to the semiconductor substrate 11 can be scattered.
Subsequently, the pn junction is formed on the semiconductor substrate 2 (
In this diffusion process, thermal diffusion is performed on the p-type monocrystalline silicon substrate in a mixed gas atmosphere of, for example, phosphorus oxychloride (POCl3) gas, nitrogen gas, and oxygen gas by a gas-phase diffusion method at a high temperature of, for example 800° C. to 900° C., for several tens of minutes, thereby uniformly forming the n-type impurity diffusion layer 3 in which phosphorus (P) is diffused in the surface layer of the p-type monocrystalline silicon substrate. When the sheet resistance of the n-type impurity diffusion layer 3 formed on the surface of the semiconductor substrate 2 is in a range of 30 Ω/□ to 80 Ω/□, excellent electric characteristic of the solar cell can be acquired.
Subsequently, pn separation is performed for electrically insulating the back surface-side electrode 13, which is a p-type electrode, and the light-receiving surface side electrode 12, which is an n-type electrode, from each other (
Because a glassy (PSG: Phospho-Silicate Glass) layer deposited on the surface during a diffusion process is formed on the surface of the p-type monocrystalline silicon substrate immediately after formation of the n-type impurity diffusion layer 3, the phosphorus glass layer is removed by using a hydrofluoric acid solution or the like. With this process, the semiconductor substrate 11 is acquired, in which a pn junction is formed by the semiconductor substrate 2 formed of the p-type monocrystalline silicon substrate, which is a first conductivity type layer, and the n-type impurity diffusion layer 3, which is a second conductivity type layer formed on the light receiving surface side of the semiconductor substrate 2.
The anti-reflective film 4 is then formed in a uniform thickness on the light receiving surface side (the n-type impurity diffusion layer 3) of the p-type monocrystalline silicon substrate to improve the photoelectric conversion efficiency (
Electrodes are then formed by screen printing. The light-receiving surface side electrode 12 is manufactured first (before baking). Specifically, a silver paste 12a, which is an electrode material paste including a glass frit, is applied onto the anti-reflective film 4, which is the light receiving surface of the p-type monocrystalline silicon substrate, in the shape of the front silver grid electrodes 5 and the front silver bus electrodes 6 by screen printing and the silver paste 12a is dried (
Next, an aluminum paste 7a, which is an electrode material paste, is applied in the shape of the back aluminum electrode 7 and a silver paste 8a, which is an electrode material paste, is further applied in the shape of the back silver electrodes 8 to the back surface side of the p-type monocrystalline silicon substrate by screen printing, and the aluminum paste 7a and the silver paste 8a are dried (
Subsequently, by simultaneously baking the electrode pastes on the light receiving surface side and the back surface side of the semiconductor substrate 11, for example, at a temperature of 600° C. to 900° C., on the front surface side of the semiconductor substrate 11, the silver material comes in contact with silicon and is re-solidified while the anti-reflective film 4 is melting by the glass material contained in the silver paste 12a. Accordingly, the front silver grid electrodes 5 and the front silver bus electrodes 6 as the light-receiving surface side electrode 12 are acquired, and conduction between the light-receiving surface side electrode 12 and silicon of the semiconductor substrate 11 is ensured (
The aluminum paste 7a also reacts with silicon of the semiconductor substrate 11 and the back aluminum electrode 7 is acquired, and the p+ layer 9 is formed immediately below the back aluminum electrode 7. The silver material of the silver paste 8a comes in contact with silicon and is re-solidified, thereby acquiring the back silver electrodes 8 (
The solar cell 1 according to the present embodiment shown in
A forming method of the inverted pyramid texture structure is explained next with reference to
First, a silicon nitride film (SiN film) 21 is formed as a protection film to be used as an etching mask on the light receiving surface side of the p-type monocrystalline silicon substrate having undergone damage removal with a film thickness of approximately 70 nanometers to 90 nanometers by the plasma CVD method (FIGS. 3-1 and 4-1). A different film such as a silicon oxide film (SiO2 film) can be formed instead of the silicon nitride film (SiN film) 21. The silicon oxide film (SiO2 film) can be formed by, for example, the plasma CVD method or thermal oxidation.
Next, openings having a desired size are formed in the silicon nitride film (SiN film) 21 according to the size of the microasperity 2a in the inverted pyramid shape to be formed. The openings are formed by processing in two stages. Specifically, in the first processing step, first openings 21a having shapes close to the target opening shape and sizes slightly smaller than the target opening size are formed (
In the first processing step, the first openings 21a having a diameter of approximately several tens of micrometers are formed in the silicon nitride film (SiN film) 21 by using etching paste. By using etching paste, it is possible to process an etching mask having high productivity, that is, having high processing efficiency by simple and less number of processes, i.e., printing, heating up to a temperature at which etching proceeds, and cleaning. As another opening method in the first processing step, the first openings 21a having a diameter of approximately several tens of micrometers can be formed also by irradiation with a laser beam having an enlarged laser diameter obtained by converting a laser beam into a divergent beam. Etching paste and irradiation with the laser beam can be concurrently used appropriately according to the opening shape and the like. Because these methods to be used in the first processing step are inferior in controllability, that is, processing accuracy, for example, as shown in
In the second processing step, the laser beam is converged to a diameter of approximately several micrometers to be reduced to a size smaller than the first openings 21a. By irradiating the silicon nitride film (SiN film) 21 with such a small-diameter laser beam, for example, KrF excimer laser of 248 nanometers, or a frequency-doubled (532 nanometers) or frequency-tripled (355 nanometers) YAG laser, microfabrication (trimming) is performed to expand the first openings 21a up to the target opening shape, thereby forming the second openings 21b. By using the laser, processing of a fine etching mask having high controllability, that is, having high processing accuracy can be performed with a simple process.
Next, anisotropic etching is performed on the p-type monocrystalline silicon substrate with an etching solution in which IPA is added to a low-concentration alkaline solution, such as several wt % sodium hydroxide or potassium hydroxide, to form the inverted pyramid texture structure formed of the microasperity (texture) 2a in the inverted pyramid shape on the surface on the light receiving surface side of the p-type monocrystalline silicon substrate so as to expose the silicon (111) surface (
Finally, the p-type monocrystalline silicon substrate is immersed in a hydrofluoric acid solution or the like to remove the silicon nitride film (SiN film) 21, which is the remaining etching mask. With this process, as shown in
With reference to
First, a silicon nitride film (SiN film) 121, which becomes an etching mask, is formed on the light receiving surface side of a semiconductor substrate 102 (a p-type monocrystalline silicon substrate) having undergone damage removal with a film thickness of approximately 70 nanometers to 90 nanometers by the plasma CVD method (
Next, openings 121a having a desired size are formed in the silicon nitride film (SiN film) 121 according to the size of a microasperity 102a in the inverted pyramid shape to be formed (
Next, etching of the silicon nitride film (SiN film) 121 via the openings 121a using an alkaline aqueous solution and photoresist removal are sequentially performed (
As described above, in the manufacturing method of a solar cell according to the first embodiment, the process of forming the openings in the etching mask at the time of forming the inverted pyramid texture structure is performed by dividing the process into two stages, i.e., the first processing step of forming the first openings 21a having shapes close to the target opening shape and sizes slightly smaller than the target opening size by a method having relatively high productivity, that is, having high processing efficiency and the second processing step of forming the second openings 21b by expanding the first openings 21a up to the target opening shape by a method having relatively high processing controllability, that is, having high processing accuracy. With this process, the openings can be formed in the etching mask accurately in a short time and with simple and less number of processes.
Therefore, according to the manufacturing method of a solar cell of the first embodiment, the inverted pyramid texture structure can be formed with good productivity and with high accuracy, and the solar cell having excellent photoelectric conversion efficiency can be manufactured with good productivity.
In a second embodiment, an explanation will be made of a method of forming the inverted pyramid texture structure and forming a selective emitter by changing the impurity concentration of the n-type impurity diffusion layer in a region under the light-receiving surface side electrode 12 to a high concentration. By this method, the contact resistance between the light-receiving surface side electrode 12 and the n-type impurity diffusion layer 3 can be reduced, and the photoelectric conversion efficiency of the solar cell can be improved. Because the basic configuration of a solar cell formed according to the second embodiment is the same as that of the solar cell 1 in the first embodiment except for the structure of the n-type impurity diffusion layer 3, reference is made to the explanations and the drawings in the first embodiment.
A manufacturing method of a solar cell according to the second embodiment is explained with reference to
First, similarly to the case of the first embodiment, the p-type monocrystalline silicon substrate having a thickness of, for example, several hundreds of micrometers, is prepared as the semiconductor substrate 2 and a damaged area is removed. Subsequently, a high-concentration (low-resistance) n-type impurity diffusion layer 31 having a thickness of several hundreds of nanometers is formed on the surface of the light receiving surface side of the p-type monocrystalline silicon substrate by the method similar to that in the first embodiment. In the impurity diffusion at this time, phosphorus (P) is diffused in a high concentration (first concentration) so that the sheet resistance of the n-type impurity diffusion layer 31 becomes approximately 30 Ω/□ to 50 Ω/□.
Because a glassy (PSG: Phospho-Silicate Glass) layer deposited on the surface during a diffusion process is formed on the surface of the p-type monocrystalline silicon substrate immediately after formation of the n-type impurity diffusion layer 31, the phosphorus glass layer is removed by using a hydrofluoric acid solution or the like. Because the impurity diffusion is performed again in the subsequent processes, pn separation is not performed here.
The silicon nitride film (SiN film) 21, which becomes an etching mask, is then formed on the n-type impurity diffusion layer 31 with a film thickness of approximately 70 nanometers to 90 nanometers by the plasma CVD method (
Next, openings having a desired size are formed in the silicon nitride film (SiN film) 21 according to the size of the microasperity 2a in the inverted pyramid shape to be formed. The openings are formed by performing processing in two stages. Specifically, in the first processing step, the first openings 21a having shapes close to the target opening shape and sizes slightly smaller than the target opening size are formed (
In the first processing step, the first openings 21a having a diameter of approximately several tens of micrometers are formed in the silicon nitride film (SiN film) 21 by using etching paste. By using the etching paste, it is possible to process an etching mask having high productivity, that is, having high processing efficiency by simple processes, i.e., printing, heating up to a temperature at which etching proceeds, and cleaning. Because these methods to be used in the first processing step are inferior in controllability, that is, processing accuracy, for example, as shown in
In the second processing step, by irradiating the silicon nitride film (SiN film) 21 with a laser beam with a diameter being focused to approximately several micrometers, such as KrF excimer laser of 248 nanometers, or a frequency-doubled (532 nanometers) or frequency-tripled (355 nanometers) YAG laser, microfabrication (trimming) is performed to expand the first openings 21a up to the target opening shape, thereby forming the second openings 21b. By using the laser beam, processing of a fine etching mask having high controllability, that is, having high processing accuracy can be performed with a simple process.
In the second embodiment, in the region where the light-receiving surface side electrode 12, which includes the front silver grid electrodes 5 and the front silver bus electrodes 6, is formed in the subsequent processes, as shown in
Next, anisotropic etching is performed on the p-type monocrystalline silicon substrate with an etching solution in which IPA is added to a low-concentration alkaline solution, such as several wt % sodium hydroxide or potassium hydroxide, to form the inverted pyramid texture structure formed of the microasperity (texture) 2a in the inverted pyramid shape on the surface on the light receiving surface side of the p-type monocrystalline silicon substrate so as to expose the silicon (111) surface (
The silicon nitride film (SiN film) 21, which is the remaining etching mask, is then immersed in a hydrofluoric acid solution or the like and removed (
A low-concentration (high-resistance) n-type impurity diffusion layer 32 having a thickness of several hundreds of nanometers is then formed on the exposed surface of the p-type monocrystalline silicon substrate in the microasperity (texture) 2a in the inverted pyramid shape by performing the impurity diffusion process again (
Next, similarly to the case of the first embodiment, pn separation is performed for electrically insulating the back surface-side electrode 13, which is a p-type electrode, and the light-receiving surface side electrode 12, which is an n-type electrode, from each other. The phosphorus glass layer formed on the surface of the p-type monocrystalline silicon substrate at the time of forming the low-concentration (high-resistance) n-type impurity diffusion layer 32 is removed by using a hydrofluoric acid solution or the like. With this process, the semiconductor substrate 11 is acquired, in which a pn junction is formed by the semiconductor substrate 2 formed of the p-type monocrystalline silicon substrate, which is a first conductivity type layer, and the n-type impurity diffusion layer 3, which is a second conductivity type layer formed on the light receiving surface side of the semiconductor substrate 2 and includes the high-concentration (low-resistance) n-type impurity diffusion layer 31 and the low-concentration (high-resistance) n-type impurity diffusion layer 32 (not shown).
Thereafter, similarly to the case of the first embodiment, the anti-reflective film 4, the light-receiving surface side electrode 12, and the back surface-side electrode 13 are formed to complete a solar cell having the inverted pyramid texture structure.
As described above, in the manufacturing method of a solar cell according to the second embodiment, the process of forming the openings in the etching mask at the time of forming the inverted pyramid texture structure is performed by dividing the process into two stages, i.e., the first processing step of forming the first openings 21a having shapes close to the target opening shape and sizes slightly smaller than the target opening size by a method having relatively high productivity, that is, having high processing efficiency and the second processing step of forming the second openings 21b by expanding the first openings 21a up to the target opening shape by a method having relatively high processing controllability, that is, having high processing accuracy. With this process, the openings can be formed in the etching mask accurately, in a short time, and with simple and less number of processes.
Therefore, according to the manufacturing method of a solar cell of the second embodiment, the inverted pyramid texture structure can be formed with good productivity and with high accuracy, and the solar cell having excellent photoelectric conversion efficiency can be manufactured with good productivity.
Furthermore, in the manufacturing method of a solar cell according to the second embodiment, the inverted pyramid texture structure is formed and the selective emitter is also formed by changing the impurity concentration of the n-type impurity diffusion layer in the region under the light-receiving surface side electrode 12 to a high concentration. With this process, the contact resistance between the light-receiving surface side electrode 12 and the n-type impurity diffusion layer 3 can be reduced, and the photoelectric conversion efficiency of the solar cell can be improved.
By forming a plurality of solar cells having the configuration explained in the above embodiments, and electrically connecting adjacent solar cells, a solar cell module having an excellent optical confinement effect and excellent photoelectric conversion efficiency can be realized. In this case, it suffices that the light-receiving surface side electrode 12 of one of the adjacent solar cells and the back surface-side electrode 13 of the other one of the solar cells are electrically connected.
As described above, the manufacturing method of a solar cell according to the present invention is useful for improving the productivity of a solar cell having an inverted pyramid texture structure and excellent photoelectric conversion efficiency.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2012/056330 | 3/12/2012 | WO | 00 | 8/20/2014 |
