Patent Grant
12046687
The present invention relates to a solar cell with high photoelectric conversion efficiency and a method for manufacturing a solar cell with high photoelectric conversion efficiency.
Solar cell structures with relatively high photoelectric conversion efficiency which use single crystal or polycrystalline semiconductor substrates include a back-surface-electrode-type solar cell, in which all of the positive and negative electrodes are provided on the non-light-receiving surface (the back surface). The schematic view of the back surface of the back-surface-electrode-type solar cell 1000 is shown in
The cross sectional structure of the back-surface-electrode-type solar cell 1000 is schematically shown in
Such a structure is known in Patent Document 1, which discloses an example of a method for producing the solar cell having the structure. The outline of the process flow is shown in
Patent Document 2 discloses another example of the production method. The outline of the process flow is shown in
The foregoing known methods have been regarded to have a problem of including many steps. The mask forming step and the opening step have to be performed in a pair without exception, thereby increasing the production cost. The methods include many thermal treatment steps, in which a substrate is exposed to higher temperature, causing to reduce the lifetime of the minority carriers of the substrate.
In either of the methods, texture has to be formed only at one side of a substrate, which necessitates to form a mask only at one side. This requires procedures such as forming a silicon nitride film or the like on one side, as well as forming silicon oxide films on the both sides followed by forming resist on the entire surface of the backside, and dipping the substrate to HF to remove the silicon oxide film only at the light-receiving surface, increasing not only the number of steps but also the materials to be used. These methods also have a problem that the formation of uniform mask in a substrate surface becomes difficult when the thickness of the mask is intended to be a minimum.
In either of the methods, the emitter layer and the base layer are formed, with the back surface being flattened. Accordingly, these layer become difficult to electrically connect with electrodes, increasing the contact resistance between the substrate and the collecting electrode to decrease the conversion efficiency.
Regarding these problems, Patent Document 3 discloses an effective method with relatively few steps in which electrical connection of the base layer is improved. This method, however, also includes a step to remove a silicon oxide film on the light-receiving surface alone (i.e., to form a mask only at one side), causing the problems described above.
The present invention was accomplished in view of the above-described problems. It is an object of the present invention to provide a method for manufacturing a solar cell that can bring high photoelectric conversion efficiency while decreasing the number of steps. It is also an object of the present invention to provide a solar cell in which the contact resistance is decreased to improve the photoelectric conversion efficiency.
To solve the problems described above, the present invention provides a method for manufacturing a solar cell, including the steps of:
forming unevenness on both of main surfaces of a semiconductor substrate of a first conductivity type;
forming a base layer of the first conductivity type, having a dopant concentration higher than in the semiconductor substrate, on a first main surface of the semiconductor substrate;
forming a diffusion mask on the base layer;
removing the diffusion mask in a pattern to have a remaining diffusion mask at other than a portion where the diffusion mask have been removed;
forming an emitter layer of a second conductivity type which is an opposite conductivity type to the first conductivity type, on the portion of the first main surface where the diffusion mask have been removed;
removing the remaining diffusion mask;
forming a dielectric film on the first main surface;
forming a base electrode on the base layer; and
forming an emitter electrode on the emitter layer.
The inventive method can decrease the step of forming and opening a diffusion mask to one time. In the first step, fine unevenness (e.g., texture) is formed, and it is not necessary to form a mask on only one side. In a solar cell manufactured by the inventive method, fine unevenness such as texture is formed on the base layer region (the base layer). This can decrease the contact resistance between the base layer and an electrode (i.e., the base electrode) connected therewith to improve the conversion efficiency.
It is preferable that the surface of the semiconductor substrate be subjected to etching on the portion where the diffusion mask have been removed after the step of removing the diffusion mask in a pattern and before the step of forming the emitter layer.
The conversion efficiency can be improved by forming the emitter layer after etching the base layer of the opening (the portion where the mask have been removed) as described above.
After forming the emitter layer, the film thickness of a silicon oxide film on the emitter layer can be 95 nm or less.
In the inventive method, the mask formation is not necessary after forming the emitter layer, and it is not necessary to form silicon oxide after forming the emitter layer.
It is preferable that the first conductivity type be P-type, and the second conductivity type be N-type; in the step of forming the base layer, a glass layer be formed on the first main surface simultaneously with forming the base layer; and in the step of forming the diffusion mask, the diffusion mask be formed on the base layer with the glass layer being left.
The minority carrier lifetime of the substrate can be kept high by forming the diffusion mask while leaving the glass layer formed simultaneously with forming a P-conductivity type layer as the base layer.
It is also appropriate that the first conductivity type be P-type, and the second conductivity type be N-type; and the base electrode and the emitter electrode be formed after forming the dielectric film without removing the dielectric film.
Forming the P-type base layer on the portion where the texture have been formed makes it possible to realize lower contact resistance between the base (P-type) layer and the collecting electrode without opening the dielectric film.
It is also preferable that the first conductivity type be N-type, and the second conductivity type be P-type; and the step of forming the dielectric film be a step of forming an aluminum oxide film to cover the base layer and the emitter layer and forming a silicon nitride film on the aluminum oxide film.
In back-surface-electrode-type solar cells, most of the back surfaces are normally emitter layers. When the emitter layer is P-type, high photoelectric conversion efficiency can be given in a convenient way by covering the back surface with an aluminum oxide film, which is effective as P-type passivation.
In the step of forming the base layer, the base layer is preferably formed on an entire surface of the first main surface.
Such a method for manufacturing a solar cell can easily manufacture a solar cell in which the base layer and the emitter layer are contiguous to each other.
It is preferable that the unevenness be texture.
Such a method for manufacturing a solar cell shows higher productivity.
The present invention also provides a solar cell including:
a semiconductor substrate of a first conductivity type;
a base layer of the first conductivity type, having a dopant concentration higher than in the semiconductor substrate, and an emitter layer of a second conductivity type, being an opposite conductivity type to the first conductivity type, each of the layer being provided on a first main surface of the substrate;
a dielectric film provided on the base layer and the emitter layer;
a base electrode electrically connected with the base layer; and
an emitter electrode electrically connected with the emitter layer;
wherein, a surface of the semiconductor substrate is provided with unevenness formed at least at the contact interface between the base electrode and the base layer; and
the first main surface has a recess in a pattern, with the surface of the recess being flat, and the emitter layer is formed on the surface of the recess.
By forming the fine unevenness (e.g., texture) on the portion being in contact with the electrode of the base region as described above, it is possible to realize lower contact resistance between the base layer and the collecting electrode without opening the dielectric film. As a result, the contact resistance can be decreased to make the solar cell have improved photoelectric conversion efficiency. As described above, the inventive solar cell also has an emitter region, the surface of which is recessed from its surroundings, with the surface being flat. The flat surface decreases the rate of re-combination of minority carriers on the surface, which contributes to improve the photoelectric conversion efficiency. It is to be noted that such a structure can be produced by removing the diffusion mask in a pattern and etching the base layer at a portion where the mask have been removed, followed by forming the emitter layer as described above.
It is preferable that the first conductivity type be N-type, and the second conductivity type be P-type; and the dielectric film have a layered structure of an aluminum oxide film and a silicon nitride film, with the aluminum oxide film being in contact with the first main surface.
In back-surface-electrode-type solar cells, most of the back surfaces are normally emitter layers. When the emitter layer is P-type, high photoelectric conversion efficiency can be given in a convenient way by covering the back surface with an aluminum oxide film, which is effective as P-type passivation.
It is preferable that the base layer and the emitter layer be contiguous to each other.
Such a solar cell can be easily fabricated.
It is preferable that the semiconductor substrate be provided with unevenness formed on a second main surface of the substrate.
In such a solar cell, the second main surface can have more decreased reflectance.
It is preferable that the unevenness be texture.
Such a solar cell can be easily fabricated.
The present invention further provides a photovoltaic module including the solar cell of the present invention described above built-in.
As described above, the solar cell according to the present invention can be installed in a photovoltaic module.
The present invention further provides a solar photovoltaic power generation system including the photovoltaic module of the present invention described above.
As described above, the photovoltaic module with the built-in solar cell of the present invention can be used for a solar photovoltaic power generation system.
By the inventive method, a solar cell with high photoelectric conversion efficiency can be manufactured while largely decreasing the number of steps. This makes it possible to manufacture a back-surface-electrode-type solar cell with high photoelectric conversion efficiency at low cost. The inventive solar cell has lower contact resistance and excellent conversion efficiency.
Hereinafter, the present invention will be described more specifically.
As described above, it has been required for a method of manufacturing a solar cell that can give high photoelectric conversion efficiency while decreasing the number of steps.
The present inventors have diligently investigated to achieve the forgoing object. As a result, the inventors have found that the foregoing object can be solved by a method for manufacturing a solar cell, including the steps of:
forming unevenness on both of main surfaces of a semiconductor substrate of a first conductivity type;
forming a base layer of the first conductivity type, having a dopant concentration higher than in the semiconductor substrate, on a first main surface of the semiconductor substrate;
forming a diffusion mask on the base layer;
removing the diffusion mask in a pattern to have a remaining diffusion mask at other than a portion where the diffusion mask have been removed;
forming an emitter layer of a second conductivity type, being an opposite conductivity type to the first conductivity type, on the portion of the first main surface where the diffusion mask have been removed;
removing the remaining diffusion mask;
forming a dielectric film on the first main surface;
forming a base electrode on the base layer; and
forming an emitter electrode on the emitter layer; thereby brought the present invention to completion.
In addition, it has been required for a solar cell in which the contact resistance is decreased to improve the photoelectric conversion efficiency as described above.
The present inventors have diligently investigated to achieve the forgoing object. As a result, the inventors have found that the foregoing object can be solved by a solar cell including:
a semiconductor substrate of a first conductivity type;
a base layer of the first conductivity type, having a dopant concentration higher than in the semiconductor substrate, and an emitter layer of a second conductivity type, being an opposite conductivity type to the first conductivity type, each of the layer being provided on a first main surface of the substrate;
a dielectric film provided on the base layer and the emitter layer;
a base electrode electrically connected with the base layer; and
an emitter electrode electrically connected with the emitter layer;
wherein, a surface of the semiconductor substrate is provided with unevenness formed at least at the contact interface between the base electrode and the base layer; and
the first main surface has a recess in a pattern, with the surface of the recess being flat, and the emitter layer is formed on the surface of the recess; thereby brought the present invention to completion.
In the following detailed description, to understand the overall invention and show how the invention is carried out in a given specific example, many given details will be explained. However, it can be understood that the present invention can be carried out without these given details. To avoid obscureness of the present invention, a known method, a procedure, and technologies will not be described in detailed hereinafter. Although a given specific example of the present invention will be described with reference to given drawings, the present invention is not restricted thereto. The drawings described herein are schematic, and do not restrict the scope of the present invention. Further, in the drawings, for the purpose of illustration, sizes of several elements are exaggerated, and hence a scale may not be correct.
[Solar Cell]
Hereinafter, the inventive solar cell will be described by referring to the drawings, but the present invention is not limited thereto.
As shown in
As shown in
In addition to the foregoing structure, the inventive solar cell is provided with the unevenness 168 formed on the surface of the semiconductor substrate at least the contact interface between the base electrode 123 and the base layer 113 as shown in
In the inventive solar cell, the first main surface has a recess, the surface of which is flat (see the recess 158 in a pattern of
The height of the unevenness is not particularly limited, but can be 1 to 50 μm, for example. In the range of 1 to 50 μm, the antireflection effect becomes large, and the formation can be performed relatively easily.
Each of the unevenness 168 and 169 is preferably texture. Such a solar cell can be easily fabricated.
It is also preferable that the first conductivity type be N-type, the second conductivity type be P-type, and the dielectric film 144 have a layered structure of an aluminum oxide film and a silicon nitride film in which the aluminum oxide film is in contact with the first main surface. In back-surface-electrode-type solar cells, most of the back surfaces are normally emitter layers. When the emitter layer is P-type, high photoelectric conversion efficiency can be given in a convenient way by covering the back surface with an aluminum oxide film, which is effective as P-type passivation.
Illustrative examples of the N-type dopant include P (phosphorus), Sb (antimony), As (arsenic), and Bi (bismuth). Illustrative examples of the P-type dopant include B (boron), Ga (gallium), Al (aluminum), and In (indium).
The dopant concentration of the semiconductor substrate 110 having the first conductivity type is not particularly limited, but can be 8×1014 atoms/cm3 or more and 1×1017 atoms/cm3 or less, for example. The thickness of the semiconductor substrate 110 is not particularly limited, but can be a thickness of 100 to 300 μm, for example. The dopant concentration of the base layer 113 can be any value higher than that of the semiconductor substrate 110, but can be 1.0×1018 atoms/cm3 or more and 2.0×1021 atoms/cm3 or less, for example. The dopant concentration of the emitter layer 112 is not particularly limited, but can be 1.0×1018 atoms/cm3 or more and 7.0×1020 atoms/cm3 or less, for example.
It is also preferable that the base layer 113 and the emitter layer 112 adjoin with each other. Such a solar cell can be easily fabricated.
[Method for Manufacturing Solar Cell]
An outlined flow diagram of a process of the inventive method is shown in
First, an N-type as-cut single crystal silicon substrate having plane orientation of {100} is prepared, with the specific resistance is set to 0.1 to 5 Ω·cm by doping high-purity silicon with a pentavalent element such as phosphorus, arsenic, or antimony. The single crystal silicon substrate can be prepared by either a CZ method or an FZ method. The substrate does not necessarily have to be a single crystal silicon, but may be a polycrystalline silicon.
Subsequently, each fine unevenness 168 and 169 called texture is formed on both of the main surface of the semiconductor substrate 110 as shown in
The semiconductor substrate 110 subjected to texture formation as described above is cleaned in acidic aqueous solution including hydrochloric acid, sulfuric acid, nitric acid, hydrofluoric acid, or mixture thereof. It is also possible to mix hydrogen peroxide to improve the cleanliness.
Then, on the first main surface of this semiconductor substrate 110, the base layer 113 is formed as shown in
It is to be noted that the base layer 113 is preferably formed on the entire surface of the first main surface in the step of
After forming the base layer 113, the diffusion masks (alias: barrier film, hereinafter also referred to as a “mask” simply) 156 are formed on both of the main surfaces as shown in
Next, the mask is opened at a portion to be an emitter region (mask opening 157) as shown in
Subsequent to opening the mask, the semiconductor substrate 110 may be dipped into aqueous solution containing alkali such as KOH and NaOH in high concentration (preferably in a concentration higher than the concentration when the texture is formed, e.g., 10 to 30%, preferably 20 to 30%) heated to 50 to 90° C. for 1 to 30 minutes as shown in
The diffusion mask 156 also functions as a mask for alkali etching in this step (
It is to be noted that the position (height) of the recess 158 in a pattern can be a lower (deeper) position than the standard position, which is based on the position of the recess of the back surface unevenness 168, as shown in
Then, the emitter layer 112 is formed on the opening as shown in
After forming the diffusion layer, the diffusion mask 156 and glass formed on the surface are removed by hydrofluoric acid or the like (see
Subsequently, the dielectric film 144 is formed on the first main surface of the semiconductor substrate 110 as shown in
As the antireflection coating 145 on the second main surface, a silicon nitride film or a silicon oxide film can be used. In an instance of the silicon nitride film, a film of about 100 nm is formed by using a plasma CVD apparatus. As the reaction gas, mixed gas of monosilane (SiH4) and ammonia (NH3) is often used, but nitrogen can be used instead of the NH3. The reaction gas may be mixed with hydrogen to control the process pressure, to dilute the reaction gas, or to accelerate the bulk passivation effect of a substrate when a polycrystalline silicon is used as the substrate. The silicon oxide film can be formed by a CVD method, but the film formed by a thermal oxidation method achieves higher properties. In order to improve the effect for protecting the surface, the silicon nitride film or the silicon oxide film may be formed after forming an aluminum oxide film on the substrate surface previously.
The dielectric film 144 of a silicon nitride film or a silicon oxide film can be utilized for the first main surface as a surface protective film. The dielectric film 144 preferably has a film thickness of 50 to 250 nm. As on the second main surface (the light-receiving surface), the silicon nitride film can be formed by a CVD method, and the silicon oxide film can be formed by a thermal oxidation method or a CVD method. When the substrate is N-type as in this instance, the silicon nitride film or the silicon oxide film can be formed onto the substrate surface that has a previously formed aluminum oxide film, which is effective as a passivation of a P-type layer. In the method of
Then, the base electrode 123 is formed on the base layer 113 by screen printing method, for example, as shown in
After the foregoing electrode printing, thermal treatment (firing) is performed to promote sintering of the electrode, together with electrical connection between the substrate and the electrode. The firing is commonly performed by treating at a temperature of 700 to 850° C. for 1 to 5 minutes. It is to be noted that the electrode for a base layer and the electrode for an emitter layer can be also fired separately.
Subsequently, the step of forming a bus bar electrode will be described by reference to
Lastly, the base bus bar electrode 233 and the emitter bus bar electrode 232 are formed to make a structure shown in
By the inventive method shown in
The inventive method can eliminate the step of forming a mask only at one side as described above, thereby making it possible to manufacture a solar cell at lower cost compared to the method of Patent Document 3. In the solar cell of the Patent Document 3, the emitter layer 112 protrudes over the base layer 113; the inventive method can manufacture a solar cell in which the base layer 113 protrudes over the emitter layer 112 as shown in
The solar cell manufactured by the foregoing method can be used for fabricating a photovoltaic module.
In the photovoltaic module 560, several to several tens of the contiguous solar cells 500 are electrically connected with each other in series to constitute a series circuit called string. The schematic view of the string is shown in
A cross sectional schematic view of the photovoltaic module 560 is shown in
This photovoltaic module can be used for fabricating and constituting a solar photovoltaic power generation system.
Hereinafter, the present invention will be described in more specifically by showing Examples and Comparative Example, but the present invention is not limited the following Examples.
Solar cells were manufactured by using the inventive method.
First, eight pieces of N-type as-cut silicon substrates with the plane orientation of {100} doped with phosphorus having a thickness of 200 μm and a specific resistance of 1 Ω·cm were prepared. Each of these silicon substrate was subjected to dipping into 2% aqueous potassium hydroxide/2-propanol solution at 72° C. to form textures onto the both surfaces of the substrate, and then cleaned in mixed solution of hydrochloric acid/hydrogen peroxide heated to 75° C. (see
Subsequently, the substrates were subjected to thermal treatment at 870° C. for 40 minutes in a phosphorus oxychloride atmosphere, with the light-receiving surfaces being stacked with each other to form phosphorus diffusion layers (base layers) (see
This was subjected to thermal oxidation at 1000° C. for 3 hours in an oxygen atmosphere to form masks (see
The mask on the back surface was opened with laser (see
This was dipped into aqueous KOH solution with the concentration of 24% at 80° C. to remove the base layer at the openings (see
Then, the substrates were placed in a thermal treatment furnace with the two pieces thereof being stacked with each other as a pair, and were subjected to thermal treatment at 1000° C. for 10 minutes, while introducing a mixed gas of BBr3, oxygen, and argon. In this way, an emitter layer was formed (see
Subsequently to the foregoing treatment, aluminum oxide films and silicon nitride films were formed on the both surfaces as dielectric films by using a plasma CVD apparatus (see
Next, Ag paste was printed by using a screen printing machine on the base layer and dried; on the emitter layer, it was printed along the opening regions of the aluminum oxide/silicon nitride film and dried (see
Onto this substrate, insulator material was printed in a pattern by using a screen printing machine. As the insulator material, silicone manufactured by Shin-Etsu Chemical Co., Ltd. was used. This was cured in a belt furnace at 200° C. for 5 minutes.
Lastly, low-temperature curing Ag paste was printed in a shape of six lines so as to intersect to the finger electrodes that had been already fabricated at right angles by using a screen printing machine, and then cured in a belt furnace at 300° C. for 30 minutes to form bus bars.
Solar cells were manufactured by the same method as in Example 1 up to the laser opening and from the step of boron diffusion without performing the step of dipping to aqueous KOH solution at 80° C.
The same treatment as in Example 1 was performed up to the laser opening and the step of dipping to aqueous KOH solution at 80° C. Subsequently, thermal treatment was performed at 1000° C. for 10 minutes in an atmosphere of a mixed gas of BBr3, oxygen, and argon, with the light-receiving surfaces being stacked with each other, followed by oxidation treatment at 1000° C. in an oxygen atmosphere to form a boron diffusion layer and 100 nm of a silicon oxide film at the openings. From the step of removing the surface glass by dipping into 25% hydrofluoric acid, the same procedure as in Example 1 was performed.
The inventive method was applied to P-type silicon substrates with the plane orientation of {100} doped with boron. Texture was formed on the both surfaces of the substrate and cleaned. Then, the substrates were placed in a thermal treatment furnace with the two pieces thereof being stacked with each other as a pair, and were subjected to thermal treatment at 1000° C. for 10 minutes, while introducing a mixed gas of BBr3, oxygen, and argon, followed by thermal oxidation at 1000° C. for 3 hours in an oxygen atmosphere to form masks.
The mask on the back surface was opened with laser, and the boron diffusion layer at the openings was removed by dipping into aqueous KOH solution.
Subsequently, thermal treatment was performed at 870° C. for 40 minutes in a phosphorus oxychloride atmosphere to form phosphorus diffusion layer at the openings.
The surface glass was removed by dipping into hydrofluoric acid, and then silicon nitride films were formed on the both surfaces.
The step of forming electrodes was performed in the same way as in Example 1.
The same treatment as in Example 4 was performed up to the formation of silicon nitride films on the both surfaces. Subsequently, the electrodes were formed without opening the silicon nitride film at the regions to be in contact with emitter electrodes when forming the electrodes.
For comparison, solar cells without having texture on the surface of each base layer were manufactured.
First, slice damage on the substrate was etched with 25% aqueous KOH solution at 70° C. After cleaning, about 50 nm of a silicon nitride film was formed as a texture mask on the one side only, using a CVD apparatus.
After forming the texture in the same way as in Examples, the silicon nitride film was removed with 25% aqueous hydrofluoric acid solution, followed by cleaning. It was observed that the texture was formed on the only one side by visual inspection. After the step of phosphorus oxychloride diffusion, the same procedure as in Example 1 was performed.
On the solar cell samples of Examples 1 to 5 and Comparative Example obtained as described above, current-voltage characteristics were measured under the conditions of spectrum: AM1.5, light intensity: 100 mW/cm2, and 25° C. by using a solar simulator manufactured by Yamashita Denso Corporation to determine photoelectric conversion efficiency. The average values of the obtained results are shown in Table 1.
As shown in Table 1, Example 1, in which the number of steps was decreased, showed higher conversion efficiency compared to that of Comparative Example. Having the texture under the base electrodes, the contact resistance between the base layer and the electrodes was improved, and the fill factor was higher. Another reason will be that the occasions of contamination was decreased due to the decrease of the number of steps, thereby improving the lifetime.
The conversion efficiency of Example 2 was equivalent to that of Example 1. High conversion efficiency can be achieved even without etching after the opening.
The conversion efficiency of Example 1 was equivalent to that of Example 3. The conversion efficiency is not lowered even when the thickness of the oxide film is thin after forming the emitter layer.
Example 4 showed higher conversion efficiency compared to that of Comparative Example. The inventive method makes it possible to bring higher conversion efficiency also in a P-type substrate.
The conversion efficiency of Example 5 was equivalent to that of Example 4. When the substrate is P-type, low contact resistance and high conversion efficiency can be achieved without opening the back surface protective film, due to the texture at the contact portions of the base electrodes.
It is to be noted that the present invention is not limited to the foregoing embodiment. The embodiment is just an exemplification, and any examples that have substantially the same feature and demonstrate the same functions and effects as those in the technical concept described in claims of the present invention are included in the technical scope of the present invention.
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