JP6114205B2 - Manufacturing method of solar cell - Google Patents

Description

  The present invention relates to a method for manufacturing a highly efficient solar cell.

  A schematic diagram of a general solar cell using a single crystal or polycrystalline silicon (Si) substrate is shown in FIG. A p-type silicon substrate to which a group III element such as boron (B), gallium (Ga), or indium (In) is added is mainly used for the substrate 101, and the light receiving surface of the substrate 101 is used for light confinement. The concavo-convex structure (texture) 102 is formed. The texture 102 is obtained by immersing the substrate 101 in an acidic or alkaline solution for a certain time. In general, a mixed acid solution in which acetic acid, phosphoric acid, sulfuric acid, water, etc., is mixed with hydrofluoric acid is used as the acidic solution. When the substrate 101 is immersed in the acid solution, fine grooves on the rough surface during substrate processing are preferentially etched. As a result, a texture is formed. As the alkaline solution, potassium hydroxide, sodium hydroxide aqueous solution, or tetramethylammonium hydroxide aqueous solution is used. Alkali etching advances etching by forming Si—OH bonds, so that the etching rate depends on the crystal plane orientation, so that a texture with a surface with a slow etching rate exposed can be obtained.

  In addition, the light receiving surface of the substrate 101 is thermally diffused with a group V element such as phosphorus (P), arsenic (As), or antimony (Sb) to have a first high conductivity type opposite to that of the substrate 101. A concentration layer 103 is formed.

  Further, a protective film 104 is formed on the first high concentration layer 103 so as to cover it. The protective film 104 has the following two roles.

  The first role is a role as an antireflection film for maximally capturing light incident on the solar cell, and a dielectric having a refractive index smaller than that of crystalline silicon and larger than that of air is used. Specifically, titanium oxide, silicon nitride, silicon carbide, silicon oxide, aluminum oxide, etc. can be used, and these film thicknesses vary depending on the refractive index of the film, but in the case of a silicon nitride film, a light receiving surface is generally used. Is about 80 to 100 nm.

  The second role is to suppress carrier recombination on the silicon surface. It serves as a passivation that terminates defects on the surface of the silicon substrate that extinguish photogenerated carriers. The silicon atoms inside the crystal are in a stable state due to covalent bonding between adjacent atoms. However, an unstable energy level called a dangling bond or a dangling bond appears due to the absence of adjacent atoms to be bonded on the surface which is the terminal of the atomic arrangement. Since dangling bonds are electrically active, they capture and extinguish charges generated in the silicon, thereby deteriorating the characteristics of the solar cell. In order to suppress this loss, the solar cell is subjected to some surface termination treatment to reduce dangling bonds, or by giving an electric charge to the antireflection film, thereby increasing the concentration of either electrons or holes on the surface. The recombination of electrons and holes is suppressed by drastically decreasing. In particular, the latter is called field effect passivation. A silicon nitride film or the like is known to have a positive charge, and is well known as field effect passivation.

  Further, an electrode 105 for extracting photogenerated carriers is formed on the first high concentration layer 103 so as to penetrate the protective film 104. As a method of forming this electrode, a method of printing a metal paste obtained by mixing metal fine particles such as silver in an organic binder with a screen plate using a screen plate and bonding it to a substrate by heat treatment is widely used. Yes. The electrode is generally formed after the dielectric film is formed. Therefore, in order to bring the electrode and silicon into contact, it is necessary to remove the dielectric film between the electrode and silicon. However, the metal paste penetrates the protective film 104 by adjusting the glass component or additive in the metal paste. In other words, so-called fire-through is possible in contact with silicon.

  On the other hand, on the non-light-receiving surface opposite to the light-receiving surface, a second high-concentration layer in which impurities that express the same conductivity type as the substrate 101 are diffused at a high concentration in order to suppress recombination of photogenerated carriers. 106 is formed, and an electrode 107 is formed so as to cover the second high concentration layer 106.

  As a method for forming the second high-concentration layer 106, from the viewpoint of cost, an aluminum paste in which aluminum (Al) fine particles are mixed with an organic binder is printed on the p-type silicon substrate using a screen plate or the like. In general, heat treatment is performed at a temperature equal to or higher than the eutectic point of aluminum and aluminum (577 ° C.). When heat treatment is performed at this temperature, the silicon recrystallizes while taking in a large amount of aluminum during the cooling process, and a base layer is formed.

  In the process of heat treatment and recrystallization, most of the aluminum paste away from the contact interface with silicon remains as it is and becomes the electrode 107.

  However, on the other hand, the second high concentration layer-electrode structure using aluminum has a limited carrier recombination suppressing effect on the back surface of the solar cell, and further has a large optical loss due to a large light absorption coefficient. was there.

  Therefore, in order to avoid these problems and increase the efficiency of the solar cell, a so-called PR (Passive Rear) structure type solar cell as shown in FIG. 2 has been proposed. The PR structure is characterized in that the non-light-receiving surface of the substrate 201 is flattened, covered with a protective film 207 having a high passivation effect, and the base layer 206 and the electrode 208 are localized to reduce the surface recombination of carriers. It is.

  The light-receiving surface is textured and the non-light-receiving surface is smoothed by immersing the sliced silicon substrate in an alkali or acid solution for damage etching, and then exposing only the light-receiving surface to reactive ions or etching gas. Then, the texture etched method is used, and the substrate after damage etching is further immersed in alkali or acid solution to form texture on both sides of the substrate, and then the texture on the non-light-receiving surface is wet using a spin etcher or conveyor type single-sided etching device There is a method of etching.

  As a method of locally forming the base layer and the electrode, an opening is formed by patterning a protective film with photolithography or etching paste, impurities are added to the opening by thermal diffusion, and then aluminum or silver ( A method of forming a metal such as Ag) on the second high concentration layer by physical vapor deposition is generally used (Non-Patent Document 1).

  However, these methods have the problems of low productivity, complicated processes and high costs. Therefore, at present, by applying the fire-through, which is usually used for forming the light-receiving surface electrode of solar cells, to the non-light-receiving surface, high-cost processes such as protective film patterning and metal physical vapor deposition are omitted. Studies have been made to produce a PR structure at low cost (for example, Non-Patent Document 2).

J. et al. Knobloc, et. al. , IEEE PVSC (1993) pages 271-276 A. Weber, et. al. , Proc. the 24th EUPSEC (2009) 891-895

  However, with respect to the smoothed surface such as the PR structure shown in FIG. 2, the sinterable conductive paste does not provide sufficient adhesion and electrical contact with the substrate, and the electrode peels off, There was a problem that resistance loss increased.

  As a result of intensive investigations to achieve the above object, the present inventors smoothed only the non-electrode forming region on the non-light-receiving surface of the solar cell, thereby achieving high productivity and good output characteristics. The present inventors have found that a solar cell having high reliability can be obtained.

  That is, in the method for manufacturing a solar cell according to the embodiment of the present invention, the step of forming a concavo-convex structure on at least a part of the non-light-receiving surface of the crystalline silicon substrate, and the additive impurity concentration on at least a part of the light-receiving surface of the substrate Forming a first high-concentration layer having a conductivity type higher than that of the substrate and having a conductivity type different from that of the substrate; and an additive impurity concentration higher than that of the substrate on the non-light-receiving surface of the substrate and the same conductivity as the substrate The oxide film formed in the step of locally forming the second high concentration layer having a mold, the step of thermally oxidizing the substrate, and the step of thermally oxidizing is etched to form the second high concentration layer on the non-light-receiving surface A step of exposing the silicon surface of the substrate in at least a part of the region other than the formed region, and a collecting electrode for extracting the charge excited by the light incident on the substrate to the outside on the first high concentration layer and the second high concentration layer And a step of forming, a step of smoothing at least a portion of the concavo-convex structure in the region other than the first high concentration layer and a second region where the high concentration layer formed in the non-light-receiving surface of the substrate.

  In the present invention, in the thermal oxidation step, the oxide film is formed on the first high concentration layer and the second high concentration layer than on the region other than the region where the first high concentration layer and the second high concentration layer are formed. It is good to form thickly.

  In the present invention, in the exposing step, the oxide film formed in a region other than the region where the first high concentration layer and the second high concentration layer are formed may be selectively removed in an acid solution.

  In the present invention, the thermal oxidation step may be performed by forming an oxide film having a thickness of 20 nm or more and 200 nm or less in the region where the second high concentration layer of the non-light-receiving surface is formed.

  In the present invention, the smoothing process may include a process of etching the exposed silicon with an alkaline solution.

  In the present invention, the alkaline solution may be selected from any of sodium hydroxide, potassium hydroxide, tetramethylammonium hydroxide, or a combination thereof.

  In the present invention, the step of forming the second high concentration layer may include a step of patterning the second high concentration layer in a region where the electrode is formed.

  In the present invention, when a p-type substrate is employed, the step of forming the second high-concentration layer may include a step of adding boron to the substrate.

  In the present invention, when an n-type substrate is employed, the step of forming the second high concentration layer includes a step of adding phosphorus to the substrate, and the step of thermal oxidation includes a heat treatment at 800 ° C. or higher and 950 ° C. or lower. It may be configured as follows.

It is a figure which shows the structure of the general solar cell by a prior art. It is a figure which shows the structure of the general solar cell by a prior art. It is a figure which shows the process of the solar cell substrate which concerns on this invention.

[First Embodiment]
Hereinafter, an example of the manufacturing method of the solar cell according to the first embodiment of the present invention will be described with reference to FIG. However, the present invention is not limited to the solar cell produced by the method of the present embodiment.

  An as-cut n-type crystalline silicon substrate having a resistivity of 0.1 to 5 Ω · cm by doping high purity silicon with a group V element such as phosphorus, arsenic or antimony is prepared, Etching is performed using a high concentration alkali such as 60% sodium hydroxide or potassium hydroxide, or a mixed acid of hydrofluoric acid and nitric acid. The crystalline silicon substrate may be produced by any method of cast method, Cz method or FZ method.

Subsequently, the texture 302 is formed on both surfaces of the substrate 301 (FIG. 3A). Texture 302 can be applied in an alkaline solution (concentration 1 to 10%, temperature 60 to 100 ° C.) such as heated sodium hydroxide, potassium hydroxide, potassium carbonate, sodium carbonate, sodium hydrogen carbonate, tetramethylammonium hydroxide for 10 minutes. It is easily produced by dipping for about 30 minutes. In many cases, a predetermined amount of 2-propanol is dissolved in the solution to control the reaction. When this method is applied to a single crystal silicon substrate having a crystal plane orientation <100>, a so-called random pyramid structure in which a large number of plane orientations <111> are exposed in a pyramid shape is obtained. On the other hand, in the case of a polycrystalline silicon substrate having a random crystal plane orientation, the texture cannot be formed uniformly in the plane by the above method. For example, H ₂ , CHF ₃ , SF ₆ , CF ₄ , C ₂ F ₆ , C A method of etching silicon with reactive ions excited by a high frequency with a gas such as ₃ F ₈ or ClF ₃ , more preferably an acidic mixed solution of hydrogen fluoride, nitric acid, acetic acid, phosphoric acid, etc. A method of immersing the substrate in can be applied. As a more specific method of the latter acid etching, for example, an etchant mixed with nitric acid having a concentration of 15 to 31 wt% and hydrofluoric acid having a concentration of 10 to 22 wt% may be used. More preferably, acetic acid is further mixed in the mixed acid solution by 10 to 50 w%. By dipping the silicon substrate for about 10 to 30 minutes at a liquid temperature of 5 to 30 ° C., an arc-shaped isotropic texture is easily obtained. In order to enhance the light confinement effect, the texture height difference is generally formed to be about several μm to 10 μm.

Next, a first high concentration layer (emitter layer) 303 is formed (FIG. 3B). In general, BBr ₃ is preferably used, and boron (B) is added to the substrate 301 by a vapor phase diffusion method at 900 to 1100 ° C. Further, the present invention is not limited thereto, and boron may be added using a compound deposited by a chemical vapor deposition method or a compound that can be screen-printed or spin-coated.

  The first high-concentration layer 303 needs to be formed only on the light receiving surface. To achieve this, the non-light receiving surface is diffused with the two substrates 301 facing each other, or silicon nitride is formed on the non-light receiving surface. Thus, it is necessary to devise measures such that an additive impurity is not added to the non-light-receiving surface.

The additive impurity concentration on the surface of the first high-concentration layer 303 is preferably 1 × 10 ¹⁹ or more and 3 × 10 ²⁰ atoms / cm ³ , more preferably 5 × 10 ¹⁹ or more and 1 × 10 ²⁰ atoms / cm ³ . It is good to do. If it is less than 1 × 10 ¹⁹ atoms / cm ³ , the contact resistance between the substrate and the electrode is increased, and if it is 3 × 10 ²⁰ atoms / cm ³ or more, defects in the first high concentration layer and charge carriers due to Auger recombination The recombination becomes remarkable and the output of the solar cell decreases. After the first high concentration layer is formed, the glass formed on the surface is removed with hydrofluoric acid or the like.

Next, a second high-concentration layer (base layer) 304 is formed in an electrode formation region where the electrode 308 is formed in a subsequent process (FIG. 3C). In general, a diffusion barrier such as silicon nitride is formed on the entire surface of the non-light-receiving surface, an electrode formation region is opened with an etching paste that can be photolithography or screen-printed, and then a gas phase is formed at a temperature of 800 to 1000 ° C. using POCl _3. Phosphorus (P) is added to the substrate 301 by a diffusion method.

  In general silicon solar cells, the second high-concentration layer 304 needs to be formed only on the non-light-receiving surface, and in order to achieve this, the light-receiving surfaces of the two substrates 301 are overlapped and diffused. In addition, it is necessary to devise measures to prevent phosphorus from diffusing on the light receiving surface by forming a diffusion barrier such as silicon nitride on the light receiving surface side.

  As another method, phosphorus ionized by ion implantation through a mask patterned into the shape of the electrode formation region may be added to the substrate 301, or a compound that can be screen printed is added to the shape of the electrode formation region. In addition, the impurity may be added to the substrate 301 by heat-treating the substrate 301.

The surface impurity concentration of the second high-concentration layer 304 is preferably 1 × 10 ¹⁹ or more and 1 × 10 ²¹ atoms / cm ³ , more preferably 5 × in order to obtain good electrical contact between the substrate and the electrode. It is preferable to be about 10 ¹⁹ or more and 1 × 10 ²¹ atoms / cm ³ or less. If it is less than 1 × 10 ¹⁹ atoms / cm ³ , the contact resistance between the substrate and the electrode increases, and the output of the solar cell decreases. 1 × 10 ²¹ atoms / cm ^{3 or more} is the solid solubility limit of phosphorus in silicon.

  Next, thermal oxidation is performed to form an oxide film 305 on the substrate surface (FIG. 3D). The growth rate of the thermal oxide film 305 depends on the added impurity concentration on the surface of the silicon substrate, and has a property that the growth is promoted on the surface of the high concentration layer. In this embodiment, the first high-concentration layer is uniformly formed on the light-receiving surface, while the second high-concentration layer is locally formed on the non-light-receiving surface. In the region excluding the concentration layer formation region, the oxide film 305 is formed relatively thin. Therefore, by immersing the substrate in a hydrofluoric acid aqueous solution in the subsequent process, only the oxide film 305 on the surface of the non-diffusion region on the non-light-receiving surface is selectively removed to expose the silicon surface (FIG. 3E).

  The thermal oxide film 305 has a film thickness of 20 nm or more and 200 nm or less, preferably about 40 to 70 nm because of the necessity and productivity to sufficiently cover the surface of the second high concentration layer even after the etching process. It is good to form like this.

Not particularly limited to the method of thermal oxidation, but for example, dry O ₂ oxidation, wet oxidation or pyrogenic oxidation in a quartz tube is preferably used. Since the thickness of the oxide film 305 varies depending on the impurity concentration on the substrate surface as described above, it is in balance with the diffusion conditions. Generally, it is about 20 to 90 minutes at 700 ° C. to 950 ° C., preferably 30 to 800 ° C. to 920 ° C. It should be done for ~ 60 minutes. If the temperature is lower than 800 ° C., the growth rate of the oxide film 305 becomes slow, so that productivity is deteriorated. When the temperature is higher than 950 ° C., the film thickness of the oxide film 305 does not depend on the added impurity concentration on the silicon surface before the thermal oxidation treatment, and the oxide film thickness difference due to the concentration of the added impurity concentration cannot be obtained. This is because, due to the relationship between the solid solubility of phosphorus in silicon oxide film and silicon and the diffusion rate, phosphorus segregates on the silicon surface as the thermal oxidation progresses, increasing the vacancy concentration. It is understood that it is also promoted at the layer surface.

  The selective etching of the oxide film 305 is preferably performed at an etching rate of about 1 to 10 nm / min in order to improve controllability although it depends on the thickness of the oxide film 305. A rate in this range can be obtained by etching, for example, using a hydrofluoric acid aqueous solution having a concentration of 0.2 to 1.0 wt% and a processing temperature of 25 ° C.

  Subsequently, an alkaline aqueous solution such as sodium hydroxide, potassium hydroxide, potassium carbonate, sodium carbonate, sodium bicarbonate, tetramethylammonium hydroxide or the like having a concentration of 20 to 60% obtained by heating the substrate to a temperature of about 60 to 95 ° C., or these is used. It is immersed in the combined alkaline aqueous solution (FIG. 3 (f)).

  Since the alkaline aqueous solution does not etch the silicon oxide film 305, the non-diffusion region where silicon is exposed is etched on the non-light-receiving surface side, while the first high-concentration layer 303 and the second high-concentration layer 304 covered with the oxide film are formed. The texture 302 of the formed area is retained.

  Although the etching treatment time depends on the conditions of the alkaline solution, it may be performed for 30 seconds to 2 minutes in the above temperature and concentration range, and the texture 302 may be smoothed. More preferably, the treatment is performed for about 2 minutes to 20 minutes. And remove texture 302 completely. If the treatment is further performed for a long time, the region where the second high concentration layer 304 is formed is etched from the side surface.

  Subsequently, the oxide film 305 on the first high concentration layer 303 and the second high concentration layer 304 is removed by etching with a 1.5 to 2 wt% hydrofluoric acid aqueous solution (FIG. 3G).

The width W _t of the electrode formation region formed by the above process is generally 0.8 to 1.5 times the width W _m of the electrode 308, more preferably 1.0 to 1.. It is better to triple. If it is less than 0.8 times, there is a concern that the adhesion between the electrode and the substrate will be insufficient, and if it is more than 1.5 times, the surface recombination loss of the carriers will become apparent.

Next, protective films 306 are formed on both surfaces of the substrate 301 (FIG. 3H). As the protective film 306, a silicon nitride film or the like is formed to a thickness of about 100 nm. In the film formation, a chemical vapor deposition apparatus is used, and SiH ₄ and NH ₃ are often mixed and used as a reaction gas. However, it is also possible to use nitrogen instead of NH ₃ , and use H ₂ gas. The desired refractive index is realized by diluting the film formation species, adjusting the process pressure, and diluting the reaction gas. In order to enhance optical characteristics, the refractive index is preferably about 1.5 to 2.2. In addition to the silicon nitride film, silicon oxide, silicon carbide, amorphous silicon, aluminum oxide, titanium oxide, tin oxide, zinc oxide, or the like may be used as a single layer or a combination thereof. Different film types may be applied to the light receiving surface and the non-light receiving surface.

  Next, an electrode 307 is screen-printed on the light receiving surface of the substrate 301. After printing and drying a silver paste in which silver powder and glass frit are mixed with an organic binder, the silver powder is passed through the protective film 306 by heat treatment, and the electrode and silicon are conducted (FIG. 3 (i)).

  Further, similarly, the electrode 308 is screen-printed on the base layer of the non-light-receiving surface, a silver paste in which silver powder and glass frit are mixed with an organic binder is printed and dried, and then heat treatment at 700 to 860 ° C. is performed for 1 second to After about 5 minutes, the silver powder is passed through the protective film 306, and the electrode and silicon are made conductive (FIG. 3 (j)).

  The electrode 307 and the electrode 308 may be interchanged in the order of formation steps, or may be fired at once.

[Second Embodiment]
In the first embodiment, an example of a method for manufacturing a solar cell using an n-type silicon substrate has been described. However, the present invention can also be applied to a method for manufacturing a solar cell using a p-type silicon substrate. .

  A p-type silicon solar cell can be manufactured in the same manner as the n-type silicon solar cell. In this case, the substrate 301 is obtained by adding a group III element such as boron, aluminum, gallium, or indium to high-purity silicon. In general, those having a resistivity adjusted to 0.1 to 5 Ω · cm are used.

First, the texture 302 is formed on the substrate 301 by the same method as in the first embodiment. Next, a second high-concentration layer (base layer) 304 is formed in an electrode formation region where the electrode 308 is formed in a subsequent process. In general, a diffusion barrier such as silicon nitride is formed on the entire surface of the non-light-receiving surface, and an electrode formation region is opened with an etching paste that can be photolithographically or screen-printed, and then gas phase is used at 900 to 1100 ° C. using BBr _3. Boron is added to the substrate 301 by a diffusion method.

  In general silicon solar cells, the second high-concentration layer 304 needs to be formed only on the non-light-receiving surface, and in order to achieve this, the light-receiving surfaces of the two substrates 301 are overlapped and diffused. In addition, it is necessary to devise measures to prevent phosphorus from diffusing on the light receiving surface by forming a diffusion barrier such as silicon nitride on the light receiving surface side.

  As another method, boron ionized by ion implantation through a mask patterned in the shape of the electrode formation region may be added to the substrate 301, or a compound that can be screen printed is added to the shape of the electrode formation region. In addition, it may be heat treated and added to the substrate 301.

The surface impurity concentration of the second high-concentration layer 304 is preferably 1 × 10 ¹⁹ or more and 1 × 10 ²¹ atoms / cm ³ , more preferably 5 × in order to obtain good electrical contact between the substrate and the electrode. It is preferable to be about 10 ¹⁹ or more and 1 × 10 ²¹ atoms / cm ³ or less. If it is less than 1 × 10 ¹⁹ atoms / cm ³ , the contact resistance between the substrate and the electrode increases, and the output of the solar cell decreases. 1 × 10 ²¹ atoms / cm ³ is the solid solubility limit of boron that is most easily dissolved in silicon among group III elements.

Next, a first high concentration layer (emitter layer) 303 is formed. In general, POCl ₃ is preferably used, and phosphorus is diffused into the substrate at 800 to 1000 ° C. by a vapor phase diffusion method. Further, the present invention is not limited thereto, and a phosphorus compound that can be screen-printed or spin-coated may be used. The first high-concentration layer 303 needs to be formed only on the light-receiving surface, and in order to achieve this, diffusion is performed with the non-light-receiving surfaces of the two substrates 301 facing each other, or silicon nitride is formed on the non-light-receiving surface. It is necessary to devise a diffusion barrier such as that to prevent the added impurity from diffusing on the non-light-receiving surface.

The phosphorus concentration on the surface of the first high-concentration layer 303 is preferably 1 × 10 ¹⁹ or more and 3 × 10 ²⁰ atoms / cm ³ , more preferably 5 × 10 ¹⁹ or more and 1 × 10 ²⁰ atoms / cm ³ . It is good. If it is less than 1 × 10 ¹⁹ atoms / cm ³ , the contact resistance between the substrate 301 and the electrode increases, and if it is 3 × 10 ²⁰ atoms / cm ³ or more, defects in the first high concentration layer 303 and Auger recombination occur. Charge carrier recombination becomes prominent and the output of the solar cell decreases.

  The requirements for the thermal oxidation step are almost the same as those when an n-type substrate is used, but the heat treatment conditions are generally 800 ° C. or higher for about 20 to 90 minutes, preferably 900 ° C. to 1050 ° C. for 30 to 60 minutes. . Unlike boron, boron on the silicon surface segregates in the oxide film as the thermal oxidation progresses, so the oxide film thickness depends on the initial surface boron concentration difference even in the high temperature region as described above. Further, by performing the treatment at as high a temperature as possible, the difference in the oxide film thickness can be further increased. However, on the other hand, in the temperature range exceeding 1150 ° C., the quartz tube cannot withstand use, and furthermore, a high-heat-resistant furnace using silicon carbide or the like has a high cost. It is preferable to perform the treatment in a temperature range up to about 1100 ° C.

[Example 1]
After removing the damaged layer with a hot concentrated potassium hydroxide aqueous solution on a phosphorus-doped <100> n-type as-cut silicon substrate with a substrate thickness of 250 μm and a specific resistance of 1 Ω · cm, the texture is immersed in a potassium hydroxide / 2-propanol aqueous solution. Formation was carried out, followed by washing in a hydrochloric acid / hydrogen peroxide mixed solution.

Next, in a state where the non-light-receiving surfaces are stacked facing each other, heat treatment is performed at 980 ° C. for 30 minutes in a BBr ₃ atmosphere to form a first high-concentration layer. Subsequently, a silicon oxide film of about 200 nm is formed on the non-light-receiving surface by thermal oxidation. Formed.

Next, the oxide film on the non-light-receiving surface is opened in a grid pattern by photolithography, and heat-treated at 870 ° C. for 30 minutes in a POCl ₃ atmosphere with the light-receiving surfaces overlapped to form a second high-concentration layer. did. After diffusion, the glass layer was removed with hydrofluoric acid, washed with pure water, and dried.

  Next, dry oxidation was performed at 920 ° C. for 60 minutes using a quartz tube furnace. Thereafter, a part of the substrate was extracted, and the cross section of each part of the substrate was observed with a TEM. As a result, the oxide film thickness on the surface of each part was about 53 nm in the first high-concentration layer and the second high-concentration layer. And about 25 nm.

  Subsequently, using an aqueous hydrofluoric acid solution having a concentration of 0.5 wt%, the oxide film was etched at a liquid temperature of 25 ° C. until the surface of the non-diffusion region of the non-light-receiving surface showed water repellency, washed with water and then dried.

  Next, the substrate was immersed in an aqueous 35% potassium hydroxide solution heated to a temperature of 70 ° C. for 4 minutes to etch the non-diffusion region of the non-light-receiving surface.

  Next, a silicon nitride film having a thickness of about 100 nm was formed on the entire light receiving surface and non-light receiving surface by plasma CVD.

  Further, a silver paste was applied to the light receiving surface and the non-light receiving surface by screen printing, and after drying, electrodes were formed by heat treatment at 820 ° C. in a belt furnace.

[Example 2]
For Example 1, dry oxidation was performed at 1100 ° C. for 60 minutes in the thermal oxidation step. As a result, the oxide film thickness on the surface of each part was about 205 nm in the first high concentration layer and the second high concentration layer, and about 153 nm in the non-diffusion region of the non-light-receiving surface. Subsequently, a solar cell was manufactured through the same steps as in Example 1.

[Comparative Example 1]
Similar to Example 1, a textured substrate was prepared, and the texture of the non-light-receiving surface was removed by etching with hydrofluoric acid using a spin etching apparatus. Subsequently, a solar cell was manufactured through the same steps as in Example 1.

[Example 3]
After removing the damaged layer with a hot concentrated potassium hydroxide aqueous solution on a boron-doped <100> p-type ascut silicon substrate having a substrate thickness of 250 μm and a specific resistance of 1 Ω · cm, the substrate was immersed in an aqueous potassium hydroxide / 2-propanol solution. Texture formation was performed, followed by washing in a hydrochloric acid / hydrogen peroxide mixed solution.

Next, a silicon oxide film having a thickness of about 200 nm is formed on the non-light-receiving surface by thermal oxidation, the oxide film on the non-light-receiving surface is opened in a grid pattern by photolithography, and the BBr ₃ atmosphere is stacked with the light-receiving surfaces facing each other. Then, a second high concentration layer was formed by heat treatment at 980 ° C. for 30 minutes.

  After diffusion, the glass layer and the thermal oxide film were removed with hydrofluoric acid, washed with pure water, and dried.

Next, in a state where the non-light-receiving surfaces face each other and overlap each other, heat treatment was performed at 870 ° C. for 30 minutes in a POCl ₃ atmosphere to form a first high concentration layer.

  Next, dry oxidation was performed at 920 ° C. for 60 minutes using a quartz tube furnace. Thereafter, a part of the substrate was extracted, and the cross section of each part of the substrate was observed with a TEM. As a result, the oxide film thickness of the surface of each part was about 51 nm in the first high-concentration layer and the second high-concentration layer. And about 28 nm.

  Subsequently, using an aqueous hydrofluoric acid solution having a concentration of 0.5 wt%, the oxide film was etched at a liquid temperature of 25 ° C. until the surface of the non-diffusion region showed water repellency, washed with water, and then dried.

  Next, the substrate was immersed in an aqueous 35% potassium hydroxide solution heated to a temperature of 70 ° C. for 4 minutes to etch the non-diffusion region of the non-light-receiving surface.

  Next, a silicon nitride film having a thickness of about 100 nm was formed on the entire light receiving surface and non-light receiving surface by plasma CVD.

  Further, a silver paste was applied to the light receiving surface and the non-light receiving surface by screen printing, and after drying, electrodes were formed by heat treatment at 820 ° C. in a belt furnace.

[Comparative Example 2]
For Example 3, dry oxidation was performed at 1100 ° C. for 60 minutes in the thermal oxidation step. As a result, the oxide film thickness on the surface of each part was about 194 nm in the first high concentration layer and the second high concentration layer, and about 145 nm in the non-diffusion region of the non-light-receiving surface. Subsequently, a solar cell was manufactured through the same steps as in Example 3.

[Comparative Example 3]
Similar to Example 3, a textured substrate was prepared, and the texture of the non-light-receiving surface was removed by etching with hydrofluoric acid using a spin etching apparatus. Subsequently, a solar cell was manufactured through the same steps as in Example 3.

  The current-voltage characteristics were measured in the dark state of Examples 1 to 3 and Comparative Examples 1 to 3 and under the simulated sunlight irradiation of an air mass of 1.5 g. From these measurements, solar cell characteristics and series resistance were evaluated.

  As shown in Table 1, in Examples 1, 2, and 3 of the present invention, it is shown that the series resistance is reduced as compared with Comparative Examples 1, 2, and 3, thereby greatly improving the fill factor and the conversion efficiency. It was.

  As described above, in the solar cell manufactured by the manufacturing method described in the embodiment of the present invention, the carrier recombination loss on the surface of the non-light-receiving surface is suppressed, and good electrical contact is made between the collector electrode and the crystalline silicon substrate. It has a back surface structure.

  The method for manufacturing a solar cell according to the present invention can form a high-quality PR structure on the back surface of a crystalline silicon solar cell easily and inexpensively, and is extremely effective for increasing the efficiency and reducing the cost of the solar cell.

101, 201, 301 ... substrate 102, 202, 302 ... texture 103, 203, 303 ... first high concentration layer 104, 204 ... protective film 105, 205 ... electrode 106, 206, 304 ... second high concentration layer 107 ... electrode 207 ... protective film 208 ... electrode 305 ... oxide film 306 ... protective film 307 ... electrode 308 ... electrode

Claims (9)

Forming a concavo-convex structure on at least a part of the non-light-receiving surface of the crystalline silicon substrate;
Forming a first high-concentration layer having an impurity concentration higher than that of the substrate and having a conductivity type different from that of the substrate on at least a part of the light-receiving surface of the substrate;
Locally forming a second high-concentration layer having an additive impurity concentration higher than that of the substrate on the non-light-receiving surface of the substrate and having the same conductivity type as the substrate;
Thermally oxidizing the substrate;
Etching the oxide film formed in the thermal oxidation step to expose the silicon surface of the substrate in at least a part of the non-light-receiving surface other than the region where the second high-concentration layer is formed;
Forming a collecting electrode on the first high-concentration layer and the second high-concentration layer for extracting the charge excited by the light incident on the substrate to the outside;
Method for manufacturing a solar cell comprising a step of smoothing at least a portion of the concavo-convex structure in the region other than the region SL before Te non-light-receiving surface smell second high concentration layer is formed of the substrate.   In the thermal oxidation step, an oxide film is formed on the first high concentration layer and the second high concentration layer than on the region other than the region where the first high concentration layer and the second high concentration layer are formed. The method for manufacturing a solar cell according to claim 1, wherein:   The exposing step selectively removes an oxide film formed in a region other than a region where the first high concentration layer and the second high concentration layer are formed in an acid solution. The manufacturing method of the solar cell of 1 or 2.   The thermal oxidation step forms an oxide film having a thickness of 20 nm or more and 200 nm or less in a region where the second high-concentration layer is formed on the non-light-receiving surface. 2. A method for producing a solar cell according to item 1.   5. The method for manufacturing a solar cell according to claim 1, wherein the smoothing step includes a step of etching exposed silicon with an alkaline solution. 6. 6. The method for manufacturing a solar cell according to claim 5, wherein the alkaline solution is selected from sodium hydroxide, potassium hydroxide, tetramethylammonium hydroxide, or a combination thereof.   7. The method according to claim 1, wherein the step of forming the second high-concentration layer includes a step of patterning the second high-concentration layer in a region where the electrode is formed. Solar cell manufacturing method.   The method for manufacturing a solar cell according to claim 1, wherein the step of forming the second high concentration layer includes a step of adding boron to the substrate. The step of forming the second high concentration layer includes a step of adding phosphorus to the substrate, and the step of thermally oxidizing includes a heat treatment at 800 ° C. or higher and 950 ° C. or lower. 8. The method for producing a solar cell according to any one of 7 above.