JP4186584B2 - Solar cell production method - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing a solar cell, and in particular, to provide a manufacturing method for realizing a system with high power generation efficiency by removing a diffusion layer surface.
[0002]
[Prior art]
At present, silicon solar cells are the mainstream of electric power solar cells, but reduction of product costs is necessary for their spread. Improving the power generation efficiency of solar cells can also be said to be a means of reducing product prices. In addition, there is an additional advantage that the installation area can be reduced by using a device having high power generation efficiency.
On the other hand, conventionally, a solar cell production method is known in which a solar cell is obtained by processing a structure including a diffusion layer whose conductivity type is inverted on the surface of a semiconductor substrate. (For example, refer nonpatent literature 1).
[0003]
[Non-Patent Document 1]
Makoto Konagai, “Basics and Applications of Thin Film Solar Cells”, Ohmsha, March 2001, p. 40
[Problems to be solved by the invention]
In the conventional solar cell production method, when a pn junction of a silicon solar cell is formed, a high impurity concentration region that reduces device performance is generated on the surface. The present invention places importance on mass productivity, and aims to improve the performance of solar cells by removing this region by a simple method.
[ 0004 ]
[Means for Solving the Problems]
According to the solar cell production method of the present invention, a structure having an n-type diffusion layer whose conductivity type is inverted by thermally diffusing phosphorus on the surface of a p-type silicon substrate is converted into a hydrofluoric acid (HF) aqueous solution. A phosphorus glass removing step that removes the phosphorous glass of the n-type diffusion layer by immersing it in a surface, and a surface of the n-type diffusion layer removed by immersing the structure after the phosphorous glass removing step in an aqueous hydrofluoric acid (HF) solution The n-type diffusion layer sheet resistance at the time when the removal of the phosphorus glass is completed is 40 to 110Ω, and the n-type diffusion layer sheet resistance at the time when the removal of the phosphorus glass is completed when the surface is removed is used as a reference , A solar cell production method for obtaining a solar cell by processing a structure having an n-type diffusion layer sheet resistance increased by 0.1 to 20Ω, wherein hydrofluoric acid (HF) aqueous solution is fluorinated in the surface removal step Hydrogen concentration is 0.1 to 0 percent by weight, the temperature of the hydrofluoric acid (HF) aqueous solution is 20 to 25 ° C., in which the processing time is within 16 minutes.
[ 0005 ]
In the surface removal step, the hydrofluoric acid (HF) aqueous solution has a hydrogen fluoride concentration of 5 to 10 % by weight, and the hydrofluoric acid (HF) aqueous solution has a temperature of 20 to 25 ° C. The time is less than 6 minutes.
[0006]
DETAILED DESCRIPTION OF THE INVENTION
Embodiment 1 FIG.
FIG. 1 is a flowchart showing a solar cell production method according to Embodiment 1 of the present invention. A p-type Si substrate 1 in FIG. 1A is a semiconductor substrate in this embodiment. In FIG. 1B, for example, a diffusion layer whose conductivity type is inverted by thermally diffusing phosphorus is formed on the surface of the p-type Si substrate 1. In this embodiment, the n-type diffusion layer 2 corresponds to the diffusion layer. This diffusion layer is formed by a thermal diffusion method (diffusion layer forming step). At this time, a compound containing phosphorus and oxygen or a part of the diffusion source remains as a by-product on the surface of the n-type diffusion layer 2 of the p-type Si substrate 1. The remaining substances are collectively called phosphorus glass 6. Further, the n-type diffusion layer 2 is formed on the entire surface of the P-type Si substrate 1 unless otherwise specified. Through the above steps, a structure including a diffusion layer with the conductivity type reversed is formed on the surface of the semiconductor substrate.
[0007]
Subsequently, in FIG. 1C, the phosphor glass 6 is removed by immersing the structure in an aqueous hydrofluoric acid (HF) solution. Since the phosphorus glass 6 is removed in a very short time, the n-type diffusion layer 2 is exposed by immersion for about 20 seconds (phosphorus glass removal step). Further, as shown in FIG. 1D, the other is etched away so as to leave the n-type diffusion layer 2 only on one main surface, and an insulating film 3 is formed on this main surface in FIG. This insulating film 3 functions as an antireflection film and improves the performance of the device. Then, the front electrode 4 and the back electrode 5 are formed in FIG.1 (f), and a solar cell device is manufactured.
[0008]
Here, in the diffusion layer forming step shown in FIG. 1B of the above structure, a method of thermally diffusing gas, liquid, or solid impurities by attaching them to the surface of the p-type Si substrate 1 is considered. In this case, however, the n-type diffusion layer 2 has a high impurity concentration on the surface and a distribution in which the impurity concentration decreases toward the inside. In order to obtain a solar cell system with high power generation efficiency, it is necessary to control the impurity concentration on the surface of the n-type diffusion layer 2 to a predetermined value.
[0009]
For the purpose of removing the surface of the n-type diffusion layer 2, that is, the high-concentration region for the purpose of the above control, dry etching using plasma or sandblasting, an aqueous solution in which hydrofluoric acid (HF) and nitric acid (HNO3) are mixed, A method of removing the film by wet etching or the like is conceivable. However, dry etching using plasma or the like has been difficult to apply to mass production because the surface of the structure is damaged, the manufacturing process becomes complicated, and the manufacturing cost increases. In addition, when using a chemical that can etch silicon, such as an aqueous solution in which hydrofluoric acid (HF) and nitric acid (HNO3) are mixed, the etching characteristics are affected by temperature changes due to reaction heat or changes over time in the liquid life. However, the amount of etching was difficult to control and the mass productivity was poor. Therefore, this high concentration region often remains in solar cells that have been mass-produced. In order to apply this technology to mass production, it is necessary to remove a minute region from nanometer to submicron stably and easily.
[0010]
In the solar cell production method according to the first embodiment, an n-type diffusion layer 2 having an inverted conductivity type is formed on the surface of a p-type Si substrate 1, and immersed in an aqueous hydrofluoric acid solution with a controlled temperature for a certain period of time. As a result, the surface of the n-type diffusion layer 2 (high impurity concentration layer) was etched (surface removal step). As a result, we succeeded in removing a minute region from nanometer to submicron stably and easily. Hereinafter, this embodiment will be described in more detail.
[0011]
FIG. 2 is a flowchart relating to a solar cell production method according to this embodiment. Hereinafter, steps 1-2-2 (3) -4-5-6-7-8-9 will be described with reference to the flowchart of FIG.
First, step 1 in FIG. 2 is a step of cleaning a single crystal manufactured by a pulling method or a polycrystalline silicon substrate manufactured by a casting method. In the case of a solar cell, a substrate that has been sliced from an ingot is used. Often used. In this case, in order to remove the mechanically altered layer and dirt on the substrate surface generated in the slicing step, an alkaline aqueous solution such as potassium hydroxide or sodium hydroxide aqueous solution or a mixed solution of hydrofluoric acid and nitric acid is used. Etch the substrate surface to a degree. Further, in order to remove heavy metals such as iron adhering to the substrate surface, a step of washing with a mixed solution of hydrochloric acid and hydrogen peroxide may be added.
[0012]
Subsequently, in step 2, if the substrate to be used is p-type, an n-type layer is formed to form a pn junction (diffusion layer forming step). The n-type layer is formed by thermal diffusion using phosphorus oxychloride (POCl3). Other methods include, for example, SOD (Spin-On-Dopant), PSG (Phospho-Slicate-Glass), phosphoric acid (H3PO4), phosphorous acid (H3PO3), hypophosphorous acid (H3PO2), metaphosphoric acid Using a phosphoric acid aqueous solution such as (HPO 3) or a film diffusion source as a diffusion supply source, impurities including phosphorus are attached to the substrate surface by an appropriate method and thermally diffused. Moreover, any method such as gas (gas), spin coating, spraying, roll coating, or dipping may be used for the attachment. In any case, when thermal diffusion is performed by attaching a diffusion source to the substrate surface as described above, the surface of the diffusion layer has a high phosphorus concentration, and the phosphorus concentration decreases as the distance from the surface increases. It becomes.
[0013]
Next, step 3 is a step of removing the phosphor glass 6 shown in FIG. 1B remaining on the substrate surface after diffusion (phosphorus glass removing step). As described above, when thermal diffusion is performed, a compound containing phosphorus and oxygen or a part of the diffusion source remains as the phosphor glass 6 on the surface of the n-type diffusion layer 2. The phosphorous glass remaining on the surface of the n-type diffusion layer 2 is removed by the hydrofluoric acid aqueous solution in this phosphorous glass removing process, and the n-type diffusion layer 2 of the structure is exposed.
[0014]
Subsequently, in step 4, the structure is immersed in a temperature-controlled hydrofluoric acid aqueous solution for a predetermined time, and the surface of the n-type diffusion layer 2 having a high phosphorus concentration is removed by etching (surface removal step). Here, it is generally said that hydrofluoric acid aqueous solution cannot etch silicon. For this reason, normally, the process in the phosphorus glass removing step 3 in the previous stage is expected to be completed in a time (about 20 seconds) necessary for removing the phosphorus glass 6 remaining on the surface of the n-type diffusion layer 2. However, as a result of experiments, the inventor has immersed the structure in a temperature-controlled hydrofluoric acid aqueous solution for a certain period of time, thereby etching a region having a high phosphorus concentration on the surface of the n-type diffusion layer 2 and improving the performance of the solar cell. I found out. For this reason, in this embodiment, the surface of the n-type diffusion layer 2 is etched by continuing to immerse the structure in the temperature-controlled hydrofluoric acid aqueous solution even after the phosphorus glass removing step is completed, so that the performance of the solar cell is improved. We are going to improve. The processing time required for this surface removal process is affected by the temperature of the hydrofluoric acid aqueous solution, but there is almost no reaction heat of itself and the etching rate is close to zero. It can be said to be a simple and easy method.
[0015]
In particular, the case where the sheet resistance of the n-type diffusion layer 2 corresponds to 40 to 110Ω at the end of the phosphorus glass removing step 3 is suitable for the solar cell production method according to this embodiment. In such a case, the phosphorus concentration on the surface of the n-type diffusion layer 2 is as high as 1E + 20 cm −3 or more, causing an increase in the carrier recombination rate and a decrease in the amount of incident light, leading to a decrease in the performance of the solar cell device. Therefore, by immersing in a hydrofluoric acid aqueous solution having a controlled temperature for a certain period of time, the surface region having a high phosphorus concentration is removed by etching, and the performance can be improved.
[0016]
Here, the sheet resistance can be measured by, for example, a four-probe method in a dark place using a specific resistance measuring device VR-70 manufactured by Kokusai Electric Co., Ltd.
[0017]
As described above, the phosphor glass 6 can be removed with a hydrofluoric acid aqueous solution in a short time. For this reason, the phosphorus glass removal process 3 can be performed in combination with the surface removal process 4, and it is not necessary to perform both separately. That is, the phosphorus glass removing step 3 and the surface removing step 4 can be performed as a series of steps (phosphor glass removing and surface removing step).
[0018]
The hydrofluoric acid aqueous solution used in this embodiment has a hydrogen fluoride concentration of 0.1 to 20% by weight, and the temperature of the aqueous solution is stable at a room temperature of 23 ° C. When the liquid temperature is 20 to 25 ° C., for example, the processing time required for the phosphorus glass removal and surface removal step is suitably within 20 minutes. More preferably, the hydrogen fluoride concentration is 5 to 10% by weight and the treatment time is within 10 minutes. These predetermined values were determined as conditions that do not damage the n-type diffusion layer 2.
[0019]
Moreover, it can be said that it is a simple method to measure the increase amount of sheet resistance as a standard of the etching amount in the surface removal process 4. That is, as the increase amount of the sheet resistance increases, the etching amount increases, which means that the phosphorus concentration on the surface decreases. As a result, the desired effect is increased, but there is a disadvantage that good contact with the electrode cannot be obtained. Therefore, generally, the amount of increase in sheet resistance in the surface removal step 4 may be 0.1 to 20Ω within a range that does not damage the n-type diffusion layer 2, but more preferably 3 to 6Ω. Gave better results when the increase was about 4Ω. In addition, the reference value of the increase amount indicates the time point when the phosphorus glass removing step 3 is completed.
[0020]
Further, as a simple index representing the end of the phosphorus glass removing step 3, when the phosphor glass 6 is present, the surface of the structure exhibits a hydrophilic surface, and when the phosphor glass 6 is removed, a hydrophobic surface appears. To do. When the structure is lifted from the hydrofluoric acid aqueous solution, it is hydrophilic when the hydrofluoric acid aqueous solution is close to the surface of the structure, and is hydrophobic when it is not close. In the experiment, a hydrophobic surface appeared within 1 minute. In view of the process margin of the phosphorus glass removal step 3, the immersion time in the hydrofluoric acid aqueous solution having a liquid temperature of 20 to 25 ° C. was set to 4 minutes. 3 was the time required. That is, the time required for removing phosphorus glass is within 4 minutes.
Therefore, in the case of the phosphorus glass removing and surface removing step, 4 to 20 minutes or 4 to 10 minutes are required in consideration of the time required for removing the phosphorus glass.
In addition, the hydrofluoric acid aqueous solution which does not heat is enough for the phosphorus glass removal process 3.
[0021]
The pn junction separation in step 5 is a step performed so that the p electrode and the n electrode are not short-circuited, and the n-type silicon layer may be removed in accordance with a desired electrode pattern. As the removal method, in addition to the method described in Step 1, a mechanical method such as plasma etching or sand blasting may be used. Further, when the diffusion region is selectively performed, such as diffusion represented by a method of attaching SOD or the like only to one surface of the substrate, the pn junction isolation step can be omitted.
[0022]
Step 6 is a step of forming an antireflection film. Since this antireflection film reduces the surface reflectance with respect to the incident light of the solar cell, the generated current can be increased. For example, when a silicon nitride film is applied to the antireflection film, the formation method is formed using a low pressure thermal CVD method or a plasma CVD method. In the case of the thermal CVD method, dichlorosilane (SiCl2H2) and ammonia (NH3) are often used as raw materials. For example, the gas flow ratio is NH3 / SiCl2H2 = 10 to 20, the pressure in the reaction chamber is 0.2 to 0.5 Torr, and the temperature is 760. Film formation is performed under the condition of ° C. Next, a mixed gas of SiH4 and NH3 is generally used as a raw material gas when the plasma CVD method is used. As the film forming conditions, for example, the gas flow ratio NH3 / SiH4 = 0.5 to 1.5, the pressure in the reaction chamber is 1 to 2 Torr, the temperature is 300 to 550 ° C., and the frequency of the high frequency power source necessary for plasma discharge is several hundreds. kHz or more is appropriate.
[0023]
Steps 7 to 9 show electrode formation for extracting current from the solar cell. The electrode forms a predetermined pattern by screen printing with silver aluminum or aluminum paste for p-type silicon and silver paste for n-type silicon, for example, at a temperature of 650 to 900 ° C. for several tens of seconds to several By firing for a minute, ohmic contact with the n-type and p-type substrates is obtained.
[0024]
FIG. 3 is a comparison table showing device characteristics of the solar cell according to Embodiment 1 of the present invention and the solar cell created by the conventional method. Voc is the open circuit voltage, Jsc is the short circuit current, FF is the fill factor, and Eff. Is the power generation efficiency. It is considered that etching the region having a high phosphorus concentration in the diffusion layer reduces the surface recombination rate of carriers and increases the open circuit voltage. In addition to the decrease in the surface recombination rate, it is considered that the short-circuit current has increased due to the increase in the amount of incident light. Thus, the power generation efficiency is increased and the performance of the solar cell device is improved.
[0025]
In this embodiment, as described above, since the surface removal process or the phosphorus glass removal / surface removal process is adopted, a very small amount of nanometer to submicron which is the surface of the n-type diffusion layer 2 (region with high phosphorus concentration). It is possible to remove the region stably and easily. For this reason, the solar cell production method according to this embodiment is suitable for mass productivity and can improve the performance of the solar cell by a simple method.
[0026]
Further, in the solar cell production method according to this embodiment, the surface of the diffusion layer having the conductivity type reversed on the semiconductor substrate is etched to reduce the surface recombination rate of the carrier and the amount of incident light. Increase in power generation efficiency, and a device with high power generation efficiency can be realized. That is, the product cost can be reduced. In addition, there is an effect that desired power can be obtained in a small area. Further, since the phosphorus glass removal and surface removal process also serves as the phosphorus glass removal process, it can be realized with a small number of processes, and has an effect of increasing the manufacturing throughput and reducing the apparatus cost. Further, the surface removal treatment shown in this embodiment is easy to control the etching amount and easily realize a predetermined sheet resistance of the solar cell device, and thus has an effect of increasing the degree of freedom in electrode material and electrode pattern design. . Furthermore, since the formation of the diffusion layer can be realized by general thermal diffusion, there is an effect that it can be easily applied to mass production. In addition, since the semiconductor substrate can be realized by silicon, there is an effect that can be applied to most devices of power solar cells.
[0027]
Embodiment 2. FIG.
In the first embodiment, the example in which the surface of the n-type diffusion layer 2 having a high phosphorus concentration is etched by extending the processing time in the step 4 has been described. In this embodiment, it will be clarified that the etching amount depends on the temperature of the hydrofluoric acid aqueous solution.
FIG. 4 is a table showing the relationship between the temperature of the hydrofluoric acid aqueous solution and the increase in sheet resistance. The table shown in FIG. 4 shows that the amount of etching with the hydrofluoric acid aqueous solution is affected by the temperature of the hydrofluoric acid aqueous solution in terms of the increase in sheet resistance. The immersion time is standardized to 4 minutes at any temperature. The standard sheet resistance value uses a hydrofluoric acid aqueous solution that is stable at a liquid temperature of 22 ° C., and can be regarded as a sample subjected to the phosphorus glass removal step for the reason described in Example 1. . FIG. 4 shows that the temperature of the hydrofluoric acid aqueous solution may be controlled in order to adjust the phosphorus concentration on the surface of the diffusion layer 2 to a desired value by the surface removal step.
[0028]
As described above, the surface removal step using the hydrofluoric acid aqueous solution may be performed as follows. That is, when the aqueous solution not heated is used, it is realized by making the immersion time longer than 4 minutes. Even when the immersion time is adjusted to the processing time of the phosphorus glass removing step 3 or shorter than that, it is realized by heating the hydrofluoric acid aqueous solution. The temperature range of the hydrofluoric acid aqueous solution is 10 to 80 ° C., more preferably 30 to 40 ° C. Here, the minimum temperature of 10 ° C. mainly means the lower limit of room temperature. In particular, although there is little need to cool the hydrofluoric acid aqueous solution, the controllability can be improved by reducing the sheet resistance increase rate (reducing the etch rate) by cooling. Further, since the etching rate increases in proportion to the increase in the liquid temperature, the treatment time can be shortened by heating the hydrofluoric acid aqueous solution to room temperature or higher. For example, when the temperature of the hydrofluoric acid (HF) aqueous solution is heated to 30 ° C. or higher, the time required for the phosphorus glass removal and surface removal process is shorter than 4 to 20 minutes. In this embodiment, since the etching amount is adjusted by the temperature of the hydrofluoric acid aqueous solution, a desired phosphorus concentration can be obtained on the surface of the n-type diffusion layer 2.
[0029]
Further, as described above, when the surface removal step is realized for a long time, there is no need for heating and it can be realized at room temperature, so that there is an effect that the apparatus cost can be reduced. On the other hand, when it is realized at a high temperature, since it is a short-time process, there is an effect that the manufacturing throughput can be increased.
[0030]
In this embodiment, parts different from those in the first embodiment are described, and descriptions of the same or corresponding parts as those in the first embodiment are omitted.
【The invention's effect】
As described above, the solar cell production method according to the present invention is a method for obtaining a solar cell by processing a structure having a diffusion layer having a conductivity type inverted on the surface of a semiconductor substrate. Since the surface removal step of removing the surface of the diffusion layer of the structure body by immersing in an aqueous solution of hydrogen acid (HF) is provided, the performance of the solar cell can be improved by removing this region by a relatively simple method. it can.
[Brief description of the drawings]
FIG. 1 is a flowchart showing a solar cell production method according to Embodiment 1 of the present invention.
FIG. 2 is a flowchart relating to a solar cell production method according to this embodiment.
FIG. 3 is a comparison table showing device characteristics of the solar cell according to the first embodiment of the present invention and a solar cell created by a conventional method.
FIG. 4 is a table showing the relationship between the temperature of an aqueous hydrofluoric acid solution and the increase in sheet resistance.
[Explanation of symbols]
1 p-type Si substrate, 2 n-type diffusion layer, 3 insulating film, 4 surface electrode, 5 back electrode, 6 phosphorus glass

Claims (2)

A structure provided with an n-type diffusion layer whose conductivity type is inverted by thermally diffusing phosphorus on the surface of the p-type silicon substrate is immersed in a hydrofluoric acid (HF) aqueous solution to form the n-type diffusion layer. A phosphorus glass removing step for removing the phosphorus glass;
A surface removing step of removing the surface of the n-type diffusion layer by immersing the structure after the phosphorus glass removing step in a hydrofluoric acid (HF) aqueous solution;
The n-type diffusion layer sheet resistance at the time when the removal of the phosphorus glass is finished is 40 to 110Ω,
A solar cell is fabricated by processing the structure in which the n-type diffusion layer sheet resistance is increased by 0.1 to 20Ω with respect to the n-type diffusion layer sheet resistance at the time when the removal of the phosphor glass is completed at the time of surface removal. A solar cell production method to obtain,
In the surface removal step, the hydrofluoric acid (HF) aqueous solution has a hydrogen fluoride concentration of 0.1 to 20% by weight,
The temperature of the aqueous hydrofluoric acid (HF) solution is 20-25 ° C .;
A solar cell production method, wherein the treatment time is within 16 minutes. A structure provided with an n-type diffusion layer whose conductivity type is inverted by thermally diffusing phosphorus on the surface of the p-type silicon substrate is immersed in a hydrofluoric acid (HF) aqueous solution to form the n-type diffusion layer. A phosphorus glass removing step for removing the phosphorus glass;
A surface removing step of removing the surface of the n-type diffusion layer by immersing the structure after the phosphorus glass removing step in a hydrofluoric acid (HF) aqueous solution;
The n-type diffusion layer sheet resistance at the time when the removal of the phosphorus glass is finished is 40 to 110Ω,
A solar cell is fabricated by processing the structure in which the n-type diffusion layer sheet resistance is increased by 0.1 to 20Ω with respect to the n-type diffusion layer sheet resistance at the time when the removal of the phosphor glass is completed at the time of surface removal. A solar cell production method to obtain,
In the surface removal step, the hydrofluoric acid (HF) aqueous solution has a hydrogen fluoride concentration of 5 to 10 % by weight,
The temperature of the aqueous hydrofluoric acid (HF) solution is 20-25 ° C .;
A solar cell production method, wherein the treatment time is within 6 minutes.

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