Method for producing a substrate for a solar cell
Patent Information
- Authority / Receiving Office
- DE · DE
- Patent Type
- Patents
- Current Assignee / Owner
- SHIN ETSU CHEMICAL CO LTD
- Filing Date
- 2015-11-25
- Publication Date
- 2026-07-02
AI Technical Summary
The minority carrier lifetime of silicon single crystal substrates is shortened due to oxygen-induced defects when subjected to high-temperature thermal diffusion, leading to a decrease in the efficiency of solar cells.
A method involving high-temperature heat treatment at 1200°C or more for 30 seconds or more followed by low-temperature heat treatment at 800°C or less than 1200°C to dissolve oxide precipitation nuclei, preventing the growth of oxygen-induced defects and maintaining minority carrier lifetime.
Prevents the shortening of minority carrier lifetime, thereby improving the conversion efficiency of solar cells manufactured using such substrates.
Abstract
Description
Technical field
[0001] The present invention relates to a method for producing a substrate for a solar cell and a substrate for a solar cell. Background of the technology
[0002] General solar cells each consist of an electrode formed from Ag paste material using a screen printing process, and an antireflective coating made of SiN. xThe substrate consists of a layer formed by a CVD process and an emitter layer (n-type diffusion layer) formed by thermal diffusion when a p-type silicon substrate is used (see, for example, patent document 1). In this thermal diffusion process, the emitter layer is formed by vapor-phase diffusion of POCl3 or by coating diffusion of a phosphoric acid-based material, and the substrate is exposed to a temperature of approximately 800°C. To form a boron diffusion layer, such as a BSF layer, for improved efficiency, the substrate must be heated to approximately 1000°C. List of literature on patent literature
[0003] Patent document 1: Japanese unexamined patent application (Kokai) with publication number 2002-076388 Brief description of the invention Problem to be solved by the invention
[0004] However, if a silicon single-crystal substrate undergoes heat treatment at 800°C or higher during the thermal diffusion process described above, or if an oxide film forms on the substrate surface, defects can grow in the silicon single-crystal substrate containing a certain amount of oxygen atoms, thus reducing the minority carrier lifetime of the silicon single-crystal substrate. Consequently, a problem arises in that the properties of solar cells manufactured using such substrates deteriorate. This deterioration of properties is evidently due to the substrate having a higher oxygen concentration.
[0005] The present invention was conceived in view of the problem described above. One object of the present invention is to provide a method for producing a substrate for a solar cell that can prevent a reduction in the minority carrier lifetime of the substrate, even if the substrate has a higher oxygen concentration. Means to solve the problem
[0006] To solve the problems described above, the present invention provides a method for producing a substrate for a solar cell formed from single-crystal silicon, the method comprising the following steps: Manufacturing a silicon single crystal ingot; Cutting a silicon substrate from the silicon single-crystal ingot; and Subjecting the silicon substrate to low-temperature heat treatment at a temperature of 800°C or more and less than 1200°C, wherein the silicon single crystal ingot or the silicon substrate is subjected to high-temperature heat treatment at a temperature of 1200°C or more for 30 seconds or longer prior to low-temperature heat treatment.
[0007] As described above, by subjecting a silicon single-crystal ingot or silicon substrate to high-temperature heat treatment at 1200°C or more for 30 seconds or longer prior to low-temperature heat treatment at 800°C or more and less than 1200°C, it is possible to pre-dissolve oxide deposition nuclei that can be the origin of oxide deposition defects. This prevents the growth of oxygen-induced defects even after low-temperature heat treatment in the subsequent manufacturing process. Accordingly, it is possible to produce a substrate in which a reduction in minority carrier lifetime is prevented, thus improving the conversion efficiency of the solar cell produced using the manufactured substrate.In the explanation of the present invention, the heat treatment at a temperature of 800°C or more and less than 1200°C is referred to for the sake of simplicity as ‘low temperature heat treatment’ to distinguish it from the ‘high temperature heat treatment’ at a temperature of 1200°C or more.
[0008] In this case, it is preferable that the silicon substrate be subjected to high-temperature heat treatment after cutting the silicon substrate from the silicon single-crystal ingot.
[0009] By subjecting a silicon substrate to high-temperature heat treatment, as described above, it is possible to reliably dissolve oxide deposition nuclei that are the origin of oxide deposition defects, thereby making it possible to reliably prevent a reduction in the minority carrier lifetime of the substrate.
[0010] In this case, low-temperature heat treatment may include dopant diffusion treatment or oxygen treatment.
[0011] In the production of a substrate for a solar cell, many doping diffusion and oxygen treatments are carried out in the low-temperature heat treatment range. The present invention can be suitably applied when the doping diffusion or oxygen treatment, which performs such low-temperature heat treatment, is carried out.
[0012] In this case, the silicon substrate can have an oxygen concentration of 12 ppm or more.
[0013] When the oxygen concentration of a silicon substrate is 12 ppm or higher, the deterioration of the properties of a solar cell is particularly significant with conventional methods, and the present invention can be suitably applied. Furthermore, the oxygen concentration in a silicon substrate in the explanation of the present invention is an atomic ratio (in this case, the unit is also described as 'ppma') based on a new ASTM standard.
[0014] In this case, the silicon single-crystal ingot can be doped with phosphorus.
[0015] If a silicon single-crystal ingot is n-type phosphor-doped, the present invention can be suitably applied.
[0016] In this case, it is preferable that the silicon single-crystal ingot is doped with gallium and that the high-temperature heat treatment is carried out for 30 minutes or less.
[0017] By using gallium as a p-type dopant to dope a single-crystal silicon ingot, it is possible to more effectively prevent a reduction in the minority carrier lifetime of a substrate. If the high-temperature heat treatment lasts 30 minutes or less, gallium evaporation from the substrate surface can be prevented, thus preventing an increase in surface resistance. This, in turn, prevents a decrease in the fill factor of a solar cell fabricated using such a substrate.
[0018] In this case, the high-temperature heat treatment is preferably carried out in an atmosphere containing phosphorus oxychloride.
[0019] If the high-temperature heat treatment is carried out in an atmosphere containing phosphorus oxychloride, a reduction in the minority carrier lifetime of the substrate due to the strong gettering effect of phosphorus can be more effectively prevented.
[0020] The present invention also provides a substrate for a solar cell, which is produced by the method for producing a substrate for a solar cell as described above.
[0021] Such a substrate for a solar cell makes it possible to prevent the reduction of the minority carrier lifetime of the substrate. This makes it possible to improve the conversion efficiency of the solar cell produced using such a substrate. Effect of the invention
[0022] As described above, the inventive method for producing a substrate for a solar cell enables the production of a substrate in which the reduction of the minority carrier lifetime is prevented. This makes it possible to improve the conversion efficiency of the solar cell produced using the manufactured substrate. The inventive substrate for a solar cell enables the reduction of the minority carrier lifetime of the substrate to be prevented. This makes it possible to improve the conversion efficiency of the solar cell produced using such a substrate. Brief description of the drawings
[0023] Fig. Figure 1 is a flowchart showing an example of the embodiment of the inventive method for producing a substrate for a solar cell;
[0024] Fig.Figure 2 is a flowchart showing another example of the embodiment of the inventive method for producing a substrate for a solar cell;
[0025] Fig. Figure 3 is a sectional view showing an example of the substrate according to the invention for a solar cell;
[0026] Fig. 4 is a sectional view showing an example of a solar cell made using the substrate for a solar cell made of Fig. 3 is produced, shows;
[0027] Fig. 5 is a flowchart that shows a process flow for manufacturing the substrate for a solar cell from Fig. 3 shows;
[0028] Fig. 6 is a flowchart that shows a process flow for manufacturing the solar cell from Fig. 4 shows;
[0029] Fig. 7 is a sectional view showing another example of the substrate according to the invention for a solar cell;
[0030] Fig. Figure 8 is a sectional view showing another example of a solar cell produced using the substrate according to the invention. Fig. 7 shows;
[0031] Fig. Figure 9 is a flowchart that shows a process flow for manufacturing the substrate for a solar cell. Fig. 7 shows;
[0032] Fig. 10 is a flowchart that shows a workflow for manufacturing the solar cell from Fig. 8 shows;
[0033] Fig. Figures 11(a) and (b) are diagrams showing EL images of solar cells, each produced using the substrate for a solar cell in Example 1 or Comparative Example 1. Description of embodiments
[0034] The present invention is described below with reference to the figures specifically as an example of the embodiment, although the present invention is not limited thereto.
[0035] As described above, defects can occur when a silicon single-crystal substrate is subjected to heat treatment at 800°C or higher and below 1200°C, and the like. In these cases, the silicon single-crystal substrate, containing a certain amount of oxygen atoms, grows due to the oxygen, thus shortening the minority carrier lifetime of the silicon single-crystal substrate. Consequently, a problem arises in that the properties of solar cells manufactured using such substrates deteriorate.
[0036] Accordingly, the inventors of the present invention have thoroughly investigated a method for producing a substrate for a solar cell that can prevent a reduction in the minority carrier lifetime of the substrate, even if the substrate has a higher oxygen concentration.Accordingly, the inventors of the present invention have discovered that the substrate in which a reduction of the minority carrier lifetime is prevented can be produced by subjecting a silicon single-crystal ingot or a silicon substrate to high-temperature heat treatment at a temperature of 1200°C or more for 30 seconds or longer prior to medium-temperature heat treatment at a temperature of 800°C or more and less than 1200°C, thereby making it possible to dissolve oxide deposition nuclei, which may represent the origin of oxide deposition defects, beforehand in order to prevent the growth of oxygen-induced defects even after low-temperature heat treatment in the subsequent manufacturing process; thus completing the present invention.
[0037] An example of the embodiment of the inventive method for producing a substrate for a solar cell (of the first embodiment) is described below by reference to Fig. 1 described.
[0038] First, a silicon single crystal ingot is produced (see step S11 in Fig. 1) Specifically, a silicon single-crystal ingot is produced, for example, by a CZ process (Czochralski process). In this phase, the silicon single-crystal ingot can be doped with n-type or p-type dopant to achieve a desired conductivity type. It should be noted that when growing a silicon single crystal using a CZ process, oxygen from a quartz crucible, which holds the raw material melt, is introduced into the single crystal.
[0039] Then the silicon single-crystal ingot produced in step S11 is sliced into a wafer of a silicon substrate (see step S12 in Fig.1) In particular, a silicon substrate of a certain thickness is cut from the silicon single crystal into a wafer shape using a cube saw, a wire saw, etc.
[0040] The silicon substrate cut in step S12 is then subjected to high-temperature heat treatment for 30 seconds or longer at a temperature of 1200°C or more (see step S13 in Fig. 1) Here, the high-temperature heat treatment temperature means the maximum temperature applied to a silicon substrate during the heat treatment; the high-temperature heat treatment time means the time during which the temperature is maintained at 1200°C or higher. Furthermore, the high-temperature heat treatment can be carried out using a lamp curing device, a horizontal furnace, a vertical furnace, etc. The upper limit of the high-temperature heat treatment is theoretically the melting point of silicon.
[0041] After step S13, the silicon substrate undergoes low-temperature heat treatment at a temperature of 800°C or more and less than 1200°C in the process of manufacturing a substrate for a solar cell (see step S14 in Fig. 1).
[0042] As described above, a silicon substrate undergoes high-temperature heat treatment at 1200°C or higher for 30 seconds or more prior to low-temperature heat treatment. This pre-dissolves oxide deposition nuclei (forming a solid solution of oxide deposition nuclei) that are the origin of oxide deposition defects. This prevents the growth of oxygen-induced defects, even after low-temperature heat treatment in the subsequent manufacturing process. Consequently, it is possible to produce a substrate that prevents a reduction in minority carrier lifetime. This, in turn, improves the conversion efficiency of the solar cell manufactured using the produced substrate.
[0043] Subsequently, a further example of the embodiment of the inventive method for producing a substrate for a solar cell (the second embodiment) is described with reference to Fig. 2 described. The explanation that overlaps with the first embodiment is accordingly omitted.
[0044] First, a silicon single crystal ingot is produced (see step S21 in Fig. 2).
[0045] The silicon single-crystal ingot produced in step S21 is then subjected to a high-temperature heat treatment at a temperature of 1200°C or more for 30 seconds or longer (see step S22 in Fig. 2).
[0046] The silicon single crystal ingot, which has undergone high-temperature heat treatment, is then cut into a slice of silicon substrate (see step S23 in Fig. 2).
[0047] The silicon substrate is then subjected to low-temperature heat treatment at a temperature of 800°C or more and less than 1200°C in a process for manufacturing a substrate for a solar cell (see step S24 in Fig. 2).
[0048] As above using Fig. As described in section 2, the high-temperature heat treatment in the present invention can be carried out on the silicon single-crystal ingot.
[0049] In both the first and second embodiments described above, the low-temperature heat treatment can include dopant diffusion treatment or oxygen treatment. In the production of a substrate for a solar cell, many dopant diffusion and oxygen treatments are performed as the low-temperature heat treatment described above. The present invention can be suitably applied if each of the dopant diffusion and oxygen treatments is performed as a low-temperature heat treatment.
[0050] In this process, the oxygen concentration of the silicon substrate can be 12 ppm (12 ppma according to the new ASTM standard) or higher. The present invention can be suitably applied when the silicon substrate has an oxygen concentration of 12 ppm or higher. In particular, a silicon single-crystal ingot produced by a CZ process (a CZ crystal) tends to form a silicon substrate, when cut from the ingot, with a high oxygen concentration of 12 ppm or higher. The oxygen concentration tends to be particularly high in an initial stage of drawing the CZ crystal (at one cone face). If the oxygen concentration varies depending on the position within a silicon single-crystal ingot, as described above, it is possible to form a substrate for a solar cell without excluding a silicon substrate with a higher oxygen concentration.
[0051] In this process, the silicon single-crystal ingot can be doped with phosphorus. When the silicon single-crystal ingot is n-type doped with phosphorus, the present invention can be suitably applied.
[0052] In this process, it is preferable to dope the silicon single-crystal ingot with gallium and to perform the high-temperature heat treatment for 30 minutes or less. Using gallium as a p-type dopant to dope a silicon single-crystal ingot more effectively prevents a reduction in the minority carrier lifetime of the substrate. If the high-temperature heat treatment lasts 30 minutes or less, gallium evaporation from the substrate surface can be prevented, thus preventing an increase in the substrate surface resistance. This, in turn, prevents a decrease in the fill factor of a solar cell fabricated using such a substrate.
[0053] In this process, it is preferable that the high-temperature heat treatment be carried out in an atmosphere containing phosphorus oxychloride. When the high-temperature heat treatment is performed in an atmosphere containing phosphorus oxychloride, a reduction in the minority carrier lifetime of the substrate due to the strong gettering effect of phosphorus can be more effectively prevented. In this atmosphere containing phosphorus oxychloride, the substrate is doped with phosphorus. This does not cause any problems, since a phosphorus-doped substrate, for example, was originally doped with phosphorus; a gallium-doped substrate will be doped with an n-type dopant anyway by forming a pn junction.
[0054] Below is an example of the embodiment of the substrate according to the invention for a solar cell with reference to Fig. 3 described.
[0055] The substrate for a solar cell 10 out of Fig. 3 is achieved by the above using Fig. 1 and Fig. Two described methods for producing a substrate for a solar cell were used. The substrate for a solar cell 10 For example, the phosphorus-doped silicon substrate 100 , the emitter layer (boron diffusion layer) 110 , which are located on the light-receiving surface of the phosphor-doped silicon substrate 100 is planned, and the BSF layer (phosphorus diffusion layer) 111 , which are located on the back side of the phosphorus-doped silicon substrate 100 is planned. The emitter layer 110 is with the anti-reflective coating 120 the light-receiving surface on the light-receiving surface of the same and the BSF layer 111 is equipped with the anti-reflective coating on the back 121 provided on the reverse side of the same.
[0056] The substrate for a solar cell 10 The silicon single-crystal ingot or silicon substrate was fabricated by performing high-temperature heat treatment at a temperature of 1200°C or higher for 30 seconds or longer prior to low-temperature heat treatment at a temperature of 800°C or higher but less than 1200°C. This process dissolved any oxide deposition nuclei, which are the origins of oxide deposition defects. This prevents the growth of oxide deposition defects even after the low-temperature heat treatment in the subsequent fabrication process. The low-temperature heat treatment includes a boron diffusion heat treatment, for example, to form the emitter layer. 110 The substrate for a solar cell 10is a substrate in which the reduction of a minority carrier lifetime as described above is prevented, making it possible to improve the conversion efficiency of the solar cell manufactured using such a substrate.
[0057] Below is an example of the embodiment of a solar cell manufactured using the substrate for a solar cell with reference to Fig. 4 described.
[0058] The solar cell 11 out of Fig. 4 is connected to the electrode 130 the light-receiving surface on the light-receiving surface of the emitter layer 110 of the substrate for a solar cell 10 provided and with the rear electrode 131 on the reverse side of the BSF layer 111 of the substrate for a solar cell 10 planned. Furthermore, it shows Fig. 4. An example of a solar cell 11 , in which the electrode 130the light-receiving surface electrically connected to the emitter layer 110 , which are due to the antireflective coating 120 the light-receiving surface, is connected, and is the rear electrode. 131 electrically with the BSF layer 111 , which is due to the rear surface anti-reflective layer 121 It happened, it's connected.
[0059] An example of the manufacturing process according to the invention for the substrate for a solar cell is then given. 10 out of Fig. 3 (an embodiment using a phosphorus-doped substrate) with reference to Fig. 5 described. The present invention is not limited to the following method for producing the substrate for a solar cell.
[0060] First, the phosphorus-doped silicon substrate cut from a silicon single crystal ingot is 100 cleaned after removal of the damaged layer (see Fig.5(a)). The removal of the damaged layer can be carried out by removing the phosphorus-doped silicon substrate. 100 for example, immersed in a hot, concentrated aqueous potassium hydroxide solution.
[0061] The phosphorus-doped silicon substrate is then 100 , after the damaged layer has been removed, subjected to high-temperature heat treatment at a temperature of 1200°C or more for 30 seconds or longer (see Fig. 5(b)). In this process, the high-temperature heat treatment can be carried out using a lamp curing device, a horizontal furnace, a vertical furnace, etc.
[0062] The phosphorus-doped silicon substrate is then 100 after high-temperature heat treatment, subjected to texture etching, followed by cleaning (see Fig.5(c)). Texture etching can be carried out, for example, by immersion in an aqueous potassium hydroxide / 2-propanol solution. It should be noted that texture etching allows the formation of a fine roughness, referred to as texture, to reduce the reflectivity of the light-receiving surface. Furthermore, this texture etching process can be performed prior to high-temperature heat treatment. In this case, it is also possible to perform the high-temperature heat treatment and the diffusion mask formation step of the subsequent step continuously, using a horizontal furnace, etc.
[0063] Then, on the phosphorus-doped silicon substrate 100 , which was subjected to texture etching to form the emitter layer 110 a diffusion mask formed (see Fig. 5(d)). The formation of a diffusion mask can be carried out, for example, by using the phosphorus-doped silicon substrate100 The material is placed in a horizontal furnace, and oxide films are grown by thermal oxidation, followed by etching of one side of the oxide film. In this phase, the oxidation is preferably carried out by wet oxidation, which can grow an oxide film faster than dry oxidation, and also with regard to cost. This thermal oxidation can be performed at a temperature within the range of low-temperature heat treatment.
[0064] The phosphorus-doped silicon substrate is then 100 with the diffusion mask formed on it subjected to boron diffusion (see Fig. 5(e)). Boron diffusion can be carried out, for example, by using the phosphorus-doped silicon substrate 100It is placed in a horizontal furnace, followed by heat treatment in an atmosphere of argon and BBr3 gas. This step removes the emitter layer. 110 (Boron diffusion layer) 110 formed. This heat treatment is carried out, for example, at a temperature of approximately 1000°C (in the temperature range of the “low-temperature heat treatment”).
[0065] Then the phosphorus-doped silicon substrate is 100 treated with hydrofluoric acid to remove boron glass and a silicon oxide film formed on its surface (see Fig. 5(f)).
[0066] Subsequently, the phosphorus-doped silicon substrate is used. 100 , which is subjected to boron diffusion, a diffusion mask to form the BSF layer 111 formed (see Fig. 5(g)). The formation of the diffusion mask can be carried out by the substrate 100 with the emitter layer formed on it 110The material is placed in a horizontal furnace and the oxide film is grown by thermal oxidation, followed by etching of the oxide film on the back surface. This thermal oxidation treatment can also be carried out at a temperature within the temperature range of "low-temperature heat treatment".
[0067] Then the phosphorus-doped silicon substrate is 100 with the diffusion mask formed on it, subjected to phosphorus diffusion (see Fig. 5(h)). Phosphorus diffusion can be carried out, for example, by using the phosphorus-doped silicon substrate 100 It is placed in a horizontal furnace, followed by heat treatment in an oxygen and POCl3 atmosphere. This step creates the BSF layer (phosphorus diffusion layer). 111 formed. In this phosphorus diffusion using POCl3, the substrate is typically exposed to heat at a temperature of approximately 800°C.
[0068] Next, the phosphorus-doped silicon substrate will be 100 treated with hydrofluoric acid to remove phosphor glass and a silicon oxide film on its surface (see Fig. 5(i)). A passivation film of an oxide film or an aluminum oxide film can be applied to the surface of the phosphorus-doped silicon substrate. 100 Passivation films are formed to improve conversion efficiency. In this process, an oxide film can be formed by thermal oxidation, LPCVD, etc.; an aluminum oxide film can be formed by PECVD, ALD, etc.
[0069] Then the antireflective coating is applied 120 the light-receiving surface on the light-receiving surface of the emitter layer 110 of the phosphorus-doped silicon substrate 100 formed (see Fig. 5(j)). The formation of the antireflective coating 120The light-receiving surfaces can be modified, for example, by forming a silicon nitride film using plasma CVD.
[0070] Then the anti-reflective coating on the back is applied. 121 on the reverse side of the BSF layer 111 of the phosphorus-doped silicon substrate 100 formed (see Fig. 5(k)). The formation of the rear antireflective layer 121 This can be achieved, for example, by forming a silicon nitride film using plasma CVD.
[0071] In the manner described above, a substrate for a solar cell can be created. 10 out of Fig. 3 will be produced.
[0072] The following is an example of a method for manufacturing the solar cell. 11 out of Fig. 4 using the substrate for a solar cell 10 out of Fig. 3 with reference to Fig. 6 described.
[0073] First, the rear electrode is131 on the back surface of the rear anti-reflective coating 121 of the substrate for a solar cell 10 formed (see Fig. 6(a)). The rear electrode 131 It can be created in a desired pattern using, for example, silver paste by screen printing.
[0074] The electrode is then 130 the light-receiving surface on the light-receiving surface of the antireflective coating 120 the light-receiving surfaces of the substrate for a solar cell 10 formed (see Fig. 6(b)). The electrode 130 The light-receiving surface can be screen-printed in a desired pattern using, for example, silver paste.
[0075] Then, a solar cell is applied to the substrate. 10 , on which the rear electrode 131 and the electrode 130a firing process is carried out on the light-receiving surface (see Fig. 6(c)). The firing temperature is, for example, 600–850°C. It should be noted that the electrode 130 the light-receiving surface and the emitter layer 110 as well as the rear electrode 131 and the BSF shift 111 by passing the rear electrode 131 and the electrode 130 the light-receiving surface through the anti-reflective coating 120 the light-receiving surface and the rear anti-reflective layer 121 during the firing process without forming an opening in the antireflective coating 120 the light-receiving surfaces and the rear anti-reflective coating 121 They may be electrically connected.
[0076] The solar cell 11 out of Fig. 4 can be produced in the manner described above.
[0077] In the following, a further example of the embodiment of the substrate according to the invention for a solar cell is described with reference to Fig. 7 described.
[0078] The substrate for a solar cell 20 out of Fig. 7 is used with the in Fig. 1 and Fig. The substrate for a solar cell is produced according to the two methods described above. 20 For example, the gallium-doped silicon substrate 101 and the emitter layer (phosphorus diffusion layer) 112 on the light-receiving surface of the gallium-doped silicon substrate 101 The emitter layer 112 is with the anti-reflective coating 120 the light-receiving surfaces on the light-receiving surface of the same.
[0079] The substrate for a solar cell 20It is produced by subjecting the silicon single-crystal ingot or silicon substrate to a high-temperature heat treatment at 1200°C or higher for 30 seconds or more, prior to low-temperature heat treatment at 800°C or higher and less than 1200°C. Accordingly, oxide deposition nuclei that cause oxide deposition defects are dissolved beforehand. This prevents the growth of oxygen-induced defects even after the low-temperature heat treatment in the subsequent manufacturing process. The low-temperature heat treatment in this process is, for example, a phosphorus diffusion heat treatment to form the emitter layer. 112 The substrate for a solar cell 20is a substrate in which a reduction of the minority carrier lifetime as described above is prevented in order to improve the conversion efficiency of the solar cell produced by using the manufactured substrate.
[0080] In the following, a further example of the embodiment of the substrate according to the invention for a solar cell is described with reference to Fig. 8 described.
[0081] In the solar cell 21 out of Fig. 8 is the emitter layer 112 of the substrate for a solar cell 20 with the electrode 130 The back surface of the substrate is intended for a solar cell. 20 with the rear aluminum electrode 132 It is designed and is the back side of the substrate for a solar cell. 20 with the BSF layer (aluminum diffusion layer) 113 intended. In the solar cell21 out of Fig. 8 is the electrode 130 the light-receiving surface electrically connected to the emitter layer 112 connected, whereby they formed the antireflective coating 120 the light-receiving surface, and is the rear aluminum electrode 132 electrically with the BSF layer 113 tied together.
[0082] The following is an example of a process for manufacturing the substrate for a solar cell. 20 out of Fig. 7 (an embodiment using a gallium-doped substrate) with reference to Fig. 9 described in more detail. Explanations relating to the embodiment which uses a phosphorus-doped silicon substrate. 100 Terms that have already been used and described are omitted accordingly.
[0083] First, the gallium-doped silicon substrate is 101, which is cut from a silicon single-crystal ingot, cleaned after removal of the damaged layer (see Fig. 9(a)).
[0084] The gallium-doped silicon substrate 101 The removed damaged layer is subjected to high-temperature heat treatment at a temperature of 1200°C or more for 30 seconds or longer (see Fig. 9(b)).
[0085] Then the gallium-doped silicon substrate is 101 after high-temperature heat treatment, subjected to texture etching, followed by cleaning (see Fig. 9(c)). This texture etching step can be performed before or after the high-temperature heat treatment step.
[0086] Subsequently, the gallium-doped silicon substrate is used. 101 , which is subjected to texture etching, a diffusion mask to form the emitter layer 112 formed (see Fig.9(d)). The diffusion mask can be formed by the gallium-doped silicon substrate 101 is placed in a horizontal furnace and the oxide film is formed by thermal oxidation, followed by an etching process on one side.
[0087] Next, the gallium-doped silicon substrate will be 101 with the diffusion mask formed on it, subjected to phosphorus diffusion (see Fig. 9(e)). Phosphorus diffusion can be carried out, for example, by using the gallium-doped silicon substrate 101 It is placed in a horizontal furnace, followed by heat treatment in an oxygen and POCl3 gas atmosphere. It is also possible to reduce production costs by using two parts of the gallium-doped silicon substrate. 101Without forming the diffusion masks described above, the POCl3 gas is placed on a piece of wood or a quartz boat in such a way that it does not reach one side of each piece during diffusion, whereupon a phosphorus diffusion layer forms on the other side of each piece. The heat treatment temperature in this phase is typically around 800°C.
[0088] Then the gallium-doped silicon substrate is 101 treated with hydrofluoric acid to remove phosphor glass and a silicon oxide film formed on its surface (see Fig. 9(f)).
[0089] Then the antireflective coating is applied 120 the light-receiving surfaces on the light-receiving surface of the emitter layer 112 of the gallium-doped silicon substrate 101 formed (see Fig. 9(g)). The formation of the antireflective coating 120The light-receiving surface can be modified, for example, by forming a silicon nitride film using plasma CVD.
[0090] In the manner described above, the substrate for a solar cell can be prepared. 20 out of Fig. 7 will be produced.
[0091] The following is an example of a process for manufacturing the solar cell. 21 out of Fig. 8 using the substrate for a solar cell 20 out of Fig. 7 with reference to Fig. 10 described.
[0092] First, the rear aluminum electrode is 132 on the back surface of the substrate for a solar cell 20 except formed on the busbar electrode part (see Fig. 10(a)). The rear aluminum electrode 132 can be placed on the back of the substrate for a solar cell 20 for example, they can be formed by screen printing using aluminum paste.
[0093] Subsequently, a silver electrode can be placed on the busbar electrode portion of the substrate's back surface for a solar cell. 20 formed using silver paste by screen printing (see Fig. 10(b)).
[0094] Then the electrode 130 the light-receiving surface on the light-receiving surface of the antireflective coating 120 the light-receiving surface of the substrate for a solar cell 20 formed (see Fig. 10(c)).
[0095] Next, a solar cell will be prepared on the substrate. 20 , on which the rear aluminum electrode 132 and the electrode 130 a firing process is carried out on the light-receiving surface (see Fig. 10(d)). During this firing process, aluminum diffuses from the rear aluminum electrode. 132 to the gallium-doped silicon substrate 101, to the BSF layer (aluminum diffusion layer) 113 to form. It should be noted that the electrode 130 the light-receiving surface and the emitter layer 112 by passing the electrode 130 the light-receiving surface through the anti-reflective coating 120 the light-receiving surfaces can be electrically connected during the firing process without an opening in the anti-reflective coating 120 to form the light-receiving surfaces.
[0096] The solar cell can be constructed in the manner described above. 21 out of Fig. 8 will be produced. Examples
[0097] The present invention is described in more detail below with reference to examples and comparative examples, although the present invention is not limited thereto. (Example 1)
[0098] The substrate for a solar cell 10 out of Fig.3 was made according to the manufacturing process from Fig. 5. The prerequisite was that an n-type substrate with a resistance of 1 Ω·cm (pulled by CZ process, oxygen concentration of 17 to 18 ppm) was used as the phosphorus-doped silicon substrate. 100 was used and the high-temperature heat treatment was carried out for 5 minutes in a horizontal furnace (using a SiC tube) under conditions of 1250°C in a nitrogen atmosphere.
[0099] Using the manufactured substrate for a solar cell, 100 solar cells were produced. 11 out of Fig. 4 according to the manufacturing process Fig. 6. The EL (electroluminescence) image of the manufactured solar cell is shown in Fig. 11(a) shown. As shown from Fig.As can be seen from Figure 11(a), an oxygen-induced spiral defect, described below, is excluded. The battery characteristics (short-circuit current density, open-circuit voltage, fill factor, and conversion efficiency) were measured on the manufactured solar cells. The results are shown in Table 1. Here, the short-circuit current density is a current density value when the resistor connected to the solar cell exhibits a resistance of 0 Ω; the open-circuit voltage is a voltage value when the resistor connected to the solar cell exhibits a very high resistance; the fill factor (form factor) is the maximum electrical power produced / (short-circuit current × open-circuit voltage); and the conversion efficiency is (output from the solar cell / solar energy supplied to the solar cell) × 100. (Example 2)
[0100] The substrate for a solar cell 10 out of Fig.Example 3 was produced in the same way as in Example 1. The prerequisite was that the high-temperature heat treatment was carried out in such a way as to prevent phosphorus diffusion. 10 Minutes in a horizontal furnace (using a SiC tube) under conditions of 1200°C in a POCl3 atmosphere, resolving oxide deposition defects and performing phosphorus getters (getters with phosphorus).
[0101] Using the manufactured substrate for a solar cell, 100 solar cells were produced. 11 out of Fig. 4. The solar cells were manufactured in the same way as in Example 1. The battery properties (short-circuit current density, open-circuit voltage, fill factor, and conversion efficiency) were measured on the manufactured solar cells. The results are shown in Table 1. (Comparative example 1)
[0102] The substrate for a solar cell 10 out of Fig.Example 3 was produced in the same way as in Example 1. The prerequisite was that the high-temperature heat treatment was not carried out.
[0103] Using the manufactured substrate for a solar cell, 100 solar cells were produced. 11 out of Fig. 4. Produced in the same way as in Example 1. The EL image of the produced solar cell is shown in Fig. 11(b) shown. As shown Fig. As can be seen in Figure 11(b), an oxygen-induced spiral defect is observed. The battery properties (short-circuit current density, open-circuit voltage, fill factor, and conversion efficiency) were measured on the manufactured solar cells. The results are shown in Table 1. [Table 1] Short-circuit current density (standard deviation) Open circuit voltage (standard deviation) Fill factor (standard deviation) Conversion efficiency (standard deviation) Example 1 39.3 mA / cm 2 (0.127 mA / cm 2 ) 648 mV (1.69 mV) 0,789 (0,00350) 20,1% (0,094%) Example 2 39.6 mA / cm 2 (0.127 mA / cm 2 ) 648 mV (1.69 mV) 0,789 (0,00367) 20,3% (0,093%) Example 3 38.2 mA / cm 2 (0.379 mA / cm 2 ) 636 mV (3.96 mV) 0,794 (0,00540) 19,3% (0,385%)
[0104] As can be seen from Table 1, the conversion efficiency in Examples 1 and 2 was improved compared to the reference example 1, each using a phosphorus-doped substrate. In each of Examples 1 and 2, the variations in the solar cell properties (short-circuit current density, open-circuit voltage, fill factor, and conversion efficiency) were reduced compared to those in reference example 1. This is likely due to the high-temperature heat treatment performed in Examples 1 and 2, which prevents oxygen-induced defects compared to reference example 1 (see Fig. 11), in which the high-temperature heat treatment was not performed, thus preventing a reduction in the lifetime of the bulk portion of each substrate. It was also found that the conversion efficiency was improved in Example 2, in which the high-temperature heat treatment was performed in a POCl3 atmosphere. (Example 3)
[0105] The substrate for a solar cell 20 out of Fig. 7 was made according to the manufacturing process from Fig. 9. The prerequisite was that a p-type substrate with a resistance of 1 Ω·cm (pulled by CZ process, oxygen concentration of 17 to 18 ppm) was used as the gallium-doped silicon substrate. 101 was used and the high-temperature heat treatment was carried out for 5 minutes in a horizontal furnace (using a SiC tube) under conditions of 1250°C in a nitrogen atmosphere.
[0106] Using the manufactured substrate for a solar cell, 100 solar cells were produced. 21 out of Fig. 8 according to the manufacturing process Fig. 10 solar cells were produced. The battery properties (short-circuit current density, open-circuit voltage, fill factor, and conversion efficiency) were measured on the produced solar cells. The results are shown in Table 2. (Example 4)
[0107] The substrate for a solar cell 20 out of Fig. Example 7 was produced in the same way as in Example 3. The prerequisite was that the high-temperature heat treatment was carried out such that phosphorus diffusion was performed in a horizontal furnace (using a SiC tube) under conditions of 1200°C for 10 minutes in a POCl3 atmosphere, thereby resolving oxide deposition defects and performing phosphorus gettering.
[0108] Using the manufactured substrate for a solar cell, 100 solar cells were produced. 21 out of Fig. 8 were manufactured in the same way as in Example 3. The battery properties (short-circuit current density, open-circuit voltage, fill factor, and conversion efficiency) were measured on the manufactured solar cells. The results are shown in Table 2. (Comparative example 2)
[0109] The substrate for a solar cell 20 out of Fig. Example 7 was produced in the same way as in Example 3. The prerequisite was that the high-temperature heat treatment was not carried out.
[0110] Using the manufactured substrate for a solar cell, 100 solar cells were produced. 21 out of Fig. 8 produced in the same way as in Example 3.
[0111] The battery characteristics (short-circuit current density, open-circuit voltage, fill factor, and conversion efficiency) were measured on the manufactured solar cells. The results are shown in Table 2. (Example 5)
[0112] The substrate for a solar cell 20 out of Fig.7 was produced in the same way as in Example 3. The prerequisite was that the high-temperature heat treatment was carried out for 40 minutes in a horizontal furnace (using a SiC tube) under conditions of 1250°C in a nitrogen atmosphere.
[0113] Using the manufactured substrate for a solar cell, 100 solar cells were produced. 21 out of Fig. 8 were manufactured in the same way as in Example 3. The battery properties (short-circuit current density, open-circuit voltage, fill factor, and conversion efficiency) were measured on the manufactured solar cells. The results are shown in Table 2. [Table 2] Short-circuit current density (standard deviation) Open circuit voltage (standard deviation) Fill factor (standard deviation) Conversion efficiency (standard deviation) Example 3 38.3 mA / cm 2 (0.128 mA / cm 2 ) 640 mV (1.71 mV) 0,804 (0,00364) 19,7% (0,095%) Example 4 38.5 mA / cm 2 (0.132 mA / cm 2 ) 643 mV (1.83 mV) 0,804 (0,00371) 19,9% (0,108%) Comparative example 2 37.5 mA / cm 2 (0.372 mA / cm 2 ) 631 mV (3.93 mV) 0,799 (0,00543) 18,9% (0,377%) Example 5 38.4 mA / cm 2 (0.137 mA / cm 2 ) 638 mV (1.78 mV) 0,783 (0,00440) 19,2% (0,115%)
[0114] As can be seen from Table 2, the conversion efficiency in Examples 3 to 5 was improved compared to the reference example 2, each using a gallium-doped substrate. In each of Examples 3 to 5, the variations in the solar cell properties (short-circuit current density, open-circuit voltage, fill factor, and conversion efficiency) were reduced compared to those in reference example 2. This is likely due to the high-temperature heat treatment performed in Examples 3 to 5, which prevents oxygen-induced defects compared to reference example 2, where the high-temperature heat treatment was not performed, thus preventing a reduction in the lifetime of the bulk portion of each substrate. It was also found that the conversion efficiency was improved in Example 4, where the high-temperature heat treatment was performed in a POCl3 atmosphere.Furthermore, in Examples 3 and 4, the filling factor and conversion efficiency were improved with high-temperature heat treatment lasting 30 minutes or less compared to Example 5 with high-temperature heat treatment lasting 30 minutes or more. This is likely due to the high-temperature heat treatment being performed for 30 minutes or less in Examples 3 and 4, which prevented the removal of a gallium dopant from the substrate, thus preventing an increase in the substrate's resistance and consequently, the series resistance.
[0115] It should be noted that the present invention is not limited to the embodiment described above. The embodiment is only exemplary, and any examples that have essentially the same features and exhibit the same functions and effects as those in the technical concept described in the claims of the present invention are included within the technical scope of disclosure of the present invention.
Claims
[1] Method for producing a substrate for a solar cell formed from a single crystal silicon, the method comprising the following steps: Manufacturing a silicon single crystal ingot; Cutting a silicon substrate from the silicon single-crystal ingot; and Subjecting the silicon substrate to low-temperature heat treatment at a temperature of 800°C or more and less than 1200°C, wherein the silicon single crystal ingot or the silicon substrate is subjected to high-temperature heat treatment at a temperature of 1200°C or more for 30 seconds or longer prior to low-temperature heat treatment. [2] Method for producing a substrate for a solar cell according to claim 1, wherein the silicon substrate is subjected to high-temperature heat treatment after cutting the silicon substrate from the silicon single crystal ingot. [3] Method for producing a substrate for a solar cell according to claim 1 or 2, wherein the low-temperature heat treatment includes dopant diffusion treatment or oxygen treatment. [4] Method for producing a substrate for a solar cell according to any one of claims 1 to 3, wherein the silicon substrate has an oxygen concentration of 12 ppm or more. [5] Method for producing a substrate for a solar cell according to any one of claims 1 to 4, wherein the silicon single crystal ingot is doped with phosphorus. [6] Method for producing a substrate for a solar cell according to any one of claims 1 to 4, wherein the silicon single crystal ingot is doped with gallium and the high-temperature heat treatment is carried out for 30 minutes or less. [7] Method for producing a substrate for a solar cell according to any one of claims 1 to 6, wherein the high-temperature heat treatment is carried out in an atmosphere containing phosphorus oxychloride. [8] Substrate for a solar cell, wherein the substrate for a solar cell is produced by the method for producing a substrate for a solar cell according to any one of claims 1 to 7.