Solar cell manufacturing process

Abstract

Methods for manufacturing a solar cell, comprising the steps: (A) Preparing a silicon substrate with p-type conductivity (4); (B) Forming a passivation film (5) on a main surface of the silicon substrate; and (C) Manufacturing a solar cell using the silicon substrate, including the passivation film formed thereon, in which step (B) includes the following steps: (B-1) Forms a solution (14) containing aluminium acetylacetonate into a mist; and (B-2) Spraying the solution formed into a mist onto the main surface of the silicon substrate in the atmosphere to form the passivation film, which is an Al2O3 film; wherein in step (B-2) the silicon substrate is heated to a film formation temperature of 360°C and ozone is added to the heated silicon substrate to form the passivation film.

Description

TECHNICAL AREA

[0001] The present invention relates to a method for manufacturing solar cells, and more specifically a method for forming a passivation film on a silicon substrate. STATE OF THE ART

[0002] In the field of crystalline silicon-based solar cells, silicon substrates have been thinned to reduce the amount of silicon used and improve the conversion efficiency of the silicon substrates. Unfortunately, thinner silicon substrates exhibit a significantly lower conversion efficiency. This is because, for example, a large number of defects on the conductive front surface of the silicon substrate primarily cause a reduction in the lifetime of minority charge carriers (e.g., electrons in a p-type substrate) generated by light irradiation. Thus, reducing the loss of minority charge carriers ultimately improves the conversion efficiency of solar cells.

[0003] To regulate the reduction in lifetime of charge carriers, a passivation film is generally formed on the back surface of the silicon substrate. An aluminum oxide film, which is one of many types of passivation films, has attracted attention due to its superior passivation effect (its function in regulating lifetime reduction) on a p-type silicon substrate as described above.

[0004] The aluminum oxide films contain negative, fixed charges and are known to induce a passivation effect resulting from the field effect caused by the fixed charge. Thus, the aluminum oxide film containing negative, fixed charges is formed on the front surface of the p-type silicon substrate to regulate the diffusion of electrons, which are minority charge carriers, into the substrate surface, thereby preventing charge carrier loss.

[0005] The aluminium oxide film, which is the passivation film, is formed on the p-type silicon substrate by chemical vapor deposition (CVD) (see e.g. JP 2012 - 33 538 A). State of the art

[0006] Methods for manufacturing a solar cell with a passivation film are known, for example, from DE 11 2011 105 618 T5, US 2013 / 0 084 672 A1, NAKANO, H. [et al.]: Low cost and high performance antireflective coatings for solar cells. Optical engineering, Vol. 24, 1985, No. 1, pp. 207-212. – ISSN 0091-3286.

[0007] Furthermore, processes for the production of metal oxides are known from US 5 318 857 A. SUMMARY OF THE INVENTION Problems to be solved by the invention

[0008] Unfortunately, forming an aluminum oxide film by CVD requires a material such as trimethylaluminum (TMA), which is expensive and difficult to handle. In CVD, a film-forming area must undergo vacuum processing, increasing the cost of film formation. Furthermore, in plasma-based CVD, the silicon substrate could be damaged by the plasma.

[0009] Alternatively, an aluminum oxide film can be formed on the silicon substrate by atomic layer deposition (ALD). Unfortunately, ALD also requires TMA and vacuum processing, resulting in higher manufacturing costs. Furthermore, the significantly slower film formation rate of ALD could lead to poor production efficiency. Plasma-assisted ALD could potentially be used to improve the film formation rate. Unfortunately, the silicon substrate could be damaged in plasma-assisted ALD.

[0010] The present invention therefore aims to provide a method for manufacturing solar cells that enables the formation of an aluminum oxide film, which is a passivation film, with a high degree of production efficiency at lower manufacturing costs, without causing damage to a silicon substrate. Means to solve the problems

[0011] To solve the problem described above, a method according to the invention for manufacturing solar cells comprises the steps (A) preparing a silicon substrate with p-type conductivity, (B) forming a passivation film on a major surface of the silicon substrate, and (C) manufacturing a solar cell using the silicon substrate comprising the passivation film formed thereon. Step (B) comprises the steps (B-1) atomizing a solution containing aluminum acetylacetonate, and (B-2) spraying the atomized solution onto the major surface of the silicon substrate in the atmosphere to form the passivation film, which is an Al₂O₃ film. In step (B-2), the silicon substrate is heated to a film-forming temperature of 360 °C, and ozone is added to the heated silicon substrate to form the passivation film. Effects of the invention

[0012] The inventive process for manufacturing solar cells comprises the steps (A) preparing the silicon substrate with p-type conductivity, (B) forming the passivation film on the main surface of the silicon substrate, and (C) manufacturing the solar cell using the silicon substrate comprising the passivation film formed thereon. Step (B) comprises the steps (B-1) atomizing the solution containing aluminum acetylacetonate, and (B-2) spraying the atomized solution onto the main surface of the silicon substrate in the atmosphere to form the passivation film, which is an Al₂O₃ film. In step (B-2), the silicon substrate is heated to a film-forming temperature of 360 °C, and ozone is added to the heated silicon substrate to form the passivation film.

[0013] This enables the formation of a back-surface passivation film, consisting of an aluminum oxide film, on the p-type silicon substrate using a cost-effective and easily handled material. The process eliminates the need for, for example, vacuum processing, thus reducing manufacturing costs. The film formation process does not damage the p-type silicon substrate. The process also improves manufacturing efficiency.

[0014] These and other tasks, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when considered in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 A cross-sectional view of a solar cell configuration. Fig. 2 A view of a configuration of a film formation device for implementing an exemplary film formation process. Fig. 3 A view of a configuration of a film formation device for implementing a film formation process according to an embodiment of the invention. DESCRIPTION OF EXECUTION FORMS

[0015] Fig. Figure 1 is a cross-sectional view of a basic configuration of a solar cell.

[0016] As in Fig. As shown in Figure 1, a silicon layer (3) (hereinafter referred to as the n-type silicon layer (3)) with n-type conductivity is formed on the upper surface (the front surface) of a silicon substrate (4) (hereinafter referred to as the p-type silicon substrate (4)) with p-type conductivity. A transparent front surface passivation film (such as a silicon oxide film or a silicon nitride film) (2) is formed on the upper surface (the front surface) of the n-type silicon layer (3). A front surface electrode (1) is formed in the front surface passivation film (2) and is connected to the n-type silicon layer (3).

[0017] A back-surface passivation film (5) is formed on the lower surface (the back surface) of the p-type silicon substrate (4). An aluminum oxide film (AlOx) is used as the back-surface passivation film (5). A back-surface electrode (6), which is connected to the p-type silicon substrate (4), is formed on the back-surface passivation film (5).

[0018] In the Fig. In the solar cell shown, light shining from the side of the front surface passivation film (2) reaches a pn junction between the silicon layer (3) and the silicon substrate (4) (the n-type silicon layer (3) and the p-type silicon substrate (4)) to generate charge carriers so that electricity is produced, and then the produced electricity is carried away through the electrodes 1 and 6.

[0019] As described above, the passivation films (2) and (5) are formed to regulate the reduction in charge carrier lifetimes. A large number of defects (such as lattice defects) occur on the main surfaces of the silicon layer (3) and the silicon substrate (4), and thus minority charge carriers generated by light irradiation are recombined by these defects. The passivation films (2) and (5) are formed on the main surfaces of the silicon layer (3) and the silicon substrate (4) to regulate charge carrier recombination, thereby improving the charge carrier lifetimes.

[0020] The present invention relates to a method for forming an aluminum oxide film (5) as a back-surface passivation film (5) on the p-type silicon substrate (4). The present invention is described in detail below with reference to the drawings, which illustrate one embodiment thereof.

[0021] Fig. Figure 2 is a view of a schematic configuration of an exemplary film-forming device capable of implementing a method for forming an aluminum oxide film (5).

[0022] As in Fig. As shown in Figure 2, the film-forming device comprises a reactor (11), a heating device (13), a solution container (15) and a fog-forming device (16).

[0023] In the film-forming device, an aluminum acetylacetonate solution (14), which has been atomized or formed into a mist, is sprayed onto the back surface of the p-type silicon substrate (4), thereby forming the aluminum oxide film (5), which is an Al2O3 film, on the back surface of the p-type silicon substrate (4).

[0024] While the p-type silicon substrate (4) is placed on the heating device (13), mist (the aqueous solution (14) with a small particle diameter) is introduced into the reactor (11) in the atmosphere, and then the aluminum oxide film (5), which is an Al₂O₃ film, is formed on the back surface of the p-type silicon substrate (4) as a result of a given reaction. Note that the front surface of the p-type silicon substrate (4) is placed on the heating device (13).

[0025] The heating device (13) can, for example, be a heating device capable of heating the silicon substrate (4) placed on the heating device (13). During film formation, the heating device (13) is heated to 360°C by an external control device.

[0026] The solution vessel (15) is filled with the starting material solution (hereinafter referred to as the solution) (14) for forming the aluminum oxide film (5). The solution (14) contains aluminum acetylacetonate as the metal source.

[0027] For example, an ultrasonic atomizing device can be used as a fogging device (16). The fogging device (16), which is the ultrasonic atomizing device, sends ultrasonic waves to the solution (14) in the solution vessel (15), thereby atomizing / forming the aluminum acetylacetonate solution (14) in the solution vessel (15) into a fog. The atomized aluminum acetylacetonate solution (14) passes through a channel (L1) that leads to the back surface of the p-type silicon substrate (4) in the reactor (11).

[0028] The atomized aluminum acetylacetonate solution (14) is fed to the reactor (11), and then the solution (14) reacts on the p-type silicon substrate (4), which has been heated to 360°C in the atmosphere, forming the aluminum oxide film (5) on the back surface of the p-type silicon substrate (4). The aluminum acetylacetonate solution (14) remaining unreacted in the reactor (11) is continuously discharged from the reactor (11) through a channel (L3).

[0029] The following describes an exemplary procedure for forming a backside surface passivation film (5) (of the aluminium oxide film (5)).

[0030] First, a given impurity is introduced into a silicon substrate to produce a silicon substrate (the p-type silicon substrate (4)) with p-type conductivity. Then, the p-type silicon substrate (4) is placed on the heating device (13) in the reactor (11). At this point, the placement surface is the front surface of the p-type silicon substrate (4), and the interior of the reactor (11) is at atmospheric pressure.

[0031] The heating device (13) heats the p-type silicon substrate (4), which is placed on the heating device (13), to a film formation temperature of 360°C, at which the aluminum oxide film (5), which is an Al2O3 film, is to be formed. The p-type silicon substrate (4) is maintained at the film formation temperature.

[0032] Meanwhile, in the solution vessel (15), the solution (14) is formed into a mist by the fogging device (16). The aluminum acetylacetonate solution (14) formed into a mist (the solution (14) with a small particle diameter) flows through the channel (L1), undergoes flow adaptation, and is fed to the reactor (11). The solution (14) contains aluminum acetylacetonate as the metal source.

[0033] The solution (14), formed into a mist and subjected to flux matching, is applied to the back surface of the p-type silicon substrate (4) using atmospheric heat. The solution (14) is sprayed onto the p-type silicon substrate (4) using heat, and then the aluminum oxide film (5), which is an Al₂O₃ film, is formed on the back surface of the p-type silicon substrate (4).

[0034] Then the solar cell is connected to the Fig. The configuration shown in Figure 1 is produced using the p-type silicon substrate (4), including the back-surface passivation film (5) (of the aluminum oxide film (5)) formed thereon. Generally, the back-surface passivation film (5) is formed after the formation of the n-type silicon layer (3).

[0035] As described above, an exemplary method for forming the backside surface passivation film (5) (of the aluminium oxide film (5)) employs the misting / atomization method (the method of forming a film by spraying the aqueous solution (14) into the atmosphere) to form the aluminium oxide film (5), which is an Al2O3 film, on the backside surface of the p-type silicon substrate (4).

[0036] In, for example, CVD or ALD, the aluminum oxide film (5) is formed by adding the vaporized starting material to the p-type silicon substrate (4). In contrast, according to the invention, the aluminum oxide film (5), which is an Al₂O₃ film, is formed by spraying the atomized aqueous aluminum acetylacetonate solution (14) onto the p-type silicon substrate (4). As described above, the solution (14) contains aluminum acetylacetonate. Thus, the back-surface passivation film (5), which is made from an Al₂O₃ film, can be formed on the p-type silicon substrate (4) using a material that is inexpensive and easy to handle, instead of using a material such as TMA, which is expensive and difficult to handle.

[0037] The present invention, which is an atmospheric film-forming process, eliminates the need for vacuum processing and the like, thus reducing manufacturing costs. According to the invention, the atomized aluminum acetylacetonate solution (14) is sprayed onto the p-type silicon substrate (4) to perform the film-forming process. Thus, in the film-forming process, the p-type silicon substrate (4) is not damaged, for example, by plasma irradiation.

[0038] The aluminum oxide film (5) is formed by the atomization process at a rate of 10 to 15 nm / min, which is five times or higher than the rate at which the aluminum oxide film is formed by, for example, ALD. Thus, the use of the film formation process according to the invention can also improve production efficiency.

[0039] While a general film formation process has been described above, in step (B-2) of the process according to the invention, ozone is added to the silicon substrate, which has been heated to the film formation temperature of 360°C, to form the passivation film. The inventors have thoroughly carried out a number of different experiments, analyses, and the like, thereby successfully finding the process for forming the aluminum oxide film (5), which is an Al₂O₃ film, that can significantly improve the lifetime of charge carriers. That is, the inventors have successfully found film formation conditions for increasing the passivating effects of the aluminum oxide film (5). The film formation process is described below.

[0040] Fig. Figure 3 is a view of a schematic setup of a film-forming device with which the method for forming the aluminium oxide film (5) according to the present embodiment can be implemented.

[0041] As a comparison between Fig. 2 and Fig. As can be seen in section 3, the film-forming device according to the present embodiment has the configuration in Fig. 2 and additionally includes an ozone generator (17). The following describes the part that differs from the configuration in Fig. 2 differs.

[0042] The ozone generator (17) is capable of generating ozone. For example, in the ozone generator (17), a high voltage is applied between the parallel electrodes, which are arranged in parallel, and oxygen is passed between the electrodes to decompose oxygen molecules, which then recombine with other oxygen molecules to form ozone.

[0043] As in Fig. As shown in Figure 3, the ozone generator (17) and the reactor (11) are connected to each other by a channel (L2) that is separate from the channel (L1). Thus, the ozone generated by the ozone generator (17) is directed through the channel (L2) to be supplied to the back surface of the p-type silicon substrate (4) in the reactor (11).

[0044] The configuration, apart from the above details, is the same as described above, and a description of the same configuration is omitted.

[0045] The method for forming the backside surface passivation film (5) (of the aluminium oxide film (5)) according to the present embodiment is given below.

[0046] The p-type silicon substrate (4) is placed on the heating device (13) in the reactor (11) at atmospheric pressure. The p-type silicon substrate (4) placed on the heating device (13) is heated to the film formation temperature of 360°C, at which the aluminum oxide film (5), which is an Al2O3 film, is to be formed, and the p-type silicon substrate (4) is maintained at the film formation temperature.

[0047] Meanwhile, in the solution vessel (15), the aluminum acetylacetonate solution (14) is atomized into a mist by the fogging device (16). The aluminum acetylacetonate solution (14) formed into a mist (the aqueous solution (14) with a small particle diameter) passes through the channel (L1), undergoes flow adjustment, and is fed to the reactor (11). The solution (14) contains aluminum acetylacetonate as the metal source. For example, a methanol solution containing dissolved aluminum acetylacetonate can be used as solution (14). In the present embodiment, ozone is generated by the ozone generator (17), and the generated ozone is passed through the channel (L2) to be directed into the reactor (11).

[0048] The aluminum acetylacetonate solution (14), formed into a mist and subjected to flow adjustment, is sprayed onto the back surface of the p-type silicon substrate (4) using atmospheric heat. Ozone is then introduced, forming the aluminum oxide film (5), which is an Al₂O₃ film, on the back surface of the p-type silicon substrate (4). The aluminum acetylacetonate solution (14) and ozone, which remain unreacted in the reactor (11), are continuously discharged from the reactor (11) through channel (L3).

[0049] Then the solar cell is connected to the Fig. 1 configuration shown using the p-type silicon substrate (4) including the back surface passivation film (5) formed thereon (of the aluminium oxide film (5), which is an Al2O3 film).

[0050] As described above, in the process for forming the backside surface passivation film (5) (of the aluminum oxide film (5)) according to the present embodiment, the aluminum acetylacetonate solution (14) is sprayed into the reactor (11) by the atomization process, and then ozone is supplied to the reactor (11) so that the aluminum oxide film (5), which is an Al2O3 film, is formed on the backside surface of the p-type silicon substrate (4).

[0051] Thus, the solar cell produced by the film formation process described in the present embodiment exhibits an improved lifetime of charge carriers compared to the solar cell produced by the film formation process described in embodiment 1. That is, the addition of ozone gas during the formation of the aluminum oxide film (5) can increase the passivation effect of the aluminum oxide film (5).

[0052] The p-type silicon substrate (4) was produced using a float zone (FZ) technology. The p-type silicon substrate (4) has a resistivity of 3 Ω-cm. The p-type silicon substrate (4) had a thickness of 280 µm.

[0053] The film formation process according to the preceding general description was implemented on a p-type silicon substrate (4), resulting in the formation of an aluminum oxide film (Al₂O₃) with a film thickness of 60 nm on the p-type silicon substrate (4). The film formation process according to the embodiment of the invention was implemented on another p-type silicon substrate (4), resulting in the formation of an aluminum oxide film (Al₂O₃) with a film thickness of 60 nm on the other p-type silicon substrate (4). Both of these film formation processes exhibited the same film formation conditions, except with respect to the presence or absence of ozone input.

[0054] Then the microwave photoconductivity decay (µ-PCD) technique was implemented on one p-type silicon substrate (4) including the Al2O3 formed thereon, and on the other p-type silicon substrate (4) including the Al2O3 formed thereon to measure the lifetime of charge carriers.

[0055] It was ultimately found that the lifetime of charge carriers in the other p-type silicon substrate (4) was four to five times longer than the lifetime of charge carriers in the one p-type silicon substrate (4). In other words, a significant improvement in the lifetime of charge carriers resulting from the ozone supply during the formation of the aluminum oxide film (5) was experimentally verified. EXPLANATION OF REFERENCE SYMBOLS 4. p-type silicon substrate 5 Backside surface passivation film (aluminum oxide film) 11 Reactor 13 Heating device 14 Aluminum acetylacetonate solution 15 solution containers 16 Fog generation device 17 Ozone generator L1, L2, L3 channel

Claims

[1] Method for manufacturing a solar cell, comprising the steps: (A) Preparing a silicon substrate with p-type conductivity (4); (B) Forming a passivation film (5) on a main surface of the silicon substrate; and (C) Manufacturing a solar cell using the silicon substrate, including the passivation film formed thereon, in which step (B) includes the following steps: (B-1) Forms a solution (14) containing aluminium acetylacetonate into a mist; and (B-2) Spraying the solution formed into a mist onto the main surface of the silicon substrate in the atmosphere to form the passivation film, which is an Al2O3 film; wherein in step (B-2) the silicon substrate is heated to a film formation temperature of 360°C and ozone is added to the heated silicon substrate to form the passivation film.

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