Semiconductor device, solar cell, and method for manufacturing semiconductor device

Abstract

The present invention realizes a hole-selective film having both hole selectivity and passivation characteristics. A method for manufacturing a semiconductor device includes a step of forming a titanium oxide film on a crystalline silicon layer by thermal atomic layer deposition and a step of subjecting the titanium oxide film to hydrogen plasma treatment.

Description

Technical Field

[0001] The present invention relates to semiconductor devices, solar cells, and manufacturing techniques for semiconductor devices, which can be effectively applied to, for example, semiconductor devices constituting solar cells and their manufacturing techniques. Background Art

[0002] In Non-Patent Document 1, it is described that a titanium oxide film (TiOx film) formed on a buffer layer made of hydrogenated amorphous silicon functions as an electron-selective film or a hole-selective film depending on manufacturing conditions and the like.

[0003] In Non-Patent Documents 2 and 3, it is described that a titanium oxide film directly formed on a crystalline silicon layer as a photoanode for hydrolysis functions as a hole-selective film.

[0004] In Non-Patent Documents 4 to 8, as films constituting a transparent hole-selective film in a solar cell using crystalline silicon, molybdenum oxide films (MoOx films), vanadium oxide films (V2Ox films), tungsten oxide films (WOx films), etc. are described.

[0005] In Non-Patent Document 9, a solar cell using a titanium oxide film formed on a crystalline silicon layer by thermal atomic layer deposition as an electron-selective layer is described.

[0006] In Non-Patent Document 10, it is described that in a heterojunction solar cell, a passivation film is used, which is a passivation film formed by sequentially laminating a hydrogenated amorphous silicon film (a-Si:H) and a hydrogenated amorphous silicon nitride film (a-SiNx:H) on the surface of a crystalline silicon layer.

[0007] In Non-Patent Document 11, a technique of using a titanium oxide film as a passivation film formed on the light-receiving surface of a crystalline silicon solar cell is described.

[0008] Prior Art Documents

[0009] Non-Patent Documents

[0010] Non-Patent Document 1: T. Matsui et al., Energy Procedia 124(2017)628

[0011] Non-Patent Document 2: Y. W. Chen et al., Nature Materials 10(2011)539

[0012] Non-Patent Document 3: S. Hu et al., Science 344(2014)6187

[0013] Non-Patent Document 4: C. Battaglia et al., Nano Letters 14(2014)967

[0014] Non-Patent Document 5: J. Bullock et al., Energy Procedia 77(2015)446

[0015] Non-Patent Document 6: L.G. Gerling et al., Solar Energy Materials & Solar Cells, 145(2016)109

[0016] Non-Patent Document 7: L.G. Gerling et al., Energy Procedia 124(2017)584

[0017] Non-Patent Document 8: M. Bivour et al., Solar Energy Materials & Solar Cells 142(2015)34

[0018] Non-Patent Document 9: X. Yang et al., Advanced Materials 28(2016)5891

[0019] Non-Patent Document 10: A. Descoeudres et al., Prog Photovolt Res Appl. 1-9(2019). DOI:10.1002 / pip.3227

[0020] Non-Patent Document 11: J. Cui et al., Solar Energy Materials & Solar Cells 158(2016)115 Summary of the Invention

[0021] Problems to be Solved by the Invention

[0022] For example, in recent years, the development of carrier-selective membranes with carrier selectivity has been promoted. Specifically, as the carrier-selective membrane, a hole-selective membrane that selectively allows holes to pass through and an electron-selective membrane that selectively allows electrons to pass through can be cited. For such a carrier-selective membrane, it is desired to improve the carrier selectivity that selectively allows carriers to pass through, and it is desired to improve the passivation characteristics that suppress the recombination of electrons and holes at the interface between the carrier-selective membrane and the base membrane. That is, for the carrier-selective membrane, it is desired to simultaneously achieve an improvement in carrier selectivity and an improvement in passivation characteristics.

[0023] In particular, when focusing on the hole-selective film in the carrier-selective film, a hole-selective film that combines good hole selectivity and good passivation characteristics has not been obtained so far. Therefore, in particular, it is desirable to conduct research to obtain a hole-selective film that combines good hole selectivity and good passivation characteristics.

[0024] Other problems and novel features will become clear from the description of this specification and the accompanying drawings.

[0025] Solutions to problems

[0026] A semiconductor device according to an embodiment includes: a crystalline silicon layer having a main surface; an intermediate film provided on the main surface and containing silicon, titanium, and oxygen; and a titanium oxide film provided on the intermediate film. Among them, the intermediate film further contains 1.5 atomic% or more of hydrogen.

[0027] In addition, a method for manufacturing a semiconductor device according to an embodiment includes (a) a step of forming a titanium oxide film on a crystalline silicon layer by thermal atomic layer deposition and (b) a step of performing hydrogen plasma treatment on the titanium oxide film.

[0028] Effects of the invention

[0029] According to an embodiment, a hole-selective film that combines hole selectivity and passivation characteristics can be realized. Description of the drawings

[0030] Figure 1 It is a diagram showing a schematic device structure of a carrier-selective solar cell unit.

[0031] Figure 2 It is a diagram showing a schematic device structure of a carrier-selective solar cell unit in which random pyramid textures are formed on both the front and back surfaces of a crystalline silicon layer.

[0032] Figure 3 It is a flowchart for explaining the manufacturing process of the carrier-selective solar cell of Embodiment 1.

[0033] Figure 4 It is a graph showing the current-voltage characteristics of a carrier-selective solar cell using a crystalline silicon layer having a flat (100) surface.

[0034] Figure 5 It is a graph showing the film thickness dependence of a sample in which a titanium oxide film is formed by thermal atomic layer deposition in a carrier-selective solar cell using a crystalline silicon layer having a flat (100) surface. This graph particularly shows the relationship between the following values and the film thickness of the titanium oxide film: (a) short-circuit current density, (b) open-circuit voltage, (c) fill factor, (d) conversion efficiency.

[0035] Figure 6 It is a graph showing the film formation temperature dependence in the characteristics of a carrier - selective solar cell. This graph particularly shows the relationships between the following values and the film formation temperature during the formation of a titanium oxide film using thermal atomic layer deposition: (a) short - circuit current density, (b) open - circuit voltage, (c) fill factor, (d) conversion efficiency.

[0036] Figure 7 It is a graph showing the relationship between the characteristics of a carrier - selective solar cell and the hydrogen plasma treatment time. This graph particularly shows the relationships between the following values and the hydrogen plasma treatment time: (a) short - circuit current density, (b) open - circuit voltage, (c) fill factor, (d) conversion efficiency.

[0037] Figure 8 It is a photograph showing a part of the cross - sectional structure of a carrier - selective solar cell fabricated using a crystalline silicon layer with a flat (100) surface.

[0038] Figure 9 It is a graph (vertical axis: linearly represented) showing the results of quantitative analysis of each element contained in the intermediate film for a sample having the same structure as a carrier - selective solar cell fabricated using a crystalline silicon layer with a flat (100) surface, performed using Rutherford backscattering spectrometry (RBS) and elastic recoil detection analysis (ERDA).

[0039] Figure 10 It is a graph in which the vertical axis of Figure 9 is represented logarithmically to clarify the difference in hydrogen concentration.

[0040] Figure 11 It is a table comparing the results of quantitative analysis of each element contained in the intermediate film performed using Rutherford backscattering spectrometry (RBS) and elastic recoil detection analysis (ERDA) with the characteristics of a solar cell.

[0041] Figure 12 It is a graph showing the relationship between the wavelength of incident light and the external quantum efficiency.

[0042] Figure 13 It is a table comparing an example of a representative prior study of a front - emitter type in which a hole - selective film is provided on the light - incident surface side of an n - type crystalline silicon layer (light - absorption layer) in a solar cell unit using a hole - selective film containing various materials with the present embodiment.

[0043] Figure 14This is a diagram showing the schematic device structure of the carrier-selective solar cell unit of Modification 1.

[0044] Figure 15 This is a diagram showing the schematic device structure of the carrier-selective solar cell unit of Modification 2.

[0045] Figure 16 This is a diagram showing the schematic device structure of the carrier-selective solar cell unit of Modification 3.

[0046] Figure 17 This is a diagram showing the relationship between the structure and the solar cell characteristics of the carrier-selective solar cell unit of Modification 4.

[0047] Figure 18 This is a diagram showing the structure of a schematic semiconductor device including a p-channel field-effect transistor.

[0048] Figure 19 This is a diagram showing the structure of the back electrode type solar cell unit of Embodiment 3.

[0049] Figure 20 This is a flowchart explaining the manufacturing process flow of the back electrode type solar cell unit of Embodiment 3.

[0050] Figure 21 This is a table explaining the superiority of the passivation characteristics of the second passivation film of Embodiment 3. Detailed Embodiments

[0051] In the following embodiments, for convenience, they are described by being divided into multiple parts or embodiments when necessary, but unless otherwise specified, they are not unrelated to each other, and the relationship between them is that one is a partial or complete modification example, detail, supplementary explanation, etc. of the other.

[0052] In addition, in the following embodiments, when referring to the number of elements, etc. (including the number, value, quantity, range, etc.), unless otherwise specified and unless it is clearly limited to a specific number in principle, it is not limited to that specific number, that is, it can be more than the specific number or less than the specific number.

[0053] And it goes without saying that in the following embodiments, its constituent elements (including element steps, etc.) are not necessarily essential unless otherwise specified or clearly essential in principle.

[0054] Similarly, in the following embodiments, when referring to the shape, positional relationship, etc. of components, etc., examples that are substantially close to or similar to such shapes, etc. should be included, unless specifically stated otherwise or clearly considered not to be the case in principle. The same applies to the above-mentioned numerical values and ranges.

[0055] In addition, in all the figures used to illustrate the embodiments, in principle, the same reference numerals are assigned to the same components, and repeated descriptions thereof are omitted. Furthermore, to make the drawings easier to understand, even in the top view, shading may sometimes be added.

[0056] (Embodiment 1)

[0057] In this Embodiment 1, a carrier-selective solar cell using a carrier-selective film will be taken as an example to illustrate the technical idea related to the carrier-selective film in this embodiment.

[0058] <Carrier-Selective Solar Cell>

[0059] In a general solar cell, a pn junction is formed by bringing a p-type semiconductor layer into contact with an n-type semiconductor layer, and the photogenerated electron-hole pairs are taken out by ohmic electrodes formed on the p-type semiconductor layer and the n-type semiconductor layer, respectively. Solar cells based on such a pn junction (pn homojunction) are currently the most popular. However, in this solar cell, due to the direct contact between the metal constituting the ohmic electrode and the silicon constituting the semiconductor layer, a large number of defects exist at the interface between the metal and the silicon. As a result, the photogenerated electron-hole pairs recombine through the defect levels, leading to a decrease in the performance of the solar cell. In addition, in this solar cell, conductive impurities are doped into silicon at a high concentration to control the characteristics of the solar cell. Therefore, due to the conductive impurities doped at a high concentration, the carrier lifetime decreases and recombination easily occurs. Therefore, the conversion efficiency of a solar cell using a pn homojunction is limited to about 23%.

[0060] On the other hand, for example, a heterojunction solar cell formed by combining hydrogenated amorphous silicon (a-Si:H) and crystalline silicon has currently achieved a relatively high conversion efficiency of over 25%. This is because amorphous silicon has excellent passivation characteristics, which are characteristics that suppress the recombination of electrons and holes caused by dangling bonds (unbonded bonds) formed on the surface of crystalline silicon. In addition, in a heterojunction solar cell, there is no contact between metal and crystalline silicon, so recombination caused by a large number of defects formed at the interface between metal and silicon is also suppressed. Furthermore, in a heterojunction solar cell, conductive impurities are introduced into amorphous silicon, while on the other hand, conductive impurities can be not introduced into crystalline silicon, so it is also possible to suppress the reduction of carrier lifetime caused by conductive impurities introduced into crystalline silicon. This means that the recombination of carriers in crystalline silicon is suppressed. For the above reasons, in a heterojunction solar cell, a higher conversion efficiency than that of a solar cell using a pn homojunction can be obtained.

[0061] However, amorphous silicon is formed, for example, using a specific high-pressure gas represented by silane (SiH4). Therefore, generally speaking, for manufacturing a heterojunction solar cell using amorphous silicon, there is a tendency for equipment investment and maintenance costs to become high. In addition, hydrogenated amorphous silicon has the property of absorbing visible light with a wavelength of 700 nm or less, and the light absorbed by hydrogenated amorphous silicon becomes a loss in energy conversion. Therefore, it can be seen that there are difficult problems to be overcome if it is necessary to reduce the manufacturing cost and improve the conversion efficiency in a heterojunction solar cell.

[0062] Regarding this point, as a new attempt, a carrier-selective solar cell using a transparent carrier-selective film has been proposed in place of a pn homojunction solar cell and a heterojunction solar cell. This carrier-selective solar cell forms a hole-selective film and an electron-selective film in such a way as to sandwich a light absorber (usually a semiconductor of a single conductivity type), and extracts a hole current through the hole-selective film and an electron current through the electron-selective film, thereby operating as a solar cell.

[0063] Such a carrier-selective solar cell has the potential to be realized by a simple film-forming technique, and thus there is a possibility of reducing the manufacturing cost of the solar cell. In addition, among the materials constituting the carrier-selective film such as the hole-selective film and the electron-selective film, there are many materials that are transparent in the wavelength range of sunlight, and the light absorption loss can be reduced, so there is a possibility of improving the conversion efficiency of the solar cell.

[0064] In view of the above situation, the development of carrier-selective solar cells is being promoted.

[0065] <Room for improvement in carrier-selective solar cells>

[0066] The key to improving the performance of a carrier-selective solar cell lies in the carrier-selective film. That is, for the carrier-selective film, a higher function of selectively extracting carriers is required. For example, for a hole-selective film, a higher function of selectively extracting holes is required. On the other hand, for an electron-selective film, a higher function of selectively extracting electrons is required.

[0067] In addition, for the carrier-selective film, excellent passivation characteristics are required. This passivation characteristic is a characteristic that suppresses the recombination of electrons and holes caused by dangling bonds present on the surface of the light absorber. This is because, as long as recombination can be suppressed, the conversion efficiency of the solar cell can be improved. Therefore, for the carrier-selective film used in a carrier-selective solar cell, a high function of selectively extracting carriers and excellent passivation characteristics are required. For example, for a hole-selective film, a higher function of selectively extracting holes and excellent passivation characteristics are required. Similarly, for an electron-selective film, a higher function of selectively extracting electrons and excellent passivation characteristics are required.

[0068] Here, when particularly focusing on the hole-selective film among the carrier-selective films used in carrier-selective solar cells, in existing hole-selective films, the situation where both excellent passivation characteristics and a high function of selectively extracting holes are achieved has not been reached. For example, molybdenum oxide, vanadium oxide, etc. have attracted attention as hole-selective films, but their passivation characteristics are not good. In order to ensure the passivation characteristics, a buffer film represented by an amorphous silicon film needs to be interposed between the light absorption layer and the hole-selective film. That is, in order to make a carrier-selective solar cell practical, a hole-selective film with both hole selectivity and passivation characteristics needs to be realized, but in existing technologies, a technology for realizing a hole-selective film with both hole selectivity and passivation characteristics has not been established. That is, when making a carrier-selective solar cell practical, there is room for improvement in terms of obtaining a hole-selective film with both hole selectivity and passivation characteristics.

[0069] Therefore, in the present Embodiment 1, research has been carried out to obtain a hole-selective film with both good hole selectivity and good passivation characteristics. Hereinafter, the technical idea of the present Embodiment 1 in which this research has been carried out will be described.

[0070] <Summary of the technical idea of Embodiment 1>

[0071] The technical idea of Embodiment 1 is as follows: A titanium oxide film (TiOx film), which was previously considered to function as an electron-selective film for a carrier-selective solar cell, is used as a hole-selective film, and the manufacturing process of the titanium oxide film is studied to realize a titanium oxide film having both good hole selectivity and good passivation characteristics.

[0072] Moreover, hereinafter, it is demonstrated that the titanium oxide film can function as a hole-selective film in a carrier-selective solar cell using crystalline silicon.

[0073] In addition, it is also explained that, according to the technical idea of Embodiment 1, better passivation characteristics and higher power generation performance can be obtained than those of the hole-selective films composed of molybdenum oxide films, tungsten oxide films, vanadium oxide films, etc., which have been reported many times before.

[0074] Furthermore, most of the previous studies on hole-selective films using oxide semiconductor films were conducted on hole-selective films that are flat surfaces and formed on the silicon (100) surface. In this regard, in this embodiment, it is also mentioned that a titanium oxide film directly formed on a silicon (111) facet with a more practical random pyramid texture structure can also achieve high power generation performance.

[0075] <Device Structure of Carrier-Selective Solar Cell>

[0076] Figure 1 It is a diagram showing a schematic device structure of a carrier-selective solar cell unit. It should be noted that in Figure 1 , the grid-shaped metal electrode portion on the light incident surface is omitted. In addition, the same omission is made in Figure 2 and Figures 14 to 16 described later.

[0077] In Figure 1 , the carrier-selective solar cell unit 100 has a crystalline silicon layer 10. The crystalline silicon layer 10 is composed of, for example, an n-type silicon layer doped with an n-type impurity such as phosphorus (P). Moreover, the front surface (first main surface) of the crystalline silicon layer 10 is a flat surface composed of the silicon (100) surface. Similarly, the back surface (second main surface) of the crystalline silicon layer 10 is also a flat surface composed of the silicon (100) surface.

[0078] Next, as shown in Figure 1 , a hole-selective film 11 is formed on the front surface of the crystalline silicon layer 10 in direct contact with the crystalline silicon layer 10. On the other hand, as shown in Figure 1As shown, on the back surface of the crystalline silicon layer 10, an electron-selective film 12 is formed in direct contact with the crystalline silicon layer 10. That is, the crystalline silicon layer 10 is sandwiched between the hole-selective film 11 and the electron-selective film 12. At this time, the hole-selective film 11 is composed of a titanium oxide film. In contrast, the electron-selective film 12 is composed of, for example, a titanium oxide film or an amorphous silicon film containing hydrogen, but in the first embodiment, it is not particularly limited.

[0079] Next, as Figure 1 shown, a transparent electrode 13 is disposed on the hole-selective film 11. The transparent electrode 13 is composed of a transparent film that is at least transparent to visible light contained in sunlight and has conductivity. On the other hand, a metal electrode 14 is disposed in contact with the electron-selective film 12. The metal electrode 14 is formed of, for example, a silver film.

[0080] In the carrier-selective solar cell unit 100 configured as described above, for example, light is incident on the carrier-selective solar cell unit 100 from above the transparent electrode 13. Then, the light passes through the transparent electrode 13 having transparency and the titanium oxide film constituting the hole-selective film 11 having transparency and is incident on the crystalline silicon layer 10. Then, electrons are excited from the valence band of the crystalline silicon layer 10 to the conduction band by light having light energy greater than the bandgap of silicon in the light incident on the crystalline silicon layer 10. As a result, electron-hole pairs are formed inside the crystalline silicon layer 10. Among the paired electrons and holes, the holes (h+) pass through the titanium oxide film serving as the hole-selective film 11 and reach the transparent electrode 13. On the other hand, among the paired electrons and holes, the electrons (e−) pass through the electron-selective film 12 and reach the metal electrode 14. Thereby, a potential difference is generated between the transparent electrode 13 and the metal electrode 14. That is, when light is irradiated on the carrier-selective solar cell unit 100, an electromotive force is generated between the transparent electrode 13 and the metal electrode 14. Therefore, when a load is connected between the transparent electrode 13 and the metal electrode 14, the electromotive force generated between the transparent electrode 13 and the metal electrode 14 can be used to drive the load. In this way, the carrier-selective solar cell unit 100 operates.

[0081] Here, for example, in Figure 1 the shown carrier-selective solar cell unit 100, the surface of the crystalline silicon layer 10 is a flat surface. Therefore, the reflection loss of light at this flat surface is large, and the short-circuit current density of the carrier-selective solar cell unit 100 is low. As a result, the performance of the carrier-selective solar cell unit 100 deteriorates. Therefore, in order to improve the performance of the carrier-selective solar cell unit 100 so that it is suitable for practical applications, for example, a random pyramid texture structure is formed on the surface of the crystalline silicon layer 10.

[0082] Specifically, Figure 2 is a schematic device structure diagram of a carrier-selective solar cell unit 200 in which a random pyramid texture structure is formed on both the front and back surfaces of the crystalline silicon layer 10. In Figure 2 , both the front and back surfaces of the crystalline silicon layer 10 are composed of silicon (111) facets, and a random pyramid texture structure is formed. Thus, in Figure 2 the carrier-selective solar cell unit 200 shown, the conversion efficiency can be improved by utilizing the reflection reduction effect and light trapping effect generated by the random pyramid texture structure. That is, according to the carrier-selective solar cell unit 200 with a random pyramid texture structure as shown in Figure 2 , a solar cell with excellent performance suitable for practical applications can be provided. Similarly, in the carrier-selective solar cell unit 200 configured in this way, among the electron-hole pairs generated in the crystalline silicon layer 10 by the incidence of light, holes (h+) pass through the titanium oxide film constituting the hole-selective film 11 and reach the transparent electrode 13. On the other hand, among the electron-hole pairs, electrons (e−) pass through the electron-selective film 12 and reach the metal electrode 14. Thus, in Figure 2 the carrier-selective solar cell unit 200 shown, when a load is connected between the transparent electrode 13 and the metal electrode 14, the electromotive force generated between the transparent electrode 13 and the metal electrode 14 can be utilized to drive the load.

[0083] <Characteristics of the device structure>

[0084] Next, the characteristic points of the device structure of the carrier-selective solar cell unit in Embodiment 1 will be described. The characteristic points in Embodiment 1 are, for example, that the hole-selective film 11 shown in Figure 1 is composed of a titanium oxide film. Figure 2 Thus, according to Embodiment 1, a hole-selective film 11 having both hole selectivity and passivation characteristics can be obtained.

[0085] Here, "hole selectivity" means the property of selectively allowing holes to pass through while selectively blocking electrons. For example, good "hole selectivity" means excellent performance of selectively allowing holes to pass through while selectively blocking electrons.

[0086] In addition, "passivation characteristics" mean suppressing the presence in the base layer in contact with the carrier-selective film (for example, Figure 1 、 Figure 2The function of the recombination of electrons and holes caused by dangling bonds on the surface of the crystalline silicon layer 10). In particular, focusing on the "passivation characteristics" of the hole-selective film 11 means the function of suppressing the recombination of electrons and holes caused by dangling bonds on the surface of the base layer in contact with the hole-selective film 11. For example, in the hole-selective film 11 with good "passivation characteristics", the dangling bonds on the surface of the base layer are terminated chemically, so that the function of suppressing the recombination of electrons and holes caused by the dangling bonds is excellent. Moreover, in the hole-selective film 11 with good "passivation characteristics", the fixed charges existing inside the hole-selective film 11 generate a field effect that repels electrons from the dangling bonds on the surface of the base layer, so that the function of suppressing the recombination of electrons and holes caused by the dangling bonds is excellent. In particular, in the present embodiment, the feature is that a hole-selective film made of a titanium oxide film achieves passivation characteristics equal to or better than those of the commonly used electron-selective film. In addition, in this specification, the hole-selective film made of a titanium oxide film having good passivation characteristics means that a hole-selective film made of a titanium oxide film can achieve passivation characteristics equal to or better than those of the commonly used electron-selective film.

[0087] As the hole-selective film 11 having such good hole selectivity and good passivation characteristics, a titanium oxide film is used in the first embodiment. This is a feature of the first embodiment. However, it should be noted that not just any titanium oxide film will do. That is, although the feature of the first embodiment is that the hole-selective film 11 is made of titanium oxide, by specifically studying the manufacturing method of titanium oxide, a titanium oxide film with both good hole selectivity and good passivation characteristics can be obtained. The following explains this point.

[0088] For example, a titanium oxide film can be formed by using plasma-enhanced atomic layer deposition or thermal atomic layer deposition. Here, a titanium oxide film formed by plasma-enhanced atomic layer deposition using oxygen plasma tends to have electron selectivity. On the other hand, a titanium oxide film formed by thermal atomic layer deposition using water vapor tends to have hole selectivity. That is, the titanium oxide film has electron selectivity or hole selectivity depending on the manufacturing method used to form the titanium oxide film. Therefore, in the first embodiment, the titanium oxide film formed by thermal atomic layer deposition is used as the hole-selective film 11. This is because the titanium oxide film formed by thermal atomic layer deposition has a high tendency of hole selectivity. However, the inventors newly found through research that not all titanium oxide films formed by thermal atomic layer deposition can be titanium oxide films with both good hole selectivity and good passivation characteristics. Then, the inventors newly found that on the premise of forming a titanium oxide film by thermal atomic layer deposition, further research on the manufacturing process is required to obtain a titanium oxide film with both good hole selectivity and good passivation characteristics.

[0089] Hereinafter, the research on the manufacturing method for obtaining a titanium oxide film with both good hole selectivity and good passivation characteristics will be described.

[0090] <Manufacturing Method of Carrier Selective Solar Cell>

[0091] Figure 3 It is a flowchart for explaining the manufacturing process of the carrier selective solar cell of the first embodiment. First, an n-type silicon substrate is prepared (S101). For example, the (100) plane of this silicon substrate is used as the front surface, the resistivity is 2 Ωcm, and the thickness is 280 μm. Next, anisotropic etching is performed on the (100) plane of the silicon substrate to form a random texture structure on the silicon substrate (S102). For example, a (111) facet is formed on the front surface of the silicon substrate by wet etching using a solution mainly composed of potassium hydroxide (KOH). As a result, a random texture structure composed of (111) facets is formed on the front surface of the silicon substrate.

[0092] Here, an example of forming a random texture structure to reduce the light reflectance and improve the light trapping effect is described. However, in the manufacturing process of the carrier selective solar cell for evaluation, it is also possible not to form a random texture structure on the front surface of the silicon substrate. However, similarly in the manufacturing process of the carrier selective solar cell for evaluation, forming a random texture structure on the front surface of the silicon substrate enables evaluation corresponding to an actual solar cell.

[0093] Next, after cleaning the silicon substrate, for example, dilute hydrofluoric acid is used to remove the natural oxide films formed on the front and back surfaces of the silicon substrate (S103). Thereafter, an electron-selective film having good passivation characteristics is formed on the back surface (the side opposite to the light incident surface) of the silicon substrate (S104). Here, the electron-selective film having good passivation characteristics can be composed of, for example, a stacked film (a-Si:H i-n) of an intrinsic amorphous silicon film doped with hydrogen and an n-type amorphous silicon film doped with hydrogen. The intrinsic amorphous silicon film doped with hydrogen and the n-type amorphous silicon film doped with hydrogen can be formed, for example, by using plasma CVD (Chemical Vapor Deposition). At this time, the intrinsic amorphous silicon film doped with hydrogen functions as a passivation film, while the n-type amorphous silicon film doped with hydrogen functions as an electron-selective film. As a result, the stacked film of the intrinsic amorphous silicon film doped with hydrogen and the n-type amorphous silicon film doped with hydrogen becomes an electron-selective film having good passivation characteristics.

[0094] In addition, in the first embodiment, an amorphous silicon film commonly used in a heterojunction solar cell is used as the electron-selective film having good passivation characteristics. However, the electron-selective film having good passivation characteristics in this embodiment is not limited thereto, and can be composed of, for example, a film other than the stacked film of the intrinsic amorphous silicon film doped with hydrogen and the n-type amorphous silicon film doped with hydrogen. For example, as the electron-selective film having good passivation characteristics in the first embodiment, a titanium oxide film manufactured by plasma-enhanced atomic layer deposition can also be used. In this case, since a specific high-pressure gas represented by silane (SiH4) is not required, it is possible to suppress equipment investment and maintenance costs.

[0095] Next, dilute hydrofluoric acid is used again on the silicon substrate to remove the native oxide film (S105). After that, a titanium oxide film is formed on the front surface, which is the light incident surface of the silicon substrate (S106). The titanium oxide film is formed by thermal atomic layer deposition. In addition, in the first embodiment, FlexAL of Oxford Instruments was used as the atomic layer deposition apparatus. At this time, as the precursor of titanium, TTIP (Titanium isopropoxide) was used, and as the oxygen source, water vapor (H2O) was used. The dosing time of TTIP for each cycle was set to 1.2 seconds, and for water, the dosing of 1.2 seconds was repeated 5 times. In addition, the film formation temperature of titanium oxide was set within the range of 120 to 350°C. By repeating this ALD cycle 128 times, a titanium oxide film with a thickness of approximately 5 nm was formed on the surface of the silicon substrate. For comparison, a solar cell was also fabricated in which titanium oxide formed by plasma-enhanced atomic layer deposition was formed on the front surface, which is the light incident surface of the silicon substrate. At this time, as the precursor of titanium, TTIP was used in the same manner as in the thermal atomic deposition method, but as the oxygen source, oxygen plasma was used. The dosing time of TTIP for each cycle was set to 1.2 seconds, and the dosing time of oxygen plasma was set to 6 seconds. By repeating this ALD cycle 100 times, a titanium oxide film with a thickness of approximately 5 nm was formed on the front surface of the silicon substrate.

[0096] Next, after forming the titanium oxide film on the front surface of the silicon substrate, a hydrogen plasma treatment for irradiating the front surface of the titanium oxide film with hydrogen plasma is performed (S107), and this hydrogen plasma treatment is performed within a time range of 60 minutes or less. Using the inductively coupled plasma source attached to the above atomic layer deposition apparatus, hydrogen plasma is generated under the conditions of a hydrogen flow rate of 50 sccm, a pressure of 10 Pa, and a discharge power of 600 W. After that, a transparent electrode is formed on the back surface of the electron selective film and the front surface of the titanium oxide film, for example, by sputtering (S108). This transparent electrode is at least transparent to visible light. For example, it is composed of indium-tin oxide (ITO) with a thickness of 70 nm to 150 nm. After that, annealing is performed in a furnace at a temperature of 180°C for 2 hours (S109). Here, annealing is performed in an oxygen-containing atmosphere such as in a low vacuum or in the atmosphere.

[0097] Finally, a silver film as a metal electrode is formed on both the back surface of the transparent electrode formed on the back surface of the electron selective film and the front surface of the transparent electrode formed on the front surface of the titanium oxide film (S110). At this time, the silver film formed on the light incident side is processed into a grid shape in order to ensure a light transmission area. For example, the area of the silver film in the cell unit area is about 4%.

[0098] As described above, the carrier-selective solar cell of Embodiment 1 can be manufactured. Moreover, in the carrier-selective solar cell thus manufactured, the titanium oxide film directly formed on the surface of the silicon substrate (crystalline silicon layer) becomes a hole-selective film having good hole selectivity and good passivation characteristics. That is, in the present embodiment, there are the following characteristic points in the manufacturing method: (1) directly forming a titanium oxide film on the surface of crystalline silicon by thermal atomic layer deposition method, (2) subjecting the titanium oxide film to hydrogen plasma treatment, and (3) annealing in an oxygen-containing atmosphere after forming a transparent electrode on the titanium oxide film. As a result, the titanium oxide film directly formed on crystalline silicon becomes a hole-selective film having good hole selectivity and good passivation characteristics. Hereinafter, the verification results regarding this point will be described.

[0099] <Verification Results>

[0100] Figure 4 It is a graph showing the current-voltage characteristics of a carrier-selective solar cell using, for example, a crystalline silicon layer having a flat (100) plane.

[0101] Here, in Figure 4 the graph (1) shows the current-voltage characteristics of a carrier-selective solar cell having a structure in which a titanium oxide film is not formed on the flat (100) surface of the crystalline silicon layer, but a transparent electrode is formed. Figure 4 The open-circuit voltage of the solar cell shown in the graph (1) of

[0102] is significantly low (0.2 V), which is known to be unfavorable for the performance of the carrier-selective solar cell. Figure 4 On the other hand, in Figure 4 the graph (2) shows the current-voltage characteristics of a carrier-selective solar cell having a structure in which a titanium oxide film is formed on the flat (100) surface of the crystalline silicon layer by thermal atomic layer deposition method at a film formation temperature of 200 °C, and a transparent electrode is formed on the titanium oxide film. When comparing the graph (2) of Figure 4 with the graph (1) of Figure 4 it can be seen that in the solar cell shown in the graph (2) of

[0103] the open-circuit voltage (0.66 V) increases dramatically, and the fill factor (FF) is greatly improved. Figure 4 In contrast, in Figure 4 the graph (3) shows the current-voltage characteristics of a carrier-selective solar cell having a structure in which a titanium oxide film is formed on the flat (100) surface of the crystalline silicon layer by plasma-enhanced atomic layer deposition method at a film formation temperature of 200 °C, and a transparent electrode is formed on the titanium oxide film. It can be seen that the carrier-selective solar cell shown in the graph (3) ofFigure 4 Compared with the carrier - selective solar cell without the titanium oxide film shown in the graph (1), the performance further deteriorates.

[0104] From this, it can be seen that the titanium oxide film formed by plasma - enhanced atomic layer deposition functions not as a hole - selective film but as an electron - selective film, while the titanium oxide film formed by thermal atomic layer deposition functions as a hole - selective film.

[0105] Next, in Figure 4 the carrier - selective solar cell shown in the graph (2), the fill factor (FF) is relatively improved, but it is not sufficient for the performance of the solar cell. Regarding this point, in Figure 4 , the graph (4) shows the current - voltage characteristics of a carrier - selective solar cell with a structure in which a titanium oxide film is formed on a flat (100) surface of a crystalline silicon layer by thermal atomic layer deposition at a film - forming temperature of 200 °C, and after subjecting the titanium oxide film to hydrogen plasma treatment at the same temperature as the film - forming temperature, a transparent electrode is formed on the titanium oxide film. In Figure 4 the carrier - selective solar cell shown in the graph (4), it can be seen that by subjecting the titanium oxide film to hydrogen plasma treatment, compared with Figure 4 the carrier - selective solar cell shown in the graph (2), the fill factor (FF) is significantly improved.

[0106] Based on this, it can be known that by combining directly forming a titanium oxide film on the surface of crystalline silicon by thermal atomic layer deposition with subjecting the titanium oxide film to hydrogen plasma treatment, the performance of a carrier - selective solar cell using the titanium oxide film as a hole - selective film can be significantly improved.

[0107] Next, Figure 5 is a graph showing the film - thickness dependence of a carrier - selective solar cell using, for example, a crystalline silicon layer having a flat (100) surface. Here, a titanium oxide film is formed by thermal atomic layer deposition at a film - forming temperature of 200 °C. In particular, Figure 5 is a graph showing the relationship between the following values and the film thickness of the titanium oxide film: (a) short - circuit current density, (b) open - circuit voltage, (c) fill factor, (d) conversion efficiency.

[0108] In addition, the "film thickness of the titanium oxide film" mentioned here is used with the intention of including an intermediate film (interface layer) formed at the interface between the titanium oxide film and the crystalline silicon layer. That is, in this specification, in particular, the titanium oxide film and the intermediate film are often referred to as the titanium oxide film together, but when it is necessary to distinguish between the titanium oxide film and the intermediate film, the titanium oxide film and the intermediate film are written as distinct films.

[0109] Here, in Figure 5 the “black circle symbol” represents a sample on which hydrogen plasma treatment (HPT) was not performed, while on the other hand, the “× symbol” represents a sample on which hydrogen plasma treatment (HPT) was performed at a temperature of 200 °C, which is the same as the film formation temperature, for 15 minutes.

[0110] First, as shown in (a) of Figure 5 , it can be seen that even if the film thickness of the titanium oxide film is changed, the value of the short-circuit current density is almost constant. In addition, it can be seen that even if hydrogen plasma treatment is performed, the short-circuit current density does not change (since the × mark overlaps behind the black circle symbol, the × symbol is not clearly shown). On the other hand, as shown in (b) of Figure 5 , it can be seen that compared with the case where no titanium oxide film is formed (0 nm), the open-circuit voltage increases dramatically by forming a titanium oxide film. In particular, when the film thickness of the titanium oxide film is 5 nm, the open-circuit voltage shows the maximum value.

[0111] Next, as shown in (c) of Figure 5 , it can be seen that when the film thickness of the titanium oxide film is less than 3 nm, the fill factor deteriorates instead when hydrogen plasma treatment is performed. On the other hand, in (c) of Figure 5 , it can be seen that when the film thickness of the titanium oxide film is greater than 3 nm, the fill factor is improved by performing hydrogen plasma treatment.

[0112] In addition, as shown in (d) of Figure 5 , it can be seen that when the film thickness of the titanium oxide film is greater than 3 nm, the conversion efficiency is higher when hydrogen plasma treatment is performed than when it is not performed.

[0113] From this, it can be known that, for example, when the film thickness of the titanium oxide film is greater than 3 nm, by performing hydrogen plasma treatment on the titanium oxide film, the performance of the carrier-selective solar cell represented by the open-circuit voltage, fill factor, and conversion efficiency can be improved compared with the case where hydrogen plasma treatment is not performed. In addition, for example, when the film thickness of the titanium oxide film is less than 8 nm, higher performance can be obtained.

[0114] Next, here, the verification results of, for example, both a carrier-selective solar cell using a crystalline silicon layer having a flat (100) plane and a carrier-selective solar cell using a crystalline silicon layer having a (111) facet will be described.

[0115] Figure 6 is a graph showing the film formation temperature dependence of the solar cell characteristics in the carrier-selective solar cell. It should be noted that in Figure 6 , the results of performing hydrogen plasma treatment for 15 minutes at the same temperature as the film formation temperature in all samples are shown. In particular,Figure 6 It is a graph showing the relationship between the following values and the film formation temperature during the formation of a titanium oxide film using the thermal atomic layer deposition method: (a) short-circuit current density, (b) open-circuit voltage, (c) fill factor, and (d) conversion efficiency.

[0116] Here, in Figure 6 the "black circle symbol" represents a sample with a (100) plane, and the "white triangle symbol" represents a sample with a (111) facet.

[0117] In addition, the "white circle symbol" is the open-circuit voltage (implied Voc) obtained from the evaluation of the carrier lifetime in a crystalline silicon layer with a (100) plane, reflecting the passivation characteristics. In particular, the "white circle symbol" corresponds to the open-circuit voltage when the carrier selectivity is ideal.

[0118] Similarly, the "black triangle symbol" is the open-circuit voltage (implied Voc) obtained from the evaluation of the carrier lifetime in a crystalline silicon layer with a (111) facet, reflecting the passivation characteristics. In particular, the "black triangle symbol" corresponds to the open-circuit voltage when the carrier selectivity is ideal.

[0119] First, as shown in (a) to Figure 6 (d) of Figure 6 it can be seen that in the samples using the (100) plane, high-performance carrier-selective solar cells are achieved in the range of 200 °C to 260 °C for the film formation temperature of the titanium oxide film. On the other hand, it can be seen that when the film formation temperature of the titanium oxide film is 120 °C or lower, although the passivation characteristics are high, the carrier selectivity (hole selectivity) and carrier transportability (carrier transport property) are impaired, resulting in a significant decrease in the open-circuit voltage and fill factor. In contrast, in the samples using the (111) facet, the optimal film formation temperature for improving the performance of the carrier-selective solar cell is 260 °C to 280 °C. That is, it can be seen that in the samples using the (111) facet, compared with the samples using the (100) plane, the optimal temperature range shifts to the higher temperature side, and the optimal temperature range becomes narrower. That is, the optimal film formation temperature varies greatly depending on the surface flatness and plane orientation of the crystalline silicon layer.

[0120] Next, here, for example, the verification results of a carrier-selective solar cell using a crystalline silicon layer with a flat (100) plane, a carrier-selective solar cell using a crystalline silicon layer with a (111) facet, and a carrier-selective solar cell using a crystalline silicon layer with a flat (111) plane will be described.

[0121] Figure 7This is a graph showing the relationship between the characteristics of a carrier-selective solar cell with a titanium oxide film formed by thermal atomic layer deposition at a film-forming temperature of 280°C and the hydrogen plasma treatment time. It should be noted that the hydrogen plasma treatment was carried out at 280°C, which is the same as the film-forming temperature of the titanium oxide film. In particular, Figure 7 This is a graph showing the relationship between the following values and the hydrogen plasma treatment time: (a) short-circuit current density, (b) open-circuit voltage, (c) fill factor, and (d) conversion efficiency.

[0122] Here, in Figure 7 the "black circle symbol" represents a sample of the (100) plane, and the "white triangle symbol" represents a sample of the (111) facet. In addition, the "white square symbol" represents a sample of the flat (111) plane.

[0123] As shown in Figure 7 (b) of Figure 7 and (c) of

[0124] It can be seen that if the hydrogen plasma treatment is carried out, the open-circuit voltage and fill factor of all samples are improved. In particular, for the samples of the (111) facet and the flat (111) plane, there is a tendency for the open-circuit voltage to be dramatically improved by the hydrogen plasma treatment. That is, the difference in the open-circuit voltage between the solar cell with the (111) facet and the solar cell with the flat (100) plane is clearly due to the difference in the plane orientation of the crystalline silicon layer. On the other hand, when the hydrogen plasma treatment time is too long (more than 30 minutes), in all samples, there is also an effect of slightly reducing the fill factor. From this, it can be known that the hydrogen plasma treatment time that can obtain the maximum conversion efficiency depends on the plane orientation and shape of the crystalline silicon, but is approximately 30 minutes.

[0125] Regarding this point, different from the plasma-enhanced atomic layer deposition method using oxygen plasma, in the thermal atomic layer deposition method, water vapor (H2O) is used as the oxygen source for forming the titanium oxide film. Therefore, it is considered that hydrogen elements generated by the water vapor are mixed into the titanium oxide film. In addition, it is considered that hydrogen is also mixed into the titanium oxide film by performing hydrogen plasma treatment. Therefore, it can be estimated that due to the mixing of hydrogen into the titanium oxide film, the titanium oxide film has good hole selectivity and good passivation characteristics. In particular, it is estimated that, for example, dangling bonds existing at the interface between the crystalline silicon layer and the titanium oxide film are terminated by the hydrogen mixed in the titanium oxide film, and as a result, the recombination of carriers caused by the dangling bonds is suppressed, thereby improving the passivation characteristics of the titanium oxide film.

[0126] Moreover, according to the research of the present inventors, it is known that: (1) when annealing is performed in an oxygen-containing atmosphere after forming a transparent electrode on the titanium oxide film, the passivation characteristics and hole selectivity of the titanium oxide film are improved. Regarding this point, the following results are obtained: (2) when annealing is performed in an oxygen-containing atmosphere before forming a transparent electrode on the titanium oxide film, although the passivation characteristics of the titanium oxide film are greatly improved, the fill factor of the solar cell is significantly decreased. On the other hand, the following results are also obtained: (3) when annealing is performed in an oxygen-free atmosphere before forming a transparent electrode on the titanium oxide film, the passivation characteristics are hardly improved. In addition, it is found that: (4) even after forming a transparent electrode on the titanium oxide film, if annealing is performed in an oxygen-free atmosphere, the improvement of the passivation characteristics of the titanium oxide film is higher than that in the case of (3), but lower than that in the case of (1). Therefore, it can be estimated that there is a possibility that, for example, the delicate oxygen concentration at the interface between the crystalline silicon layer and the titanium oxide film contributes to the improvement of the passivation characteristics and hole selectivity of the titanium oxide film.

[0127] That is, according to the research of the present inventors, it can be estimated that for the titanium oxide film formed by film formation according to the characteristic points of the manufacturing method of the present embodiment described above, the elemental composition at the interface between the crystalline silicon layer and the titanium oxide film has an important role. Specifically, based on the above research results, it can be estimated that at least the hydrogen element concentration and oxygen element concentration at the interface between the crystalline silicon layer and the titanium oxide film contribute greatly to the improvement of the hole selectivity of the titanium oxide film and the passivation film.

[0128] Therefore, in order to investigate the influence of the interface structure between the crystalline silicon layer and the titanium oxide film on the hole selectivity and passivation characteristics of the titanium oxide film, the interface structure between the crystalline silicon layer and the titanium oxide film was investigated in detail. Hereinafter, the investigation results will be described.

[0129] <Investigation Results of Interface Structure>

[0130] Figure 8This is a photograph showing a part of the cross-sectional structure of a carrier-selective solar cell fabricated using a crystalline silicon layer having a flat (100) surface. In Figure 8 it shows the cross-section of the carrier-selective solar cell observed by high-resolution transmission electron microscopy. As Figure 8 shown, an intermediate film 21 is formed on the crystalline silicon layer 20, and a titanium oxide film 22 is formed on the intermediate film 21. In addition, here, the titanium oxide film 22 and the intermediate film 21 are distinguished as different films. Moreover, a transparent electrode 23 is formed on the titanium oxide film 22. That is, as Figure 8 shown, it can be confirmed that the film thickness of the formed titanium oxide film 22 is about 4 nm, and an intermediate film (interface layer) 21 with a thickness of about 1 nm is formed at the interface between the crystalline silicon layer 20 and the titanium oxide film 22. The composition of the intermediate film 21 was evaluated by energy-dispersive X-ray analysis, and as a result, it was found that the intermediate film 21 is a film containing silicon element (Si), titanium element (Ti), and oxygen element (O). In addition, as shown below, it was found that depending on the film formation conditions, the intermediate film 21 may be a film containing hydrogen (H) in addition to the above elements. Therefore, in order to analyze the composition of the intermediate film 21 formed at the interface between the crystalline silicon layer 20 and the titanium oxide film 22 in more detail, quantitative analysis of each element contained in the intermediate film 21 was carried out using Rutherford backscattering spectrometry (RBS) and elastic recoil detection analysis technique (ERDA).

[0131] Figure 9 and Figure 10 are graphs showing the results. In Figure 9 and Figure 10 the dotted line represents the case where the titanium oxide film was formed by plasma-enhanced atomic layer deposition method (p-ALD). On the other hand, the dash-dotted line represents the case where the titanium oxide film was formed by thermal atomic layer deposition method and no hydrogen plasma treatment was performed (t-ALD). In contrast, the solid line represents the case where the titanium oxide film was formed by thermal atomic layer deposition method and hydrogen plasma treatment was performed (t-ALD+HPT).

[0132] In addition, RBS and ERDA analyses were carried out after annealing a sample composed of a transparent electrode (ITO film) (10 nm) / titanium oxide film (5 nm) / crystalline silicon layer in a low vacuum at a temperature of 180 °C, and the structure of this sample is close to the structure of an actual solar cell. It has been confirmed that even for the above-mentioned special sample structure with a thin ITO film, the performance of the solar cell can be reproduced by adding an ITO film after annealing. That is, this measurement is equivalent to the measurement of the composition of the titanium oxide film and the intermediate film of an actual solar cell.

[0133] As Figure 9 and Figure 10As shown, in the titanium oxide film formed under any film-forming conditions, the oxygen / titanium ratio (O / Ti ratio) of the titanium oxide film is approximately 2, which is close to the stoichiometric ratio. On the other hand, it is known that the intermediate film formed at the interface between the crystalline silicon layer and the titanium oxide film is composed of a film containing titanium element (Ti), oxygen element (O), and silicon element (Si), and this composition depends greatly on the film-forming conditions and hydrogen plasma treatment. Moreover, for example, as Figure 9 shown, it can be seen that in the titanium oxide film (including the intermediate film) formed by thermal atomic layer deposition method, compared with the titanium oxide film (including the intermediate film) formed by plasma-enhanced atomic layer deposition method, the oxygen concentration in the intermediate film is lower, while the silicon concentration increases instead. In addition, the following tendency can be confirmed, that is, by performing hydrogen plasma treatment, the distribution of titanium element and oxygen element in the intermediate film expands in the depth direction. In addition, as Figure 10 shown, it is known that in the titanium oxide film (including the intermediate film) formed by thermal atomic layer deposition method, hydrogen element is mainly contained in the intermediate film, and by performing hydrogen plasma treatment, the hydrogen concentration in the intermediate film will increase. On the other hand, in the titanium oxide film (including the intermediate film) formed by plasma-enhanced atomic layer deposition method, almost no hydrogen is detected.

[0134] Next, in order to verify the composition of titanium oxide formed on the crystalline silicon layer with (111) facets, RBS and ERDA were used to perform the composition analysis of the titanium oxide film formed on the crystalline silicon layer with a flat (111) surface. In addition, according to Figure 7 , the passivation performance and hole selectivity of the titanium oxide film are in the relationship of flat (100) surface > (111) facets > flat (111) surface, so it can be easily estimated that the composition of the titanium oxide film formed on the (111) facets will also be in this relationship.

[0135] Figure 11 is a table comparing the composition of the intermediate film in the titanium oxide film formed on the crystalline silicon layer with different surface orientations by different methods and the results of the solar cell characteristics. It should be noted that when the maximum titanium concentration of the titanium oxide film is set to 1, the composition of the intermediate film is defined by the oxygen concentration, silicon concentration, and hydrogen concentration at the depth where the titanium composition ratio becomes 0.5. In addition, in Figure 11 , "Jsc", "Voc", "FF", "Eff." respectively represent the "short-circuit current density (mA / cm2)", "open-circuit voltage (V)", "fill factor", and "conversion efficiency (%)" of the solar cell characteristics.

[0136] According to Figure 11, it can be considered that the following conclusion can be drawn: The main reason for obtaining a titanium oxide film with excellent hole selectivity and excellent passivation characteristics by using the manufacturing method of Embodiment 1, that is, for example, forming a titanium oxide film by thermal atomic layer deposition and performing hydrogen plasma treatment, is as follows: In the interface structure (intermediate film) between the crystalline silicon layer and the titanium oxide film, (1) the oxygen concentration in the intermediate film is low and the silicon concentration is high, and (2) the hydrogen element is contained in the intermediate film. Specifically, by adopting the characteristic points of the manufacturing method of Embodiment 1, the following interface structure is realized: In the intermediate film formed at the interface between the crystalline silicon layer and the silicon oxide layer, the intermediate film contains 1.5 atomic% or more of the hydrogen element, and when the maximum titanium concentration of the titanium oxide film is set to 1, the oxygen concentration at a depth where the composition ratio of titanium is 0.5 is 45 atomic% or less, and the silicon concentration at a depth where the composition ratio of titanium becomes 0.5 is 36 atomic% or more. As a result, according to Embodiment 1, by using the combination of the intermediate film and the titanium oxide film, a hole-selective film having excellent hole selectivity for selectively passing holes generated in crystalline silicon and excellent passivation characteristics for suppressing the recombination of electrons and holes can be realized.

[0137] <Effects in Embodiment 1>

[0138] The solar cell in Embodiment 1 is a carrier-selective solar cell, which includes: a crystalline silicon layer having a first main surface; an intermediate film provided on the first main surface and containing silicon, titanium, and oxygen; a titanium oxide film provided on the intermediate film; and a transparent electrode provided on the titanium oxide film. At this time, in the carrier-selective solar cell of this embodiment, the above intermediate film and titanium oxide film serve as a hole transport film with respect to the crystalline silicon layer. In other words, the combination of the above intermediate film and titanium oxide film functions as a hole-selective film. Moreover, in the carrier-selective solar cell of Embodiment 1, the interface structure with the composition described in the "<Investigation Results of Interface Structure>" column is formed, and as a result, a conversion efficiency of 18% or more can be obtained. That is, according to Embodiment 1, by adopting the characteristic points of the manufacturing method, a titanium oxide film including the above interface structure can be realized, and as a result, a hole-selective film having both good hole selectivity and good passivation characteristics can be realized through the titanium oxide film. Therefore, according to Embodiment 1, an excellent carrier-selective solar cell with a conversion efficiency of up to 18% or more can be provided in a carrier-selective solar cell using a titanium oxide film as a hole-selective film.

[0139] For example, currently, among solar cells using a crystalline silicon layer, the heterojunction solar cell achieves the highest conversion efficiency. This is mainly due to the excellent passivation characteristics of the hydrogen-containing amorphous silicon film used in the heterojunction solar cell and the carrier-selective controllability achieved through doping. However, the bandgap of amorphous silicon is about 1.7 eV, so for the amorphous silicon film, the light absorption coefficient in the visible light region is high. Moreover, most of the light absorbed by the amorphous silicon film becomes energy conversion losses.

[0140] In contrast, in the carrier-selective solar cell of Embodiment 1, there is no amorphous silicon buffer layer between the crystalline silicon layer and the titanium oxide film. More specifically, there is no amorphous silicon buffer layer between the crystalline silicon layer and the intermediate film. This is because, according to the combination of the titanium oxide film and the intermediate film of Embodiment 1, not only good hole selectivity is achieved, but also good passivation characteristics are achieved, so there is no need to interpose an amorphous silicon buffer layer to improve the passivation characteristics. Moreover, the bandgap of the titanium oxide used in the carrier-selective solar cell of Embodiment 1 is about 3.4 eV, and it is almost completely transparent in the visible light region. Therefore, due to the properties of this titanium oxide, in a carrier-selective solar cell using a titanium oxide film as a hole-selective film, excellent short-wavelength sensitivity can be obtained. That is, in the carrier-selective solar cell of Embodiment 1 using a titanium oxide film with an interface structure as a hole-selective film, no amorphous silicon buffer layer is required, so excellent short-wavelength sensitivity can be obtained, and as a result, a higher short-circuit current density can be obtained.

[0141] Specifically, Figure 12 is a graph showing the relationship between the wavelength of incident light and the external quantum efficiency.

[0142] In Figure 12 the dashed line represents the external quantum efficiency spectrum of the heterojunction solar cell. In this heterojunction solar cell, the hole-selective film is composed of a film (a-Si:H i-p) formed by laminating an intrinsic amorphous silicon film doped with hydrogen (film thickness: 6 nm) and a p-type amorphous silicon film doped with hydrogen (film thickness: 3 nm). On the other hand, the solid line is the external quantum efficiency spectrum of the carrier-selective solar cell of Embodiment 1 using a titanium oxide film as a hole-selective film. As Figure 12As shown, it can be seen that due to the light transmittance of titanium oxide in the visible light region, the short-wavelength sensitivity of the carrier-selective solar cell of Embodiment 1 represented by the solid line is significantly higher than that of the conventional heterojunction solar cell represented by the dashed line. As a result, for the carrier-selective solar cell of Embodiment 1 using a titanium oxide film as the hole-selective film, a higher short-circuit current density can be obtained compared to the conventional heterojunction solar cell using an amorphous silicon film. For example, the short-circuit current density of the conventional heterojunction solar cell is 38.8 mA / cm2, whereas the short-circuit current density of the carrier-selective solar cell of Embodiment 1 is as high as 40.2 mA / cm2. Moreover, it means that the higher the short-circuit current density of the solar cell, the better the performance of the solar cell. Therefore, the carrier-selective solar cell of Embodiment 1 using a titanium oxide film as the hole-selective film has the potential to achieve performance superior to that of the conventional heterojunction solar cell using an amorphous silicon film.

[0143] As described above, titanium oxide has light transmittance in the visible light region. In addition to titanium oxide, materials for the hole-selective film having light transmittance in the visible light region also include molybdenum oxide, tungsten oxide, and vanadium oxide. However, when these metal oxides are used as the hole-selective film, it is difficult to obtain a high open-circuit voltage due to low passivation characteristics. Therefore, when using the above metal oxides as the hole-selective film, in order to ensure the passivation characteristics, a buffer layer with excellent passivation characteristics represented by an amorphous silicon film is required between these metal oxide films and the crystalline silicon layer. This causes complication of the manufacturing process of the solar cell and means that it is difficult to improve the conversion efficiency due to the light absorption loss caused by the buffer layer. In contrast, in the present embodiment, the film formation method of the titanium oxide film is studied, and the titanium oxide film is used as the hole-selective film having both good hole selectivity and good passivation characteristics. As a result, furthermore, due to the synergistic effect of this and the light transmittance of titanium oxide in the visible light region, a carrier-selective solar cell can be realized whose conversion efficiency is superior to that of a carrier-selective solar cell using other metal oxides as the hole-selective film. In this regard, the technical idea in Embodiment 1 can be said to have great technical significance in that it has paved the way for the practical application of a carrier-selective solar cell that can achieve a very high conversion efficiency, which is higher than that of the heterojunction solar cell that has achieved the highest conversion efficiency.

[0144] Actually, Figure 13This is a table comparing an example of a representative prior study of a front-emitter type solar cell unit in which a hole-selective film containing various materials is used and a hole-selective film is provided on the light incident surface side of an n-type crystalline silicon layer (light absorption layer) with the present embodiment.

[0145] For example, the results of the following solar cell are shown, that is, corresponding to using an n-type crystalline silicon layer as the crystalline silicon layer 10 in Figure 1 (flat surface) and Figure 2 (textured surface) devices, setting a hole-selective film on the light incident side, and setting an electron-selective film using an amorphous silicon film (a-Si:H i-n), a polycrystalline silicon film (SiOx / poly-Si n) on the back side of the solar cell.

[0146] In Figure 13 , the items with shading represent the present embodiment. As Figure 13 shown, it can be seen that the performance of the solar cell unit using titanium oxide in the present embodiment is better than that of other metal oxides (molybdenum oxide, tungsten oxide, vanadium oxide, etc.) used as hole-selective films in the past. In summary, according to the present embodiment, it is of great technical significance in that it presents the possibility of achieving performance exceeding that of heterojunction solar cells in carrier-selective solar cells and inspires the practical application of the following solar cells, which can simultaneously achieve a reduction in performance cost due to the simplification of the manufacturing process and an improvement in the performance of solar cells represented by an increase in conversion efficiency.

[0147] <Variant Example>

[0148] Next, a variant example of Embodiment 1 will be described.

[0149] <<Variant Example 1>>

[0150] Figure 14 This is a diagram showing the schematic device structure of the carrier-selective solar cell unit of this Variant Example 1. In Figure 14 , in the carrier-selective solar cell unit 300 of this Variant Example 1, a titanium oxide film as the hole-selective film 11 is formed on the back surface of the crystalline silicon layer 10 on the side opposite to the front surface of the crystalline silicon layer 10 as the light incident surface side, and a second light-transmissive electrode 13' is formed in contact with the titanium oxide film. In the carrier-selective solar cell unit 300 configured in this way, the titanium oxide film can also be used as a hole-selective film with good hole selectivity and good passivation characteristics, so the performance of the carrier-selective solar cell unit 300 can be improved.

[0151] <<Variant Example 2>>

[0152] Figure 15 is a diagram showing a schematic device structure of the carrier-selective solar cell unit of this Modification 2. In Figure 15 , the carrier-selective solar cell unit 400 of this Modification 2 is configured such that light is incident from both the front surface and the back surface of the crystalline silicon layer 10. In the carrier-selective solar cell unit 400 of this Modification 2 configured in this way, since the titanium oxide film can also be used as a hole-selective film having good hole selectivity and good passivation characteristics, the performance of the carrier-selective solar cell unit 400 can be improved.

[0153] <<Modification 3>>

[0154] Figure 16 is a diagram showing a schematic device structure of the carrier-selective solar cell unit of this Modification 3. In Figure 16 , in the carrier-selective solar cell unit 500 of this Modification 3, on the front surface of the crystalline silicon layer 10 on the light incident surface side, a passivation film 15 made of, for example, an insulating film transparent to visible light is formed. On the other hand, in the carrier-selective solar cell unit 500 of this Modification 3, a titanium oxide film as a hole-selective film 11 and an electron-selective film 12 are formed on the back surface of the crystalline silicon layer 10 on the side opposite to the front surface, and a transparent electrode 13 is formed in contact with the titanium oxide film. Further, a metal electrode 14 is formed in contact with the electron-selective film 12 and the transparent electrode 13, respectively. In the carrier-selective solar cell unit 500 of this Modification 3 configured in this way, since the titanium oxide film can also be used as a hole-selective film having good hole selectivity and good passivation characteristics, the performance of the carrier-selective solar cell unit 500 can be improved.

[0155] <<Modification 4>>

[0156] In the embodiment, an example in which an n-type crystalline silicon layer is used as the crystalline silicon layer 10 as shown, for example, in Figure 1 has been described. However, the technical idea of the embodiment is not limited to this, and a p-type crystalline silicon layer can also be used as the crystalline silicon layer 10.

[0157] For example, when a p-type crystalline silicon layer is used as the crystalline silicon layer 10, it is also possible to form a carrier-selective solar cell unit having a rear-emitter structure in which a titanium oxide film is formed as a hole-selective film on the light-receiving surface side (front surface side), and a carrier-selective solar cell unit having a front-emitter structure in which a titanium oxide film is formed as a hole-selective film on the back surface side.

[0158] In Figure 17shows the performance results of a carrier - selective solar cell unit using a p - type crystalline silicon layer. The fabricated solar cell is, for example, in Figure 1 (planar) and Figure 2 (textured) devices, a p - type crystalline silicon layer is used as the crystalline silicon layer 10, and a titanium oxide film is provided on the light - incident surface side and used as a sample of "FSF (Front Surface Field)", and a titanium oxide film is provided on the back surface side and used as a sample of "BSF (Back Surface Field)". It should be noted that an amorphous silicon film (a - Si:H i - n) is used as the electron - selective film.

[0159] From Figure 13 and Figure 17 it can be seen that for the titanium oxide film formed by the manufacturing method of this embodiment, regardless of the polarity of the crystalline silicon used in the carrier - selective solar cell unit, it has good hole selectivity and good passivation characteristics.

[0160] In addition, in Figure 17 the conversion efficiency of the carrier - selective solar cell unit using a p - type crystalline silicon layer is lower than that of Figure 13 the carrier - selective solar cell unit using an n - type crystalline silicon layer, which may be due to the fact that the quality (defect density, etc.) of the p - type crystalline silicon substrate is worse than that of the n - type crystalline silicon substrate (defect density).

[0161] <<Modification 5>>

[0162] In addition, for example, a textured structure can be formed on both the front and back surfaces of the crystalline silicon layer 10 shown in Figure 1 , Figures 14 to 16 and it is not limited to this. A textured structure can also be formed on only one side of the light - incident surface side or the back surface side of the crystalline silicon layer 10. A solar cell with a textured structure formed on only one side can be used, for example, as the bottom cell of a tandem solar cell.

[0163] (Embodiment 2)

[0164] In the above - mentioned Embodiment 1, an example of using the titanium oxide film formed by the manufacturing method of Embodiment 1 as the hole - selective film of a solar cell was described, but the technical idea of Embodiment 1 is not limited to this. For example, it can also be applied to semiconductor devices.

[0165] Specifically, in this Embodiment 2, an example of applying the titanium oxide film formed by the manufacturing method of Embodiment 1 to a structure for extracting holes from the source region and drain region of a p - channel field - effect transistor is described.

[0166] Figure 18 This is a diagram showing the structure of a schematic semiconductor device including a p-channel field-effect transistor. As Figure 18 shown, for example, a p-channel field-effect transistor 600 including a gate electrode GE, a source region SR, and a drain region DR is formed on a semiconductor substrate 1S made of crystalline silicon. At this time, in the p-channel field-effect transistor 600, the source region SR and the drain region DR are each composed of a p-type silicon region. Further, an interlayer insulating film IL is formed so as to cover the p-channel field-effect transistor 600, and in this interlayer insulating film IL, a contact hole CNT is formed that penetrates the interlayer insulating film IL and reaches the source region SR and the drain region DR. A contact film 30 in contact with the source region SR and the drain region DR and a conductor film CF made of, for example, a tungsten film or the like are buried in the contact hole CNT to form a plug.

[0167] Here, in the second embodiment, a titanium oxide film formed by the manufacturing method of the first embodiment is used as the contact film 30. Hereinafter, the advantages will be described.

[0168] For example, in Figure 18 , when the conductor film CF buried in the contact hole CNT is in direct contact with the p-type silicon region constituting the source region SR, a material that forms an ohmic contact with the p-type silicon region constituting the source region SR is used as the conductor film CF. However, in reality, due to defects generated at the interface between the conductor film CF and the p-type silicon region, a good ohmic contact may not necessarily be formed between the conductor film CF and the p-type silicon region. In this case, the contact resistance between the conductor film CF and the p-type silicon region increases. This means that the performance of the semiconductor device deteriorates.

[0169] Regarding this point, as Figure 18 shown, for example, when a contact film 30 made of a titanium oxide film is interposed between the p-type silicon region constituting the source region SR and the conductor film CF, since the titanium oxide film formed by the manufacturing method of the first embodiment has good hole selectivity and good passivation characteristics, the flow of hole current between the plug and the source region SR can be made smooth. In other words, by using the titanium oxide film formed by the manufacturing method of the first embodiment as the contact film 30 and interposing it between the plug and the source region SR, the contact resistance between the plug and the source region SR can be reduced. That is, for example, a contact film 30 composed of an intermediate film and a titanium oxide film is formed between the conductor film CF and the source region SR, thereby reducing the contact resistance between the plug and the source region SR. Thus, according to the second embodiment, an improvement in the performance of the semiconductor device can be achieved by reducing the contact resistance due to the titanium oxide film.

[0170] (Embodiment 3)

[0171] <Discoveries obtained by the present inventor>

[0172] The discovery obtained from the technical idea of the first embodiment is that the passivation characteristics of the titanium oxide film are greatly affected by the hydrogen element concentration and oxygen element concentration of the intermediate film formed at the interface between the crystalline silicon layer and the titanium oxide film. Specifically, from the first embodiment, the following discovery can be grasped, that is, by increasing the hydrogen element concentration and oxygen element concentration of the intermediate film to a certain extent, the passivation characteristics of the titanium oxide film can be improved.

[0173] Regarding this point, in the first embodiment, focusing on applying the titanium oxide film as a hole-selective film that requires both improvement of passivation characteristics and improvement of hole selectivity, in this case, the oxygen element concentration of the intermediate film formed at the interface between the crystalline silicon layer and the titanium oxide film cannot be higher than the necessary level. This is because if the oxygen element concentration of the intermediate film increases, the resistance of the titanium oxide film (hole-selective film) including the intermediate film increases, and as a result, the fill factor of the solar cell unit decreases, and further the conversion efficiency decreases.

[0174] However, the present inventor has obtained the following discovery, that is, when only focusing on the improvement of passivation characteristics regardless of hole selectivity, preferably, the oxygen element concentration of the intermediate film is increased to a certain extent. For example, the research results of the first embodiment support this discovery. That is, according to the research of the present inventor, this discovery is supported by the following description: "(1) When an annealing is performed in an oxygen-containing atmosphere after forming a transparent electrode on the titanium oxide film, the passivation characteristics and hole selectivity of the titanium oxide film are improved. Regarding this point, the following results are obtained: (2) When an annealing is performed in an oxygen-containing atmosphere before forming a transparent electrode on the titanium oxide film, although the passivation characteristics of the titanium oxide film are greatly improved, the fill factor of the solar cell significantly decreases."

[0175] Therefore, for example, preferably, in a film that does not require improvement of hole selectivity but requires improvement of passivation characteristics, the oxygen element concentration of the intermediate film is made higher by a certain extent compared to a film that requires both improvement of hole selectivity and passivation characteristics.

[0176] Therefore, in the third embodiment, based on the above knowledge, for example, research mainly for improving the passivation characteristics of the passivation film is carried out, rather than research for an electron-selective film for extracting electrons or a hole-selective film for extracting holes, and the passivation film is used to cover the surface of the crystalline silicon film.

[0177] Hereinafter, the technical idea of the third embodiment in which this research has been carried out will be described.

[0178] <Basic idea of the third embodiment>

[0179] The basic idea of Embodiment 3 is the idea of performing hydrogen plasma treatment in such a way as to increase the hydrogen element concentration in the intermediate film formed at the interface between the crystalline silicon layer and the passivation film. Specifically, the basic idea of Embodiment 3 is the idea of performing hydrogen plasma treatment on the first titanium oxide film at least at a stage after the formation of the first titanium oxide film and before the formation of the second titanium oxide film, on the premise that the passivation film is composed of a stacked film of the first titanium oxide film and the second titanium oxide film.

[0180] According to such a basic idea, since the hydrogen plasma treatment is performed in a state where the film thickness of the passivation film is relatively thin, the hydrogen element concentration in the intermediate film can be increased. As a result, according to the basic idea, the passivation characteristics of the passivation film can be improved by utilizing the direct effect and the indirect effect brought about by the increase in the hydrogen element concentration in the intermediate film.

[0181] Hereinafter, first, the direct effect brought about by the increase in the hydrogen element concentration in the intermediate film will be described. For example, the dangling bonds existing at the interface between the crystalline silicon layer and the titanium oxide film are terminated by the hydrogen element mixed into the intermediate film. As a result, it is considered that the recombination of carriers caused by the dangling bonds is suppressed, and thus the passivation characteristics of the passivation film are improved. That is, if the hydrogen element concentration in the intermediate film increases, the number of hydrogen elements that terminate the dangling bonds increases. Therefore, it is considered that by significantly suppressing the recombination of electrons and holes caused by the dangling bonds, the passivation characteristics of the passivation film can be improved. This effect is the direct effect brought about by increasing the hydrogen element concentration in the intermediate film.

[0182] Next, the indirect effect brought about by the increase in the hydrogen element concentration in the intermediate film will be described. For example, when the hydrogen element is mixed into the intermediate film, the oxygen element contained in the intermediate film is reduced by the hydrogen element. As a result, the amount of oxygen element contained in the intermediate film becomes smaller. Then, for example, after the formation of the passivation film, annealing is performed in an oxygen-containing atmosphere. At this time, the smaller the amount of oxygen element contained in the intermediate film, the larger the amount of oxygen element taken into the intermediate film by performing annealing in an oxygen-containing atmosphere. This is because, regarding the amount of oxygen element taken in by annealing in an oxygen-containing atmosphere, the greater the concentration difference between the initial oxygen element concentration in the intermediate film and the oxygen element concentration in the oxygen-containing atmosphere, the more rapidly the oxygen element is taken into the intermediate film.

[0183] Here, an increase in the hydrogen element concentration in the intermediate film means that the amount of reduction of the oxygen element in the intermediate film increases. As a result, the initial oxygen element concentration in the intermediate film becomes lower. Consequently, the concentration difference between the initial oxygen element concentration in the intermediate film and the oxygen element concentration in the oxygen-containing atmosphere becomes larger. Therefore, when annealing is performed in an oxygen-containing atmosphere, oxygen elements are rapidly taken into the intermediate film. Thus, according to the basic idea, it is considered that by increasing the oxygen element concentration of the oxygen elements finally contained in the intermediate film, the passivation characteristics of the passivation film can be improved. This effect is an indirect effect brought about by increasing the hydrogen element concentration in the intermediate film.

[0184] Therefore, according to the basic idea in the third embodiment, the passivation characteristics of the passivation film can be improved by utilizing the synergistic effect of the direct effect and the indirect effect.

[0185] In particular, the basic idea of the third embodiment is a novel idea in that it can not only increase the hydrogen element concentration in the intermediate film by hydrogen plasma treatment but also increase the oxygen element concentration in the intermediate film by combining it with subsequent annealing in an oxygen-containing atmosphere. This novel idea can only be thought of after understanding the principle of obtaining the above indirect effect.

[0186] Hereinafter, a specific manner of embodying this basic idea will be described.

[0187] <Structure of the back electrode type solar cell unit>

[0188] Figure 19 is a diagram showing the structure of the back electrode type solar cell unit of the third embodiment. As Figure 19 shown, the back electrode type solar cell unit 700 has a crystalline silicon layer 10, a first passivation film 50, a non-passivating electron selective film 51, a non-passivating hole selective film 52, a cathode electrode (first electrode) 53, and an anode electrode (second electrode) 54. In Figure 19 it, for example, a film 50 is formed on the non-light-receiving surface, i.e., the back surface, of the crystalline silicon layer 10, and films 51 and 52 are formed in contact with the film 50. Moreover, electrodes 53 and 54 are formed in contact with films 51 and 52, respectively.

[0189] For example, as the non-passivating electron-selective film 51, an n-type hydrogenated amorphous silicon film (a-Si:H) or the like is used. In contrast, as the non-passivating hole-selective film 52, a p-type hydrogenated amorphous silicon film (a-Si:H) or the like is used. Further, as the cathode electrode 53 and the anode electrode 54, metal electrodes such as silver films are used, and in order to enhance the reflection of light, a metal oxide film such as an ITO electrode (transparent electrode) can be inserted at the interface between the silver film and the n-type hydrogenated amorphous silicon film and the p-type hydrogenated amorphous silicon film. As the first passivation film 50, an i-type hydrogenated amorphous silicon film (a-Si:H) is used.

[0190] In contrast, on the light-receiving surface, i.e., the front surface, of the crystalline silicon layer 10, a second passivation film 60 is formed. In addition, the polarity of the crystalline silicon layer 10 can be either n-type or p-type, and the surface structure can be either a flat structure composed of a silicon (100) plane or a random texture structure composed of (111) facets. Further, on the second passivation film 60, for example, an antireflection film composed of a silicon nitride film can also be formed.

[0191] Here, in Figure 19 the second passivation film 60 is formed on the front surface on the side opposite to the back surface where the cathode electrode 53 and the anode electrode 54 are formed. Therefore, the film 60 is not required to have the function of an electrode, and mainly requires a passivation property that suppresses the recombination of electrons and holes at the interface with the crystalline silicon film. Therefore, the basic idea of the present Embodiment 3, which is the study for improving the passivation property, can be applied to the film 60. Specifically, as Figure 19 shown, the second passivation film 60 is composed of a stacked film of a first titanium oxide film 55a and a second titanium oxide film 55b. Moreover, an intermediate film is formed between the crystalline silicon layer 10 and the first titanium oxide film 55a. The intermediate film contains silicon, titanium, and oxygen, and also contains 1.5 atomic% or more of hydrogen.

[0192] In addition, preferably, for other characteristics required for the second passivation film 60 formed on the light-receiving surface, it has transparency as much as possible in the visible light to near-infrared region. For example, in a heterojunction solar cell, a passivation film in which a hydrogenated amorphous silicon film (a-Si:H) and a hydrogenated amorphous silicon nitride film (a-SiNx:H) are sequentially stacked on the surface of a crystalline silicon layer is used (see Non-Patent Document 10). In the case of this structure, the absorption wavelength of the hydrogenated amorphous silicon film is visible light of 700 nm or less, so energy conversion loss occurs. In this regard, in the present Embodiment 3, a titanium oxide film having optical transparency is adopted as the second passivation film 60. As a result, according to the back electrode type solar cell unit 700, the light incident from the solar light-receiving surface can be effectively converted into energy.

[0193] <Method for manufacturing a back electrode type solar cell unit>

[0194] Hereinafter, a method for manufacturing the back electrode type solar cell unit 700 will be described.

[0195] Figure 20 It is a flowchart for explaining the process flow of the manufacturing process of the back electrode type solar cell unit of the third embodiment. In addition, Figure 19 The manufacturing processes of the non-passivating electron selective film 51, the non-passivating hole selective film 52, the cathode electrode 53, and the anode electrode 54 shown do not directly affect the passivation characteristics, so they are Figure 20 omitted. For example, as the manufacturing process of these films, after forming a hydrogenated amorphous silicon film (i layer) (S303), the formation of Figure 19 the films 51 and 52 shown, and after annealing (S309), the steps of forming the electrodes 53 and 54 can be cited. In addition, as the manufacturing process of these films, after annealing (S309), the formation of Figure 19 the films 51 and 52, and the electrodes 53 and 54 shown can also be cited.

[0196] In Figure 20 the formation process of the texture structure (S301) and the cleaning process of the substrate and the removal process of the oxide film (S302) are the same as those in the first embodiment, so the description is omitted.

[0197] Next, for example, a hydrogenated amorphous silicon film of i-type with good passivation characteristics is formed on the back surface (the side opposite to the light incident surface) of the crystalline silicon layer 10 by plasma CVD (Chemical Vapor Deposition) as the first passivation film 50 (S303).

[0198] Then, dilute hydrofluoric acid is used again for the crystalline silicon layer 10 to remove the natural oxide film formed on the front surface (light incident surface side) of the crystalline silicon layer 10 (S304).

[0199] After that, a second passivation film 60 is formed on the light incident surface, that is, the front surface of the crystalline silicon layer 10. Specifically, first, a first titanium oxide film 55a is formed (S305). It should be noted that for this film 60, the same method as in the first embodiment, that is, the thermal atomic deposition method, is used, and the titanium oxide film is formed under the same conditions. Among them, the "ALD cycle number" of the first titanium oxide film is set to 39 times (thickness: 1.5 nm).

[0200] Next, a first hydrogen plasma treatment is performed to irradiate hydrogen plasma on the front surface of the first titanium oxide film 55a (S306). At this time, the hydrogen flow rate and pressure are the same as those in the first embodiment, but the discharge power is set to 300 W and the irradiation time is set to 30 seconds.

[0201] Next, afterwards, the second titanium oxide film 55b is formed again by thermal atomic layer deposition to increase the thickness of the second passivation film 60 (S307). The "ALD cycle number" at this time is set to 128 - 39 = 89 times (thickness: 3.5 nm), and the sum of the "ALD cycle numbers" in the film formation processes of the first titanium oxide film 55a and the second titanium oxide film 55b is set to be the same as the "ALD cycle number" performed in the first embodiment.

[0202] After that, hydrogen plasma treatment is performed again, which is called the second hydrogen plasma treatment (S308). The conditions of the second hydrogen plasma treatment are the same as those in the first embodiment, and the irradiation time is set to 30 minutes.

[0203] Finally, annealing is performed for 2 hours using an oven at a temperature of 180°C (S309). Here, annealing is performed in an oxygen-containing atmosphere such as in a low vacuum or in the atmosphere.

[0204] In the above-described manner, the back electrode type solar cell unit 700 having the second passivation film 60 of the third embodiment can be manufactured. That is, in the third embodiment, there are the following characteristic points in the manufacturing method: (1) the first titanium oxide film 55a is directly formed on the surface of the crystalline silicon layer 10 by thermal atomic layer deposition, (2) the first hydrogen plasma treatment is performed on the first titanium oxide film 55a, (3) the second titanium oxide film 55b is formed again by thermal atomic layer deposition, (4) the second hydrogen plasma treatment is performed again, and (5) annealing is performed in an oxygen-containing atmosphere. As a result, the stacked film of the first titanium oxide film 55a and the second titanium oxide film 55b directly formed on the crystalline silicon layer 10 becomes a film having good passivation characteristics. Hereinafter, the verification results regarding this point will be described.

[0205] In addition, as described in the description of Samples #2 to #4 shown later, Figure 21 in the third embodiment, the second hydrogen plasma treatment process (S308) in the process of forming the second passivation film 60 (S305 to S308) can be omitted. However, from the viewpoint of improving the passivation characteristics of the second passivation film 60, it is preferably to perform both the first hydrogen plasma treatment process (S306) and the second hydrogen plasma treatment process (S308).

[0206] <Verification Results>

[0207] Figure 21 This is a table showing the passivation characteristics of the second passivation film in the case of using, for example, a crystalline silicon layer having a random textured structure composed of (111) facets. In the evaluation of the passivation characteristics, the element structure before the completion of the solar cell unit was used. Specifically, in Figure 20In this case, a sample having a structure in which the crystalline silicon layer 10 is sandwiched between the first passivation film 50 on the back side and the second passivation film 60 on the front side was fabricated. Then, the passivation characteristics were evaluated by measuring the hole lifetime of the sample.

[0208] In order to verify the effect of the stacked structure of the titanium oxide film and the repeated hydrogen plasma treatment, in Figure 21 the hole lifetimes (injected carrier density: 1015 / cm3) in the crystalline silicon layers of six samples, Sample #1 to Sample #6, are shown.

[0209] Sample #1 is a sample (comparative example) in which the first hydrogen plasma treatment (S306) and the second hydrogen plasma treatment (S308) were not performed in Figure 20 and the film formation process (S305) of the first titanium oxide film and the film formation process (S307) of the second titanium oxide film were continuously performed. In addition, Sample #2 is a sample in which (S306) was not performed, and (S308) was performed after continuously performing (S305) and (S307). Sample #3 is a sample in which (S308) was not performed after sequentially performing (S305) to (S307). Sample #4 is a sample in which all of (S305) to (S308) were performed.

[0210] Sample #5 is a sample as a comparative example in which a conventional i-type hydrogenated amorphous silicon film (a-Si:H) is used as the passivation film on both sides of the crystalline silicon layer. Sample #6 is a sample (comparative example) in which (S307) and (S308) were not performed after performing (S305) and (S306) in Figure 20

[0211] The difference between Sample #1 and Sample #2 lies in whether there is hydrogen plasma treatment, which corresponds to an example showing the effect of the hydrogen plasma treatment after the film formation of the titanium oxide film shown in the first embodiment. It can be seen that the hole lifetime of Sample #2 is increased to about 9 times that of Sample #1.

[0212] In addition, it can be seen that the hole lifetime of Sample #3, in which the first hydrogen plasma treatment was performed between the film formation process of the first titanium oxide film and the film formation process of the second titanium oxide film, was dramatically improved compared to Sample #2, in which the second hydrogen plasma treatment was performed after the film formation process of the second titanium oxide film.

[0213] Moreover, Specimen #4, which has undergone both the first hydrogen plasma treatment and the second hydrogen plasma treatment, exhibits the highest hole lifetime value among the four specimens (#1 to #4). This value indicates that its passivation characteristics are superior to those of the specimen (Specimen #5) in which the two sides of the crystalline silicon layer are passivated with an i-type hydrogenated amorphous silicon film (a-Si:H). From the above, it can be seen that by performing the first hydrogen plasma treatment after the formation of the first titanium oxide film and before the formation of the second titanium oxide film, and performing the second hydrogen plasma treatment after the formation of the second titanium oxide film, the maximum passivation characteristics can be obtained.

[0214] It should be noted that there is a prior art (refer to Non-Patent Document 11) that uses a titanium oxide film as a passivation film on the light-receiving surface of a crystalline silicon solar cell in the same manner as in the third embodiment. However, this titanium oxide film is produced by the method of Specimen #1 shown in Figure 21 That is, in the prior art, the first hydrogen plasma treatment and the second hydrogen plasma treatment performed in the third embodiment are not carried out, so the hole lifetime is as low as less than 1 ms. The results of this prior art are consistent with those of Specimen #1.

[0215] It can be seen that among Specimens #2, #3, and #4 that have undergone the hydrogen plasma treatment, the hole lifetimes of Specimens #3 and #4 have been significantly improved compared to Specimen #2, which is equivalent to the specimen verified in the first embodiment. It is considered that this is because in Specimen #2, the film thickness of the titanium oxide film subjected to the hydrogen plasma treatment is 5 nm (1.5 nm + 3.5 nm), while in Specimens #3 and #4, the film thickness of the first titanium oxide film subjected to the first hydrogen plasma treatment is as thin as 1.5 nm. Therefore, more atomic hydrogen (hydrogen element) reaches the intermediate film formed at the interface between the first titanium oxide film and the crystalline silicon layer, so the effect of the hydrogen plasma treatment on the composition (titanium, oxygen, silicon) and hydrogen content of the intermediate film becomes greater. That is, it can be speculated that hydrogen plasma treatment of a thinner titanium oxide film is the main factor for improving the passivation performance (increase in hole lifetime). Preferably, the film thicknesses of the first titanium oxide film and the second titanium oxide film are arranged in a distribution manner such that at least the film thickness of the first titanium oxide film is thinner than that of the second titanium oxide film. In particular, in view of the findings regarding Specimen #2, it is important that the film thickness of the first titanium oxide film is sufficiently thinner than 5 nm, and a range of approximately 0.5 nm or more and 2.5 nm or less is estimated to be preferable.

[0216] In addition, as in Specimen #3, after the first hydrogen plasma treatment, a second titanium oxide film is further stacked so that the thickness of the titanium oxide film is the same as that in Embodiment 1, thereby enabling passivation performance higher than that obtained in Embodiment 1. In fact, it has been confirmed that, as in the comparative example shown in Specimen #6, when only the first hydrogen plasma treatment is performed after the formation of the first titanium oxide film (film thickness: 1.5 nm), sufficient passivation performance cannot be obtained, and the further stacking of the second titanium oxide film is very effective.

[0217] One of the reasons is considered to be that when the film thickness of the titanium oxide film is 1.5 nm, since the film thickness is too thin, even if the atomic layer deposition method (ALD method) is implemented, the surface of the crystalline silicon layer cannot be completely coated. For example, it is presumed that there are parts such as pinholes in the titanium oxide film, and thus there are parts where the crystalline silicon layer is not coated.

[0218] In addition, as another reason, it is considered that if the film thickness of the titanium oxide film is too thin, it will affect the formation of the intermediate film. That is, it is considered that with this film thickness, almost only the intermediate film can be formed, so the chemical composition of the appropriate passivation film that can improve the passivation characteristics is not achieved. As a result, both the formation of fixed charges and the content of hydrogen are insufficient.

[0219] Furthermore, in Specimen #4, in addition to performing the first hydrogen plasma treatment, the second hydrogen plasma treatment is also performed, thereby further improving the passivation characteristics. In fact, when the processes performed in Specimen #3 and Specimen #4 are applied to the formation of the hole-selective film in Embodiment 1, the hole selectivity and passivation characteristics of the titanium oxide film are improved, and thus an open-circuit voltage higher than that obtained in Embodiment 1 can be obtained. However, due to the increase in the oxygen concentration of the intermediate film after annealing, the resistance increases, the fill factor decreases, and the conversion efficiency decreases. Therefore, preferably, the titanium oxide film in this Embodiment 3 is applied to the second passivation film of the back electrode type solar cell unit as shown in Figure 19 instead of being used as a hole-selective film. In the second passivation film of this back electrode type solar cell unit, hole selectivity is not required but passivation characteristics are required.

[0220] <Variant Example>

[0221] In addition, for example, it is also possible to use Figure 16 the structure composed of the titanium oxide film 11 and the transparent electrode 13 as shown in Figure 19 to replace the non-passivating hole-selective film 52 as shown in

[0222] As described above, the invention completed by the present inventor has been specifically described according to its embodiments. However, it goes without saying that the present invention is not limited to the described embodiments and can be variously modified without departing from its gist.

[0223] For example, in the above-described embodiment, an example in which a titanium oxide film formed by the manufacturing method according to Embodiment 1 is applied to a hole-selective film of a carrier-selective solar cell and a contact film of a semiconductor device has been described. However, the titanium oxide film formed by the manufacturing method according to Embodiment 1 is not limited thereto, and can also be applied to, for example, a photoanode for hydrolysis. It can be seen that the application uses of the titanium oxide film formed by the manufacturing method according to Embodiment 1 in this way are extensive and technically useful.

[0224] (Supplementary Note 1) A solar cell, comprising: a light absorber having a first surface and a second surface; a hole-selective film directly formed on the first surface and selectively allowing holes to pass through; and an electron-selective film directly formed on the second surface and selectively allowing electrons to pass through, wherein the hole-selective film is a titanium oxide film, and the hole-selective film has the same passivation characteristics as the electron-selective film.

[0225] (Supplementary Note 2) A semiconductor device, comprising a semiconductor layer and a plug connected to the semiconductor layer, the plug including a contact film in direct contact with the semiconductor layer, the contact film being composed of a hole-selective film that selectively allows holes to pass through, and the hole-selective film being a titanium oxide film.

[0226] (Supplementary Note 3) A method for manufacturing a semiconductor device, comprising: (a) a step of forming a first titanium oxide film on a crystalline silicon layer by thermal atomic layer deposition; (b) a step of subjecting the first titanium oxide film to a first hydrogen plasma treatment; and (c) a step of forming a second titanium oxide film on the first titanium oxide film by thermal atomic layer deposition after the step (b).

[0227] (Supplementary Note 4) The method for manufacturing a semiconductor device according to Supplementary Note 3, wherein the film thickness of the first titanium oxide film is thinner than that of the second titanium oxide film.

[0228] (Supplementary Note 5) The method for manufacturing a semiconductor device according to Supplementary Note 3, wherein after the step (c), a step of subjecting the second titanium oxide film to a second hydrogen plasma treatment is further included.

[0229] (Supplementary Note 6) The method for manufacturing a semiconductor device according to any one of Supplementary Notes 3 to 5, wherein the semiconductor device has a solar cell unit including a passivation film containing the first titanium oxide film and the second titanium oxide film.

[0230] (Supplementary Note 7) A semiconductor device includes: a crystalline silicon layer having a main surface; an intermediate film disposed on the main surface and containing silicon, titanium, and oxygen; a first titanium oxide film disposed on the intermediate film; and a second titanium oxide film disposed on the first titanium oxide film. In this semiconductor device, the intermediate film further contains 1.5 atomic% or more of hydrogen.

[0231] (Supplementary Note 8) The semiconductor device according to Supplementary Note 7, wherein the film thickness of the first titanium oxide film is smaller than that of the second titanium oxide film.

[0232] (Supplementary Note 9) The semiconductor device according to Supplementary Note 7, wherein the semiconductor device has a solar cell unit.

[0233] (Supplementary Note 10) The semiconductor device according to Supplementary Note 9, wherein the solar cell unit is a back electrode type solar cell unit provided with an anode electrode and a cathode electrode on the back side opposite to the front surface as the light incident surface, and a passivation film including the first titanium oxide film and the second titanium oxide film is provided on the front side of the back electrode type solar cell unit.

[0234] <Acknowledgements>

[0235] "A part of this research was supported by the "Nanotechnology Platform" project of the Ministry of Education, Culture, Sports, Science and Technology, and was carried out at the Nanofabrication Facility of the National Institute of Advanced Industrial Science and Technology (AIST).

[0236] Explanation of Reference Numerals

[0237] 1S Semiconductor substrate

[0238] 10 Crystalline silicon layer

[0239] 11 Titanium oxide film

[0240] 12 Electron selective film

[0241] 13 Transparent electrode

[0242] 13’ Second transparent electrode

[0243] 14 Metal electrode

[0244] 15 Passivation film

[0245] 20 Crystalline silicon layer

[0246] 21 Intermediate film

[0247] 22 Titanium oxide film

[0248] 23 Transparent electrode

[0249] 30 Contact film

[0250] 50 First passivation film

[0251] 51 Non-passivating electron-selective film

[0252] 52 Non-passivating hole-selective film

[0253] 53 Cathode electrode

[0254] 54 Anode electrode

[0255] 55a First titanium oxide film

[0256] 55b Second titanium oxide film

[0257] 60 Second passivation film

[0258] 100, 200, 300, 400, 500 Carrier-selective solar cell units

[0259] 600 p-channel field effect transistor

[0260] 700 Back contact type solar cell unit

[0261] CF Conductive film

[0262] CNT Contact hole

[0263] DR Drain region

[0264] GE Gate electrode

[0265] IL Interlayer insulating film

[0266] SR Source region

Claims

1. A semiconductor device comprising: A crystalline silicon layer having a main surface; An intermediate film disposed on the main surface and containing silicon, titanium, and oxygen; A titanium oxide film disposed on the intermediate film; A light-transmissive electrode disposed on the titanium oxide film; And An electron-selective film, In the semiconductor device, The combination of the intermediate film and the titanium oxide film is a hole-selective film, The intermediate film further contains 1.5 atomic% or more of hydrogen, The intermediate film is disposed at the interface between the crystalline silicon layer and the titanium oxide film, There is no amorphous silicon buffer layer between the crystalline silicon layer and the intermediate film, The combination of the intermediate film and the titanium oxide film has a hole selectivity that selectively allows holes in the crystalline silicon layer to pass through and a passivation characteristic that inhibits recombination of electrons and holes on the main surface.

2. The semiconductor device according to claim 1, Wherein, When the maximum titanium concentration of the titanium oxide film is set to 1, The oxygen concentration at the depth where the composition ratio of titanium in the intermediate film becomes 0.5 is 45 atomic% or less, And the silicon concentration at the depth where the composition ratio of titanium in the intermediate film becomes 0.5 is 36 atomic% or more.

3. The semiconductor device according to claim 1, Wherein, The total film thickness of the intermediate film and the titanium oxide film is greater than 3 nm and less than 8 nm.

4. The semiconductor device according to any one of claims 1 to 3, Wherein, On the main surface, a texture structure composed of (111) facets is formed.

5. The semiconductor device according to any one of claims 1 to 3, Wherein, The semiconductor device is a solar cell.

6. The semiconductor device according to claim 5, Wherein, The solar cell has: A light absorber including the crystalline silicon layer; The hole-selective film that selectively allows holes generated in the light absorber to pass through; and The electron-selective film that selectively allows electrons generated in the light absorber to pass through.

7. The semiconductor device according to claim 6, Wherein, The light-transmissive electrode is at least light-transmissive to visible light.

8. The semiconductor device according to any one of claims 1 to 3, Wherein, The crystalline silicon layer is an n-type silicon layer.

9. A method for manufacturing a semiconductor device, the semiconductor device comprising: A crystalline silicon layer having a main surface; An intermediate film disposed on the main surface and containing silicon, titanium, and oxygen; A titanium oxide film disposed on the intermediate film; A light-transmissive electrode disposed on the titanium oxide film; And An electron-selective film, In the semiconductor device, the combination of the intermediate film and the titanium oxide film is a hole-selective film, and there is no amorphous silicon buffer layer between the crystalline silicon layer and the intermediate film, The method for manufacturing the semiconductor device includes: (a) A step of directly forming a titanium oxide film on a crystalline silicon layer by thermal atomic layer deposition; And (b) A step of subjecting the titanium oxide film to hydrogen plasma treatment, In the manufacturing method, the intermediate film formed between the crystalline silicon layer and the titanium oxide film contains 1.5 atomic% or more of hydrogen, The combination of the intermediate film and the titanium oxide film has both a hole selectivity that selectively allows holes in the crystalline silicon layer to pass through and a passivation property that inhibits the recombination of electrons and holes on the main surface of the crystalline silicon layer.

10. The method of manufacturing a semiconductor device according to claim 9, wherein, further comprising: (c) a step of forming a conductor film on the titanium oxide film, (d) a step of performing heat treatment in an oxygen-containing atmosphere after the step (c).

11. The method of manufacturing a semiconductor device according to claim 10, wherein, the conductor film is a light-transmissive film that is at least light-transmissive to visible light.

12. The method of manufacturing a semiconductor device according to any one of claims 9 to 11, wherein, before the step (a), there is also a step of forming a textured structure on the surface of the crystalline silicon layer.

13. A solar cell, comprising: a crystalline silicon layer having a first main surface; an intermediate film provided on the first main surface and containing silicon, titanium, and oxygen; a titanium oxide film provided on the intermediate film; a light-transmissive electrode provided on the titanium oxide film; and an electron-selective film, the combination of the intermediate film and the titanium oxide film being a hole-selective film, there being no amorphous silicon buffer layer between the crystalline silicon layer and the intermediate film, the intermediate film further containing 1.5 atomic% or more of hydrogen, the combination of the intermediate film and the titanium oxide film having both a hole selectivity that selectively allows holes in the crystalline silicon layer to pass through and a passivation property that inhibits the recombination of electrons and holes on the first main surface, the conversion efficiency of the solar cell being 18% or more.

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