A method for forming a back reflector layer for a photovoltaic element

Abstract: An object of the present invention is to provide a back reflector layer in a texture structure with a high reflectivity by preventing oxidation of a ground metal or alloy, and a method for forming it. A further object is to provide a photovoltaic element excellent in characteristics such as a conversion efficiency, and a process for fabricating it. The back reflector layer has a metal or alloy (hereinafter referred to as a first metal), and a transparent oxide of a second metal, formed on the first metal. A photovoltaic element is formed by forming a semiconductor junction on the back reflector layer. In the back reflector layer and the photovoltaic element, an electron affinity of the second metal is 0.46 or more eV smaller than that of the first metal, and the transparent oxide is formed by a sputtering method in an atmosphere comprising at least H 2 O and an inert gas and with a target having a composition ratio of the second metal being 1.06 to 1.2 times a stoichiometric composition thereof.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention concerns a method for forming a back reflector layer for solar cells. Particularly, the invention relates to a back reflector layer having a high reflectivity and an asperity structure (hereinafter referred to as a texture structure), and a photovoltaic element high in performance, high in reliability, and capable of being mass-produced. Related Background Art At present we, human beings, consume the energy as being greatly dependent upon the thermal power generation using fossil fuels such as petroleum and coal, and the nuclear power generation. It will be, however, in serious question that we may also continue totally depending henceforth upon the fossil fuels, which cause global warming because of carbon dioxide etc. produced upon use of the thermal power generation, or depending upon the nuclear power generation, which includes a more or less danger of radiation not only in an accident, but also in normal operation, as an energy source. Thus, attention has been drawn to the solar-electric power generation using solar cells utilizing the solar energy, which has very little effect on the global environment, and further use thereof is being expected. There are, however, some problems to hinder widespread use of the solar-electric power generation at present. Single-crystal or polycrystal silicon groups have popularly been used heretofore for the solar cells for solar-electric power generation. In case of the solar cells using these single-crystal or polycrystal silicon, growth of crystal necessitates a lot of energy and time, and complex steps are necessary after the growth. Thus, they are not suitable for mass production, and it is difficult to provide them at low price. On the other hand, active research and development have been made on the so-called thin-film semiconductor solar cells using amorphous silicon (hereinafter referred to as a-Si) and compound semiconductors such as CdS and CuInSe 2 . In case of these solar cells, they can be fabricated simply by forming a necessary amount of a semiconductor layer on a cheap substrate such as glass or stainless steel, fabrication steps thereof are relatively simple, and they include a possibility of cost reduction. The thin-film semiconductor solar cells, however, have lower efficiencies of photoelectric conversion than those of single-crystal or polycrystal silicon solar cells and are not sufficient in reliability to long-term use. Thus, they are not widely used presently. A variety of ideas as discussed below have been presented in order to solve such problems and to improve the performance of the thin-film semiconductor solar cells. One of them is to provide a back reflector layer for enhancing the reflectivity of light on the surface of substrate in order to bring the light energy such as the sunlight not absorbed by the thin-film semiconductor for photoelectric conversion back again to the thin-film semiconductor, i.e., in order to effectively utilize incident light. For this purpose, where the substrate is optically transparent and the sunlight is made incident from the substrate side, after formation of the thin-film semiconductor an electrode is formed on the surface thereof, using a metal with high reflectivity such as silver (Ag), aluminum (Al), or copper (Cu). In contrast, where the sunlight is made incident from the surface of the thin-film semiconductor, a layer of the same metal as listed above is first formed on the substrate and then the semiconductor layer is formed thereon. Further, the reflectivity can be more enhanced by the multiple interference effect when a transparent layer having appropriate optical properties is interposed between the metal layer and the thin-film semiconductor layer, for example as shown in Figs. 6A and 6B. Figs. 6A and 6B show simulation results to verify that the reflectivity is improved when zinc oxide (ZnO) is interposed as a transparent layer between silicon and a metal selected from the listed metals: Fig. 6A shows reflectivities when a-Si is formed on the metals; Fig. 6B shows reflectivities when ZnO is formed on the metals and then a-Si is formed on ZnO. The use of such a transparent layer is also effective to enhancing the reliability of thin-film solar cells. For example, United States Patents No. 4,532,372 and No. 4,598,306 describe that use of a transparent layer with an appropriate resistance can prevent an excessive electric current from flowing between the electrodes even if the semiconductor layer has a short-circuited portion. Another idea to enhance the conversion efficiency of solar cell is a method (of the texture structure) for making fine asperities on the surface of solar cell and/or an interface between the back reflector layer and the semiconductor layer. According to such structure, the sunlight is scattered by the surface of solar cell and/or the interface between the back reflector layer and the semiconductor layer and further is confined inside the semiconductor (which is the light trap effect), whereby the sunlight can be effectively absorbed in the semiconductor. For example, where a transparent substrate is used and the sunlight is made incident from the transparent substrate side, a good result can be obtained by texturing the surface of the transparent electrode such as tin oxide (SnO 2 ) on the substrate. When the sunlight is made incident from the surface of the thin-film semiconductor, a good result can be obtained by texturing the surface of the metal layer used as the back reflector layer. M. Hirasaka, K. Suzuki, K. Nakatani, M. Asano, M. Yano, and H. Okaniwa disclose that the texture structure for back reflector layer can be obtained by depositing Al as adjusting the temperature of substrate and the deposition rate (Solar Cell Materials 20 (1990) pp99-110). Fig. 7 shows an example in which absorption of incident light increases by use of the back reflector layer in such a texture structure. In Fig. 7, curve (a) represents a spectral sensitivity of an a-Si solar cell using a flat Ag layer as the metal layer, whereas a curve (b) a spectral sensitivity of an a-Si solar cell fabricated in the same manner as it except that an Ag layer is formed in the texture structure as the metal layer. One of methods for obtaining the texture structure of transparent layer is the method for introducing water vapor into discharge gas (Effect of Water Vapor on the Textured ZnO-Based Films for Solar Cells by DC-Magnetron Sputtering; Tokio Nakada, Yukinobu Ohkubo and Akio Kunioka (Japanese Journal of Applied Physics Vol. 30 No 12A, December 1991, pp. 3344-3348)). Observation by SEM confirmed that the texture structure was more developed by the method for depositing a metal oxide on a glass plate as introducing water vapor than by a method for depositing the metal oxide using Ar gas as a discharge gas. Although the conventional technology required a thick film in order to obtain a necessary texture structure for raising the characteristics of solar cell, this method made it possible to decrease the film thickness. It was, however, found that the solar cells using the back reflector layer, obtained by forming a transparent layer on a metal layer as introducing water vapor, failed to show expected characteristics or on the contrary, they could lower the characteristics, for example the conversion efficiency. As described above, it is a present status that although there is a possibility of producing high-performance solar cells at low cost, they are not available for practical use yet. Banerjee A. et. al. (Journal of Applied Physics, Vol. 70, Nr. 3, pp. 1692 -1694) disclose a back reflector layer for solar cells having Ag as first metal and ZnO as a transparent oxide comprising a second metal and formed on said first metal, wherein the difference of electron affinities between both metals is not less than 0.46 eV. EP-A-0 554 877 discloses a back reflector layer for solar cells having a metal or alloy as first metal; a transparent oxide in a texture structure comprising a second metal (ZnO) and formed on said first metal; wherein the difference of electron affinities between both metals is not less than 0.46 eV. JP 06 204 527 A discloses a back reflector layer for solar cells having Ag as first metal and a transparent oxide comprising a second metal (ZnO) and formed on said first metal, wherein the difference of electron affinities between both metals is not less than 0.46 eV. SUMMARY OF THE INVENTION An object of the present invention is to provide a method for forming a back reflector layer for solar cells according to claim 1. Preferred embodiments are set forth in the dependent claims 2 to 5. A further aspect of the invention is to provide a method for forming a photovoltaic element excellent in characteristics such as the conversion efficiency. A back reflector layer of the present invention is a back reflector layer having a metal or alloy (hereinafter referred to as a first metal), and a transparent oxide comprising a second metal and formed on the first metal, wherein an electron affinity of the second metal is 0.46 or more eV smaller than that of the first metal and wherein the transparent oxide is formed by a sputtering method in an atmosphere comprising at least H 2 O and an inert gas and with a target having a composition ratio of the second metal being 1.06 to 1.2 times a stoichiometric composition thereof . The film thickness of the transparent oxide of the second metal is preferably between 5 nm and 20 nm, and a second transparent oxide is preferably further formed on the transparent oxide by a sputtering method using a target with a composition ratio of the second metal being 0.96 to 1.04 times the stoichiometric composition thereof. A photovoltaic element of the present invention is featured in that a semiconductor junction is formed on the above back reflector layer. A method for forming the back reflector layer is one in which on a metal or alloy (hereinafter referred to as a first metal) there is formed a transparent oxide of a second metal having an electron affinity 0.46 or more eV smaller than that of the first metal, wherein the transparent oxide is formed by a sputtering method in an atmosphere comprising at least H 2 O and an inert gas and with a target having a composition ratio of the second metal being 1.06 to 1.2 times a stoichiometric composition thereof. The film thickness of the transparent oxide is preferably between 5 nm and 20 nm, and another transparent oxide is preferably formed after formation of the above transparent oxide, using a target having a composition ratio of the second:metal being 0.96 to 1.04 times the stoichiometric composition thereof. In the present invention, O 2 gas is preferably added into the above atmosphere. A process for fabricating the photovoltaic element of the present invention is featured in that a semiconductor junction is formed after formation of the above back reflector layer. This process can form the photovoltaic element having excellent characteristics. BRIEF DESCRIPTION OF THE DRAWINGS Figs. 1A - 1C are diagrammatic cross sections to show an example of fabrication steps for fabricating the photovoltaic element of the present invention; Fig. 2 is a diagrammatic drawing to show an example of a sputtering apparatus suitably applicable to fabrication of the back reflector layer of the present invention; Fig. 3 is a diagrammatic drawing to show another example of the sputtering apparatus suitably applicable to fabrication of the back reflector layer of the present invention; Fig. 4 is a diagrammatic drawing to show a sputtering apparatus of a roll-to-roll type suitably applicable to fabrication of the back reflector layer of the present invention; Fig. 5 is a diagrammatic cross section to show another example of the photovoltaic element of the present invention; Figs. 6A and 6B are graphs to show the effect of ZnO against reflectivity at the interface between silicon and metals; and Fig. 7 is a graph to show an improvement in spectral sensitivity of solar cell by the texture structure. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Extensively and intensively studying the relation between the film-forming conditions of the transparent layer and the characteristics of photoelectric conversion element, the present inventor found the following problems in the cases where a back reflector layer of a transparent and electrically conductive layer having the texture structure was formed on a metal layer, using H 2 O as a reactive gas. In more detail, when a metal oxide layer was formed by sputtering etc. in an atmosphere containing H 2 O, the texture was developed in even smaller thicknesses than in case of a discharge gas including only an inert gas such as Ar, but the inventor observed such a tendency that the ground layer, the metal or alloy film, was oxidized. It was found that this oxidation lowered the reflectivity, resulting in lowering the function of the back reflector layer for photoelectric conversion element. The most significant cause of this oxidation is conceivably that when the H 2 O gas or O 2 gas as a reactive gas or a discharge gas is exposed to discharge, active oxygen is produced to oxidize the ground metal or alloy film. As the ground metal or alloy is a material more likely to be oxidized, it becomes more subject to oxidation. Namely, when the transparent oxide layer having the texture structure is formed by the sputtering method, the ground metal or alloy layer is subjected to oxidation by the H 2 O or O 2 gas in the atmosphere. The inventor of the present application found out after intensive and extensive studies that the transparent oxide with asperity was capable of being formed while preventing the oxidation of the ground metal upon sputtering and by the transparent oxide if a metal having an electron affinity 0.46 or more eV smaller than that of the ground metal (first metal) was selected as a metal (second metal) composing the transparent oxide and if the transparent oxide was formed using a target containing the metal 1.06 to 1.2 times more than the stoichiometric composition of the transparent oxide. This is considered as an effect due to a difference in electron affinity between the contained elements. The electron affinity is defined as follows. In a reaction where an atom or molecule A is coupled with an electron to form a negative ion, energy discharged at that time is defined as an electron affinity of A. Electrons become easier to move as this energy of one atom or molecule becomes smaller than that of another atom or molecule. Accordingly, using the target having a more content of the second metal than the stoichiometric composition of the transparent oxide layer, the transparent oxide layer is formed in an excessive amount of the second metal component. Thus, even if the ground metal layer is oxidized upon sputtering, the excessive second metal in the metal oxide deposited thereon can deprive the ground metal layer of oxygen. Or, it is considered that the excessive second metal is oxidized before oxidation of the ground metal layer, thereby preventing oxidation of the ground metal layer. Particularly, in applications as a photovoltaic element, a too small, excessive amount of the metal in the transparent oxide layer or a too small film thickness will result in insufficient antioxidation, or in contrast, a too great, excessive amount of the metal or a too large film thickness will lower the transmittance, thus decreasing the reflectivity itself. Therefore, the target used in forming the transparent oxide is determined to have the metal composition ratio of 1.06 to 1.2 times the stoichiometric composition. Next described referring to the drawings is a method for fabricating the back reflector layer and the photovoltaic element of the present invention. Figs. 1A - 1C are diagrammatic drawings to show fabrication steps of the photovoltaic element of the present invention. Metal Layer 101 In Figs. 1A - 1C, reference numeral 101 designates a substrate, and a layer 102 of a metal with high reflectivity is formed on the surface of the substrate. If the substrate 101 itself is made of a material being electrically conductive and having a sufficiently high reflectivity, the metal layer 102 may be omitted. Here, at least the surface of the metal layer 102 is preferably formed as a flat surface in order to enhance the reflectivity (Fig. 1A). Transparent Oxide Layer 103 A transparent oxide layer 103 having fine asperities on the surface is formed on the metal layer 102. This example shows a case of ZnO being used as the transparent oxide layer 103. This example shows a case where the transparent oxide layer 103 is composed of first and second transparent oxide layers, which are a metal oxide layer (first transparent oxide layer) 104, formed using a metal-excessive target (Zn + ZnO), and a metal oxide layer (second transparent oxide layer) 105, formed using a ZnO target. Here is described a method for forming the transparent oxide layer 103, using a DC magnetron sputter apparatus shown in Fig. 2. In Fig. 2, reference numeral 201 designates a deposition chamber, which is arranged to be evacuated to the vacuum by an exhaust pump not shown. Inside the deposition chamber, H 2 O and an inert gas such as argon (Ar) are introduced at predetermined flow rates to realize a desired mixture ratio thereof through gas inlet pipes 202a, 202b connected to gas bombs not shown, and an aperture of an exhaust valve 203 is adjusted to keep the inside of the deposition chamber 201 at a predetermined pressure. A substrate 204 having the flat metal layer 102 on the surface is fixed to a surface of anode 206 in which a heater 205 is installed. Disposed as opposed to the anode 206 is a cathode electrode 208, to a surface of which a target 207 is fixed and inside which a magnet not shown is provided. The target 207 is, for example, zinc oxide containing 56 % of zinc. A power supply 209 applies a high voltage of direct current (DC) to the cathode electrode 208 to produce plasma 210 between the cathode and the anode. An action of this plasma 210 generates active oxygen, which temporarily oxidizes the metal layer 102. However, ZnO and Zn as the second metal with an electron affinity 0.46 or more eV smaller than that of the first metal used in the metal layer 102, in the target 207 are deposited on the substrate 204, and the thus deposited Zn as the second metal deprives the metal layer 102 oxidized, of oxygen to reduce the metal layer 102. Accordingly, the metal layer 102 maintains a high reflectivity, and the second metal depriving of oxygen serves as a transparent oxide layer. Next, after deposition of the first transparent oxide, the target is changed to a ZnO target, for example, containing 50 % zinc, and a second transparent oxide is deposited on the first transparent oxide. In the thus obtained photovoltaic element in the form shown in Fig. 1, the transparent oxide 103 is transparent to light energy transmitted by a semiconductor. Further, it has an appropriate electric resistance and the surface thereof is of the texture structure. Particularly, use of water vapor as a reactive gas develops the texture structure in the same film thickness more than in the case of a discharge gas containing only the inert gas such as Ar (Fig. 1B). Semiconductor 106 Next, a semiconductor 106 is formed on the transparent oxide 103. Fig. 1C shows an example to use a pin-type a-Si semiconductor as a semiconductor junction. Namely, reference numeral 107 is an n-type a-Si semiconductor, 108 an i-type a-Si semiconductor, and 109 a p-type a-Si semiconductor. The photoelectric conversion portion may be formed using the pn junction or Schottky junction as the semiconductor 106. Instead of the a-Si semiconductor, the semiconductor 106 may be formed using microcrystalline silicon (µ-Si), a-SiGe, a-SiC, a-SiSn, a-SiN, a-SiO, µ-SiGe, µ-SiC, µ-SiSn, µ-SiN, µ-SiO, etc., or a combination of those for the respective conductors with the respective conduction types. A method for forming the semiconductor may be selected from a variety of methods including the RFCVD process, the MWCVD process, the photo-assisted CVD process, the thermal CVD process, the sputtering process, the ion plating process, etc. If the semiconductor 106 is thin enough, the entire semiconductor junction often shows the texture structure similar to the transparent oxide 103, as shown in Fig. 1C. Transparent Electrode 110 As shown in Fig. 1C, a transparent electrode 110 such as ITO or SnO 2 is used where the light is made incident from the opposite side to the substrate. Further, in order to improve capability of electron collection, a collector electrode 111, a bus-bar, or the like may be provided in a desired shape (for example, in a comb shape). The photovoltaic element fabricated by the above procedures has the following advantages. (1) By reducing the oxidation of the surface of the metal layer 102, the reflectivity of light on the metal surface can be maintained as high as the original reflectivity of the metal layer 102. In addition, the surface of the transparent oxide layer 103 (and semiconductor junction 106) is shaped in the well developed texture structure even in a thin film thickness as compared with the case using only the Ar gas, thus enhancing the light trap effect in which the semiconductor serves as a kind of waveguide tube inside the semiconductor junction 106. Thus, incident light energy is effectively absorbed so as to improve the conversion efficiency of the solar cell as being a photoelectric conversion element. (2) Since the surface of the metal layer 102 (or the substrate 101 itself) is flat, a contact area with the transparent oxide layer 103 decreases, which makes a reaction of diffusion or the like of the second metal atoms into the transparent oxide layer 103 unlikely to take place, thereby maintaining the high reflectivity of the metal layer 102. (3) Since the transparent oxide 103 is provided with an appropriate resistance even if the target 207 for forming the transparent oxide contains atoms of the second metal 1.06 to 1.2 times the stoichiometric composition, no excessive electric current flows even if the semiconductor has a defect such as a pinhole. Next explained are experiments which were done in order to show the operational effects of the present invention in more detail. (Experiment 1) On a glass substrate of 5 cm × 5 cm a transparent oxide was deposited in 400 nm by the DC magnetron sputter process in an atmosphere of Ar gas and/or H 2 O (50 sccm) and with a target of zinc oxide (Zn:O = 56:44, 1.12 times the stoichiometric composition). In order to observe the texture structure of the transparent layer, a scanning probe microscope (Nano Scope III manufactured by Desital Instruments Co.) was used to obtain a texture degree (average height of differences between crests and roots). The results are shown in Table 1. Ar:H 2 O (%) 100:0 90:10 80:20 70:30 60:40 Texture degree (nm) 68 75 95 300 320 Ar:H 2 O (%) 50:50 40:60 20:80 0:100 - Texture degree (nm) 322 323 324 325 - It is seen from Table 1 that the texture degree increases as the ratio of H 2 O in the Ar + H 2 O atmosphere increases. This confirms that introduction of H 2 O is effective to improvement in the texture structure. (Experiment 2) Using a SUS plate of 5 cm × 5 cm instead of the glass substrate, Ag was deposited in 60 nm on the SUS plate, and thereafter the transparent oxide was formed in the same manner as in Experiment 1. Then the texture degree was measured using the scanning probe microscope, and the reflectivity was also measured using a reflection spectroscope (U 4000 type automatic luminous intensity spectrophotometer manufactured by Hitachi, Ltd.) with Ag being a standard. The results are shown in Table 2. Ar:H 2 O (%) 100:0 90:10 80:20 70:30 60:40 Texture degree (nm) 80 92 106 320 340 Total reflectivity (%) 90 78 62 58 49 Irregular reflectivity (%) 20 15 8 32 33 Ar:H 2 O (%) 50:50 40:60 20:80 0:100 - Texture degree (nm) 342 343 344 345 - Total reflectivity (%) 49 45 44 41 - Irregular reflectivity (%) 34 34 32 31 - The total reflectivity indicates a rate of intensity of light reflected in all directions relative to the intensity of incident light, and the irregular reflectivity indicates a rate of intensity of reflected light except for the light in the directions of reflecting angles relative to the intensity of incident light. The texture degree increased as the rate of H 2 O increased, similarly as in the results of Experiment 1, but the total reflectivity decreased with increase of H 2 O. Samples were etched with acetic acid to observe the surface of Ag. The observation confirmed that the surface of Ag was oxidized to become black. From this, it is seen that H 2 O in the discharge gas is activated by discharge to promote oxidation of the surface of Ag. The metal layer continues becoming black with introduction of H 2 O. The texture degree is not so large until the ratio of 80:20. The texture degree steeply increases from the ratio of 70:30 and then saturates soon, thus decreasing the irregular reflectivity. (Experiment 3) Ag was deposited in 60 nm on a SUS plate of 5 cm × 5 cm, and then a first transparent oxide layer was deposited in 10 nm on the Ag layer, changing the composition ratio of ZnO in the target as listed in Table 3 as a countermeasure to prevent the oxidation of Ag observed in Experiment 2. Subsequently, a second transparent oxide was formed in 1000 nm in the same manner as in Experiment 2. Evaluation results are shown in Table 3. Here, the composition ratios of target are results of measurement by XMA (energy dispersive x-ray analyzer EMAX-5770 manufactured by Horiba Seisakusho). Also, R in the table represents a ratio of Zn composition in the target relative to the stoichiometric composition. (a) (b) (c) (d) Composition of Target (%) 50:50 53:47 56:44 60:40 R 1.0 1.06 1.12 1.2 Total reflectivity (%) 49 75 92 72 (e) (f) (g) (h) Composition of Target (%) 70:30 80:20 90:10 100:0 R 1.4 1.6 1.8 2.0 Total reflectivity (%) 43 18 2 0 From Table 3 it is seen that as the Zn component increases in the target, the total reflectivity increases and then decreases. Particularly, high reflectivities are obtained in the range of R being 1.06 to 1.2. (Experiment 4) Table 4 shows evaluation results obtained for changes of the film thickness of the first transparent oxide in forming a film of the first transparent oxide using the condition (c) in Experiment 3, i.e., using the target with the target composition being 56:44 (R = 1.12). Film thickness (nm) 0 1 2 5 10 20 30 Total reflectivity (%) 49 54 68 81 92 78 45 From Table 4, the total reflectivity took great values when the first transparent oxide had a film thickness of not less than 2 nm. This assures that the first transparent oxide is effectively functioning as an antioxidation layer if the film thickness is not less than 2 nm. With a further increase in film thickness, a color of Zn as a metal becomes outstanding, thereby decreasing the reflectivity. It is thus seen that the Zn + ZnO layer for antioxidation of the Ag layer needs to be used in an appropriate range of film thickness because of its properties. As apparent from the table, the film thickness of the first transparent oxide is more preferably 5 - 20 nm. (Experiment 5) On a SUS plate of 5 cm × 5 cm, the metal layer was formed in 60 nm using each of materials Al, Cu, and Ag as listed in Table 5, and the first transparent oxide of each of ZnO, In 2 O 3 , and SnO 2 (R = 1.12) was formed each at the temperature 400 °C of film-forming atmosphere and in the film thickness of 10 nm on the metal layer. The reflectivity was measured using the various types of back reflector layers. The results are also shown in Table 5. In the table, O represents a total reflectivity of not less than 70 %, while × does a total reflectivity of less than 70 %. Transparent oxide Zn + ZnO In + In 2 O 3 Sn + SnO 2 Metal layer Al ○ × × Cu ○ ○ × Ag ○ ○ × Atomic number 13 29 47 30 49 50 Element Al Cu Ag Zn In Sn Electron affinity (eV) 0.46 1.226 1.303 ∼0 0.30 1.25 A B Zn In Sn Al 0.46 0.16 -0.79 Cu 1.226 0.926 -0.024 Ag 1.303 1.003 0.053 From Table 5, when the transparent oxide is Zn, either of the metal layers of Al, Cu, Ag showed the high reflectivity; when the transparent oxide was In, the effect was observed in case of the metal layer of Ag or Cu to show the high reflectivity. When the transparent oxide was Sn, the total reflectivity was less than 70 % with the metal layer of Al, Cu, or Ag. Table 6 shows electron affinities of the respective elements listed (from Revised Third Edition of Handbook of Chemistry, Foundations II, Compiled by the Chemical Society of Japan and published by Maruzen). Further, Table 7 shows values indicating differences, E (A) - E(B), in electron affinity between the metals used in the metal layer and the metals used in the transparent layer. From comparison of Table 5 with Table 7, it is seen that good characteristics as a back reflector layer are achieved to show the total reflectivity of not less than 70 % when the difference of electron affinities, E(A) - E(B), is not less than 0.46 eV. (Experiment 6) On a stainless steel plate (SUS 430) of 5 cm × 5 cm Ag was deposited in 60 nm by the DC magnetron sputter process. On this occasion the temperature of substrate was the room temperature. On the Ag layer the transparent oxide was deposited in 400 nm at the temperature of substrate of 150 °C and in the Ar gas atmosphere by the DC magnetron sputter method using a target of zinc oxide (ZnO). The temperature of substrate at this time was 150 °C. Subsequently, on the back reflector layer, an a-Si layer with the n-type conductivity was deposited in 20 nm with raw gases of SiH 4 and PH 3 , an a-SiGe layer with the i-type conductivity in 400 nm with raw gases of SiH 4 and GeH 4 , and a microcrystalline (µc) Si layer with the p-type conductivity in 10 nm with raw gases of SiH 4 , BF 3 , and H 2 , in the named order by the glow discharge decomposition method to form semiconductor junctions. (Since a-Si formed by the glow discharge decomposition method of SiH 4 etc. includes about 10 % of hydrogen (H), it is generally expressed as a-Si:H; but for simplicity, it is described as a-Si in the description of the present application.) On this semiconductor an ITO film was deposited as a transparent electrode in 65 nm by the resistance heating evaporation method. Further on the ITO film a collector electrode was formed of a silver paste in the width of 300 µm. This is called as Sample 6a. Sample 6b was prepared in the same manner as Sample 6a except that the film thickness of the transparent oxide was 800 nm. Further, Samples 6c and 6d were prepared in the same manner as Samples 6a and 6b except that the discharge gas was Ar and H 2 O at a ratio of 1:1. In addition, Samples 6e and 6f were prepared in the same manner as Samples 6c and 6d by depositing the first transparent oxide layer in 10 nm by the DC magnetron sputter method in the atmosphere of Ar and H 2 O at a ratio of 1:1 and with the Zn + ZnO target (Zn:O = 56:44) and then depositing the second transparent oxide layer in 390 nm and in 790 nm, respectively, by the DC magnetron sputter method in the atmosphere of Ar and H 2 O at a ratio of 1:1 and with the ZnO target. The thus obtained samples were subjected to measurement of solar cell characteristics under the solar simulator of AM-1.5. The results are shown in Table 8. Target Discharge gas Film thickness (nm) Current value (A) Efficiency(%) 6a ZnO Ar 400 20.6 8.1 6b ZnO Ar 800 22.1 9 6c ZnO Ar+H 2 O 400 21.2 8.3 6d ZnO Ar+H 2 O 800 22.9 9.2 6e Zn+ZnO Ar+H 2 O 400 22.6 8.9 6f Zn+ZnO Ar+H 2 O 800 23.5 9.6 From comparison between Samples 6a and 6c or between Samples 6b and 6d, it is considered that with introduction of H 2 O the texture structure developed to improve the irregular reflectivity, thereby resulting in improving the current value as compared with the cases where the discharge gas included only Ar. Further, as apparent from the relation between Samples 6c and 6e or the relation between Samples 6d and 6f, it is seen that formation of the first transparent oxide layer further improves the current value, thereby improving the efficiency. The back reflector layer in the photovoltaic element of the present invention is next explained in more detail. (Substrate 101 and Metal Layer 102) A variety of metals can be suitably applicable to the substrate 101. From among them, a stainless steel plate, a galvanized sheet iron, an aluminum plate, a copper plate, and the like are preferably applicable because of relatively low prices. These metal plates may be used in a certain cut shape or in the form of an elongate sheet depending upon the thickness of plate. In the case of the sheet form, because the sheet plate can be rolled in a coil form, it well matches continuous production and facilitates storage and transportation. Depending upon applications, the substrate may be a crystal substrate of silicon etc., a glass or ceramic plate, or a plate of a resin etc. The surface of the substrate may be ground, but it may be used without grinding if finishing is acceptably fine, for example like a stainless steel plate treated by bright annealing. If the substrate is one with low reflectivity of light when not treated, for example like the stainless steel plate or galvanized sheet iron, or an insulating substrate such as glass, it is preferably used after depositing a metal with high reflectivity such as silver, aluminum, or copper on'the substrate. However, in case of use as a back reflector layer, since components of short wavelengths in the spectrum of incident light such as the sunlight have already been absorbed by the semiconductor, it is a sufficient condition that the reflectivity is high for the light of longer wavelengths than those. What wavelength is to be determined as a border for high reflectivities depends upon the light absorption coefficient and the film thickness of the semiconductor used. For example, in case of a-Si in the thickness of 400 nm, the border wavelength is about 600 nm, and thus, copper can be suitably used (Figs. 6A and 6B). Further, even the materials with low electric conduction when untreated, like glass or ceramics, can be used as the substrate by forming a metal layer thereon. Deposition of the metal layer can be effected by either one selected from the vacuum deposition by resistance heating or electron beam, the sputtering method, the ion plating method, the various CVD methods utilizing plasma, light, and thermal energy, the plating method, etc. As an example of the film-forming method, a case of the sputtering method is explained. Fig. 3 shows an example of a sputtering apparatus. Reference numeral 301 designates a deposition chamber, which is arranged to be evacuated to the vacuum by an exhaust pump not shown. Inside this chamber, an inert gas such as argon (Ar) is introduced at a predetermined flow rate through a gas inlet pipe 302 connected to a gas bomb not shown, and an aperture of an exhaust valve 303 is adjusted to keep the inside of the deposition chamber 301 at a predetermined pressure. The substrate 304 is fixed to the surface of anode 306, inside which a heater 305 is provided. A cathode electrode 308, to the surface of which a target 307 is fixed, is arranged as opposed to the anode 306. The target 307 is a block of a metal to be deposited. It is normally a pure metal having a purity of about 99.9 % to about 99.999 %, but a specific impurity may be introduced thereinto depending upon the case. The cathode electrode is connected to a power supply 309. The power supply 309 applies a high voltage of radio frequency (RF) or direct current (DC) to produce plasma 310 between the cathode and the anode. An action of this plasma makes metal atoms in the target 307 deposited on the substrate 304. The deposition rate can be enhanced by the magnetron sputtering apparatus increased in intensity of plasma by setting a magnet inside the cathode 308. (Transparent Oxide 103 and Texture Structure thereof) A method of film-forming the transparent oxide with fine asperities on the surface is next explained as to zinc oxide, referring to Fig. 2. Candidates of the target include a target having a rate of the metal component of zinc oxide being 48 % to 52 % (R = 0.96 to 1.04) and a target having a rate of the metal component of zinc oxide being 53 % to 60 % (R = 1.06 to 1.2). The former target can be fabricated by the normal method, but the latter target requires a change of the fabrication conditions. In detail, press (hot press) is conducted at high temperature in the Ar atmosphere for a long time, thereby increasing the rate of the metal component. In another method, a reducing gas such as CO may be introduced into the atmosphere, and in that case, the rate of the metal component can be changed by controlling an introducing amount of the reducing gas. In either of the cases using the two targets, the film-forming conditions of the transparent oxide are different more or less depending upon whether the power supply is of RF or of DC, whether the magnet behind the target is present or absent, the strength of a magnetic field if the magnet is set, the distance between the substrate and the target, etc.; generally, in the former (Zn = 48 to 52 %) case, an amount of making power per unit area of the target is 0.5 - 5 W/cm 2 , preferably 0.5 - 4 W/cm 2 , and more preferably 0.7, - 3 W/cm 2 . A flow rate of O 2 introduced is 0 - 10 with respect to 100 of the flow rate of the inert gas introduced, preferably 0 - 5, and more preferably 0 - 2. The pressure in the deposition chamber upon film formation is 0.5 - 50 mTorr, preferably 1 - 30 mTorr, and more preferably 2 - 20 mTorr. Similarly, in the latter (Zn = 53 to 60 %) case, an amount of making power per unit area of the target is 1 - 10 W/cm 2 , preferably 1 - 7 W/cm 2 , and more preferably 1- 5 W/cm 2 . Preferred ranges of the flow rate of O 2 introduced upon film formation and the pressure in the deposition chamber are the same as those in the case of the former target. Here is described the DC magnetron sputter process as an example. In Fig. 2, numeral 201 denotes the deposition chamber, which is arranged to be evacuated to the vacuum by an exhaust pump not shown. Inside the chamber, H 2 O and an inert gas such as argon (Ar) are introduced at predetermined flow rates to achieve a desired mixture ratio through the gas inlet pipes 202a, 202b connected to respective gas bombs not shown, and the aperture of the exhaust valve 203 is adjusted to keep the inside of the deposition chamber 201 at a predetermined pressure. The substrate 204 having a flat metal layer on the surface is fixed to the surface of the anode 206, in which the heater 205 is installed. Disposed as opposed to the anode 206 is the cathode electrode 208, to the surface of which the target 207 is fixed and in which the magnet not shown is provided. The target 207 includes the metal component of zinc oxide, i.e., Zn at a rate of 53 to 60 % (R = 1.06 to 1.2). The cathode electrode is connected to the power supply 209. The power supply 209 applies a high voltage of direct current (DC) to produce plasma 210 between the cathode and the anode. An action in this plasma causes zinc oxide of target 207 to be deposited on the substrate 204. The light transmittance of the transparent oxide with fine asperities on the surface thus obtained by the above process is generally preferred to be as high as possible in order to effectively utilize the incident light, but the transparent oxide does not have to be transparent to the light in the wavelength region absorbed by the semiconductor. It is rather better for the transparent oxide to have a resistance for restricting an electric current due to a pinhole. On the other hand, the effect of series resistance loss due to the above resistance on the conversion efficiency of solar cell must be within a negligible range. From such a point of view, the range of resistance per unit area (1 cm 2 ) is preferably from 10 -6 to 10 Ω, more preferably from 10 -5 to 3 Ω, and most preferably from 10 -4 to 1 Ω. A preferred film thickness of the transparent oxide is as thin as possible in respect of transparency, but in order to take the texture structure with high light trap effect, the average film thickness is preferably not less than 100 nm. Also, some cases require film thicknesses over it in respect of reliability. A conceivable reason of occurrence of optical confinement by the texture structure is scattering of light at the metal layer when the metal layer itself takes the texture structure. If the metal layer is flat but the transparent oxide takes the texture structure, a considerable reason is scattering due to phase shifts of incident light between recesses and projections at the surface of semiconductor and/or the interface with the transparent oxide. The pitch is preferably about 300 - 2000 nm, more preferably 400 - 1500 nm, and the difference in height is preferably 50 - 2000 nm, more preferably 70 - 1000 nm. If the surface of semiconductor takes the similar texture structure to the transparent layer, scattering of light due to phase difference of light becomes easy to occur, thus enhancing the effect of light trap. Further, formation of the transparent oxide of the present invention can prevent oxidation of the ground metal or alloy layer even if it contains, for example, a large quantity of H 2 O, O 2 , or O 3 as a reactive gas. Addition of O 2 gas into the atmosphere upon formation of the transparent oxide has an effect to smooth edges of the texture structure, which decreases recombination centers so as to improve the fill factor, resulting in improving the conversion efficiency. It can also prevent oxidation upon high-temperature film formation in the next step. The present invention will be explained in more detail with specific examples. Example 1 In this example, the solar cell shown in Fig. 5 was fabricated. First, the back reflector layer was continuously formed using the apparatus shown in Fig. 4. Here, a stainless-steel sheet roll 401 350 mm wide, 0.2 mm thick, and 500 m long after cleaned is set in a substrate feed chamber 403. From this chamber, the stainless-steel sheet 402 is fed through a metal-layer deposition chamber 404 and a transparent-oxide-layer deposition chamber 405 to a substrate winding chamber 406. The sheet 402 is arranged to be heated at desired temperature by a substrate heater 407, 408 in each deposition chamber. The target 409 in the deposition chamber 404 is Ag of 99.99 % purity, and an Ag layer 502 is deposited on the sheet 402 by the DC magnetron sputtering method. A target 410 in the deposition chamber 405 is zinc oxide of 99.5 % purity having a composition ratio of zinc and oxide being 56:44 (corresponding to Zn of 1.12 times the stoichiometric composition), and a first transparent oxide 504 is deposited by the DC magnetron sputtering method. A target 411 is zinc oxide (Zn:O = 50:50) of 99.5 % purity, and a second transparent oxide 505 is successively deposited by the DC magnetron sputtering method. The target 411 includes three targets from the relation of deposition rate and desired film thickness. The back reflector layer was formed using this apparatus. Setting the feed speed of sheet at 20 cm/min, the substrate temperature upon deposition of the transparent layer was adjusted to 100 °C, using only the substrate heater 408. Gases of H 2 O and Ar were introduced from gas introducing means not shown to achieve a ratio of H 2 O:Ar = 1:1 and the pressure of 3.0 mTorr, and a DC voltage of 500 V was applied to each cathode. A current of 6 A flowed through the target 409, and a current of 4 A flowed through each of the targets 410, 411. Checking the sheet wound, the thickness of Ag was 60 nm, the thickness of the first and second transparent oxides 503 was 800 nm in average and the surface of the transparent oxides was slightly opaque. It was checked by the scanning probe microscope that the thickness of the first transparent oxide 504 was 10 nm. On the transparent oxide, a bottom cell 506 with the pin semiconductor junction and a top cell 510 with the pin semiconductor junction were successively formed using a roll-to-roll type film-forming apparatus similar to the one as described in United States Patent No. 4,492,181. Here, numerals 507 and 511 are a-Si layers with the n-type conductivity, 509 and 513 µc-Si layers with the p-type conductivity, 508 is anti-type a-SiGe layer, and 512 an i-type a-Si layer. Subsequently, a transparent electrode 514 was deposited by the sputtering apparatus similar to the one shown in Fig. 4. The edges of the transparent electrode were subjected to patterning in order to prevent short-circuit in the subsequent steps. Further, a collector electrode 515 was formed, and thereafter the sheet 402 was cut to obtain the substrate 501. The mass-production effect was achieved by successive processing of the entire steps in this manner. A hundred samples were fabricated by this method and were subjected to evaluation of characteristics under light irradiation of AM 1.5 (100 mW/cm 2 ). Photoelectric conversion efficiencies obtained were 11.0 ± 0.3 %, and excellent conversion efficiencies were obtained with good reproducibility. After these solar cells were left under the environment of temperature 50 °C and humidity 90 % for 1000 hours, the conversion efficiencies showed little degradation as 10.4 ± 0.6 %. Another hundred samples were fabricated in the same manner by this method, and were subjected to irradiation for 600 hours with light equivalent to AM 1.5 in an open state. Conversion efficiencies obtained showed little degradation due to the light as 10.1 ± 0.4 %. As described above, the solar cell with high conversion efficiency and high reliability was achieved by the effect of the back reflector layer of the present invention. Example 2 Ten samples were produced in the same manner as in Example 1 except that O 2 was further introduced at 2 sccm upon deposition of the transparent oxide. The solar cells were subjected to evaluation of characteristics under light irradiation of AM 1.5 (100 mW/cm 2 ), and excellent conversion efficiencies were obtained as 11.5 % ± 0.3 %. The fill factors were mainly improved. Observing the back reflector layers of the samples by SEM, the edges of the texture of transparent layer were smoothed. This seems to decrease the recombination between electrons and holes, likely to occur at edge portions. Example 3 A back reflector layer was formed by the same method as in Example 1. Then Cu was deposited in 0.2 µm by the sputtering method on the substrate on which the back reflector layer was formed, and indium (In) was deposited thereon in 0.4 µm. Then this sample was transferred into a quartz glass bell jar, and hydrogen selenide (H 2 Se) diluted into 10 % with hydrogen was introduced into the bell jar with heating at 400 °C, thereby forming a thin film of CuInSe 2 (CIS). On this film a layer of CdS was deposited in 0.1 µm again by the sputtering method, and thereafter the resultant was annealed at 250 °C to form the pn junction. On this junction a transparent electrode and a collector electrode were formed in the same manner as in Example 1. This solar cell was subjected to evaluation of characteristics under light irradiation of AM 1.5 (100 mW/cm 2 ), and an excellent conversion efficiency was obtained as 9.3 % of conversion efficiency. This confirmed that the fabrication process of the present invention was also effective to the thin-film semiconductors other than a-Si. As detailed above, the present invention can prevent a reduction of reflectivity due to oxidation of the reflector metal layer, and thus, can obtain the transparent oxide having the texture structure effective to an increase in conversion efficiency. Also, the metal atoms become unlikely to diffuse into the semiconductor film, and an appropriate electric resistance suppresses a leak current even with a partially short-circuited portion in the semiconductor, thus achieving the photovoltaic element with high reliability. Further, the back reflector layer of this type can be fabricated in a part of the method suitable for mass production, such as the roll-to-roll method. Thus, the present invention will contribute greatly to widespread use of the solar-electric power generation. An object of the present invention is to provide a back reflector layer in a texture structure with a high reflectivity by preventing oxidation of a ground metal or alloy, and a method for forming it. A further object is to provide a photovoltaic element excellent in characteristics such as a conversion efficiency, and a process for fabricating it. The back reflector layer has a metal or alloy (hereinafter referred to as a first metal), and a transparent oxide of a second metal, formed on the first metal. A photovoltaic element is formed by forming a semiconductor junction on the back reflector layer. In the back reflector layer and the photovoltaic element, an electron affinity of the second metal is 0.46 or more eV smaller than that of the first metal, and the transparent oxide is formed by a sputtering method in an atmosphere comprising at least H 2 O and an inert gas and with a target having a composition ratio of the second metal being 1.06 to 1.2 times a stoichiometric composition thereof.

Claims: A method for forming a back reflector layer for a solar cells comprising forming on a metal or alloy as a first metal (102) a transparent oxide (103) comprising a second metal, wherein the difference of electron affinities between both metals is not less than 0.46 eV, wherein said transparent oxide is formed in an asperity structure by a sputtering method in an atmosphere comprising at least H 2 O and an inert gas, characterized in that the target in the sputtering process has a stoichiometric composition ratio of the second metal to the oxide being 1.06 to 1.2. The method for forming a back reflector layer according to Claim 1, wherein a film thickness of said transparent oxide is 5 nm to 20 nm. The method for forming a back reflector layer according to Claim 1, further comprising, after formation of said transparent oxide, forming another transparent oxide by the sputtering method using a target having a composition ratio of said second metal being 0.96 to 1.04 times the stoichiometric composition. The method for forming a back reflector layer according to Claim 1, wherein an oxygen gas is added into said atmosphere. A process for fabricating a photovoltaic element, further comprising forming a semiconductor junction after formation of the back reflector layer as set forth in either one of Claims 1 to 4.

Inventors: NAKAYAMA AKIYA, NAKAYAMA, AKIYA
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1995
Main classification: H01L31
IPC codes: H01L31