JP2013038367A - Thin film solar cell and manufacturing method of the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the productivity of a thin film solar cell with high conversion efficiency.
In one embodiment of the present invention, a back electrode (substrate-side electrode) 110, a semiconductor layer 120 for photoelectric conversion, a low-power transparent conductive film 132, and a high-power transparent conductive film 134 are provided on a substrate 100. A thin film solar cell 1000 provided in this order is provided. The low power transparent conductive film is formed by sputtering with a small plasma power below a certain reference value, and is placed in contact with the semiconductor layer. On the other hand, the high power transparent conductive film is formed by a sputtering method with a plasma power larger than the reference value, and is placed above the surface of the low power transparent conductive film. In this invention, the manufacturing method of this solar cell 1000 and another type of thin film solar cell are also provided.
[Selection] Figure 1

Description

  The present invention relates to a thin film solar cell and a method for manufacturing the same. More specifically, the present invention relates to a thin film solar cell that employs a transparent conductive film formed by a sputtering method and a method for manufacturing the same.

  Conventionally, a thin film solar cell has attracted attention as a solar cell with a small environmental load for production. Among thin-film solar cells, thin-film solar cells that employ non-single-crystal, that is, amorphous or microcrystalline silicon-based semiconductors as the semiconductor layer for photoelectric conversion are transparent such as tin-doped indium oxide (ITO) and zinc oxide (ZnO). A conductive film is employed. The transparent conductive film is disposed so as to be in contact with the semiconductor layer on at least one or both of the light incident surface, that is, the front surface and the reverse back surface. When these thin film solar cells are produced industrially, a sputtering method is often employed as a film forming method for forming a transparent conductive film from the viewpoint of production efficiency.

  Patent Document 1 (Japanese Patent Laid-Open No. 7-18431) discloses a technique of increasing the bias input power at the initial stage of film formation and decreasing it at the end of film formation by sputtering.

Japanese Unexamined Patent Publication No. 7-18431

  In the sputtering method for forming a transparent conductive film in a conventional thin film silicon solar cell, it is desirable to employ a film formation rate as high as possible from the viewpoint of production efficiency. However, the inventor of the present application has noticed that there is a problem that the photoelectric conversion characteristics of a thin film solar cell deteriorate in a thin film solar cell having a transparent conductive film formed at a high film formation rate.

  An object of the present invention is to solve at least some of the problems. The present invention contributes to increasing the mass productivity of a thin film solar cell with high conversion efficiency by providing a method of forming a transparent conductive film at a high film formation rate while keeping the characteristics of the semiconductor layer good. is there.

  More specifically, the inventor of the present application, in the case of forming a transparent conductive film under the conditions of a high film formation rate by a sputtering method, especially the plasma power adjusted to increase the film formation rate of sputtering, and finally It has been found that there is a relationship between the characteristics of thin film solar cells. That is, in order to obtain a high film formation rate, when the transparent conductive film at a position in contact with the semiconductor layer is increased by increasing the plasma power, the thin film solar cell is compared with the case where the transparent conductive film is formed with a weaker plasma power. Characteristics are degraded. However, if a transparent conductive film is formed with a small plasma power, a long time is required for processing.

  The inventor of the present application, as a cause of the above-described deterioration in characteristics, is that when the transparent conductive film is formed, the semiconductor layer already formed at that time is damaged by the plasma for sputtering of the transparent conductive film. I guessed. Based on this assumption, it was predicted that it would be useful to deposit a transparent conductive film in a position that affects the characteristics of the solar cell, that is, a range that affects the semiconductor layer, with weak plasma power. Furthermore, after the transparent conductive film at the position in contact with the semiconductor layer is formed to have a certain thickness or more, the transparent conductive film formed at that time is expected to play a role in protecting the semiconductor layer from plasma. it can. In short, it may be that the plasma power is weakened until a certain timing to form a transparent conductive film and the plasma power is increased after that timing does not affect the characteristics of the thin film solar cell. The present invention was created based on such an idea and further experimentally confirming the idea.

  That is, in one embodiment of the present invention, a substrate-side electrode formed on a substrate, a semiconductor layer for photoelectric conversion, and a sputtering method with a small plasma power below a certain reference value are formed and are in contact with the semiconductor layer. A low-power transparent conductive film placed on the substrate and a high-power transparent conductive film formed by sputtering with a plasma power larger than the reference value and placed above the surface of the low-power transparent conductive film. A thin-film solar cell provided in order is provided.

  In one embodiment of the present invention, a low power is achieved by forming a substrate-side electrode on a substrate, forming a semiconductor layer for photoelectric conversion, and a small plasma power below a certain reference value. A low power sputtering step in which a transparent conductive film is formed in contact with the semiconductor layer, and a high power sputtering step in which a high power transparent conductive film is formed above the surface of the low power transparent conductive film by plasma power greater than the reference value. Is provided in this order.

  In each aspect of the present invention described above, the sputtering power for forming the low-power transparent conductive film, that is, the low plasma power in the low-power sputtering process is controlled to be a certain value (reference value) or less. On the other hand, the sputtering power for forming the high power transparent conductive film, that is, the high plasma power in the high power sputtering process is set to a power value larger than the reference value for the low power transparent conductive film. And a high power transparent conductive film is arrange | positioned above the surface of a low power transparent conductive film, or the low power sputtering process and the high power sputtering process are included in this order. The reference value in each aspect of the present invention is determined by various factors such as a film forming apparatus to be used, conditions for film formation, and a film formation area. For this reason, it is not essential to express this reference value by a specific value. Rather, the reference value is between a low plasma power value for a low power transparent conductive film or low power sputtering process and a high plasma power value for a high power transparent conductive film or high power sputtering process. It only stipulates that there is a standard for relative control. Furthermore, the plasma power during the low-power transparent conductive film or the low-power sputtering process may be variously controlled within the range below the reference value. Similarly, the plasma power during the high-power transparent conductive film and the high-power sputtering process may be variously controlled within a range exceeding the reference value. Therefore, this reference value does not necessarily need to be determined in advance, and the maximum value of the plasma power at the time of the low power transparent conductive film and the low power sputtering process is specified afterwards as the reference value. be able to.

  Further, in the formed transparent conductive film, the high power transparent conductive film may be in contact with the low power transparent conductive film. As another example, after forming the low power transparent conductive film, the plasma power is gradually increased, and then the high power transparent conductive film is formed. A transparent conductive film formed by an intermediate plasma power between them may be sandwiched therebetween. In addition, when switching from a low power sputtering process for forming a low power transparent conductive film to a high power sputtering process for forming a high power transparent conductive film, it is necessary to change or interrupt conditions other than plasma power. Absent. In the switching, for example, it is not always necessary to change the conditions such as extinguishing the plasma, changing the target, changing the gas condition or the degree of vacuum, or transferring to another deposition chamber. Most typically, the low power sputtering process and the high power sputtering process are performed in the same manufacturing apparatus using the same sputtering target, under the same gas conditions and degree of vacuum. Is done.

  According to any aspect of the present invention, a thin film solar cell with high conversion efficiency can be produced by a process for forming a transparent conductive film with an increased effective film formation rate.

It is a schematic sectional drawing which shows the structure of the principal part of the substrate type thin film silicon solar cell of embodiment with this invention. It is a schematic sectional drawing which shows the structure of the principal part of the super straight type thin film silicon solar cell of embodiment with this invention. It is a flowchart which shows the preparation procedures of the thin film solar cell of an example of this embodiment in this embodiment. It is a block diagram which shows schematic structure of the manufacturing apparatus which processes the one part process which manufactures the thin film solar cell in embodiment with this invention. It is a block diagram which expands and shows the part of the manufacturing apparatus with which the semiconductor layer formation process in one embodiment of this invention is performed. It is a block diagram which expands and shows the part of the manufacturing apparatus with which the low power sputtering process and high power sputtering process in one embodiment of this invention are performed. FIG. 7 according to an embodiment of the present invention is a graph schematically illustrating how power is switched between low power and high power.

  Hereinafter, embodiments of the present invention will be described. In the following description, unless otherwise specified, common parts or elements are denoted by common reference numerals throughout the drawings. In the drawings, each element of each embodiment is not necessarily shown in a scale ratio.

<First Embodiment>
[1 Thin-film solar cell structure]
The thin film solar cell of this embodiment can be applied to a substrate type and a super straight type thin film silicon solar cell. Hereinafter, a typical structure of the thin film solar cell in the present embodiment will be described by taking a thin film silicon solar cell (substrate type, super straight type) as an example.

[1-1 Thin-film silicon solar cell]
[1-1-1 Substrate type]
FIG. 1 is a schematic cross-sectional view showing the structure of the main part of a substrate type thin film silicon solar cell. The thin film silicon solar cell 1000 includes, for example, a back electrode 110 that is a substrate side electrode formed on a substrate 100 that is a glass substrate or a flexible substrate, and a silicon-based semiconductor laminate 120 for photoelectric conversion (hereinafter, “ And a front electrode 130 are formed from the substrate 100 in this order. The front electrode 130 is made of a transparent conductive film, and a low-power transparent conductive film 132 disposed in contact with the Si-based laminate 120 and a high-power transparent conductive film formed on or above the surface of the low-power transparent conductive film 132. And a membrane 134. The low power transparent conductive film 132 is formed by a sputtering method with a small plasma power less than or equal to a certain reference value, whereas the high power transparent conductive film 134 is formed by a sputtering method with a plasma power greater than the reference value. The

  The Si-based stacked body 120 includes, for example, an n-type non-single-crystal silicon-based film 122 (first non-single-crystal silicon-based film, hereinafter referred to as “n-type Si-based film 122”) and an i-type non-single semiconductor that is an intrinsic semiconductor. Crystalline silicon-based film 124 (second non-single-crystal silicon-based film, “i-type Si-based film 124”) and p-type non-single-crystal silicon-based film 126 (third non-single-crystal silicon-based film, “p-type Si-based film”) The film 126 ") is a silicon-based semiconductor laminate in which the back electrode 110 is formed in this order. The low power transparent conductive film 132 is disposed in contact with the p-type Si-based film 126. The n-type Si-based film 122, the i-type Si-based film 124, and the p-type Si-based film 126 can all employ amorphous, microcrystalline, or polycrystalline silicon-based materials. As the p-type Si-based film 126, various silicon-based materials such as amorphous silicon, microcrystalline silicon, amorphous silicon carbide (a-SiC), and amorphous silicon germanium (a-SiGe) can be used.

Therefore, sunlight hν for power generation is incident on the Si-based laminate 120 from the side of the p-type Si-based film 126 located above the paper surface through the high-power transparent conductive film 134 and the low-power transparent conductive film 132. To do. As a whole, the thin film silicon solar cell 1000 functions as a substrate type thin film solar cell.

  In the thin film silicon solar cell 1000, a typical low power transparent conductive film 132 and a high power transparent conductive film 134 are both made of ITO (Indium Tin Oxide). Here, the interface between the low power transparent conductive film 132 and the high power transparent conductive film 134 (illustrated by a chain line) does not necessarily form a clear interface or boundary.

[1-1-2 Super straight type]
FIG. 2 is a schematic cross-sectional view showing the structure of the main part of a super straight type thin film silicon solar cell. The thin-film silicon solar cell 2000 includes a front electrode 210 that is a substrate-side electrode formed on a substrate 200 that is a glass substrate, for example, and a silicon-based semiconductor stacked body 220 (“Si-based stacked body 220”) for photoelectric conversion. The back electrode 230 is formed from the substrate 200 in this order. The back electrode 230 includes a transparent conductive film, and a low power transparent conductive film 232 disposed in contact with the Si-based laminate 220 and a high power formed on or above the surface of the low power transparent conductive film 232. A transparent conductive film 234. The back electrode 230 further includes a metal reflective electrode layer 236. The low power transparent conductive film 232 is formed by a sputtering method with a small plasma power that is less than or equal to a certain reference value, whereas the high power transparent conductive film 234 is formed by a sputtering method with a plasma power that is greater than the reference value. The

  The Si-based stacked body 220 in the thin-film silicon solar cell 2000 includes, for example, a p-type non-single-crystal silicon-based film 222 (first non-single-crystal silicon-based film, “p-type Si-based film 222”) and i that is an intrinsic semiconductor. Type non-single crystal silicon film 224 (second non-single crystal silicon film, “i-type Si film 224”) and n-type non-single crystal silicon film 226 (third non-single crystal silicon film, “n A silicon-based semiconductor laminate in which a Si-type film 226 ”) is formed in this order from the front electrode 210. The low power transparent conductive film 232 is disposed in contact with the n-type Si-based film 226.

  The front electrode 210 is formed of a suitable transparent conductive film. Further, the substrate 200 is also light transmissive. Therefore, sunlight hν is incident from the p-type Si-based film 222 side through the Si-based stacked body 220, the substrate 200, and the front electrode 210. As a whole, the thin film silicon solar cell 2000 functions as a super straight type thin film solar cell.

  In the thin film silicon solar cell 2000, a typical low power transparent conductive film 232 and a high power transparent conductive film 234 are both made of ZnO (zinc oxide). Also in the thin-film silicon solar cell 2000, the interface (shown by a chain line) between the low power transparent conductive film 232 and the high power transparent conductive film 234 is not necessarily a clear interface or boundary.

[1-2 Other configurations]
The thin film solar cell of this embodiment includes thin film solar cells having various structures such as a multi-junction type in addition to the above-described thin film silicon solar cells 1000 and 2000. Regardless of the structure, the thin film solar cell of this embodiment includes a substrate-side electrode formed on a substrate, a semiconductor layer for photoelectric conversion, a low-power transparent conductive film, and a high-power transparent conductive film. Are arranged in this order from the substrate, and a low-power transparent conductive film is formed by a sputtering method with a small plasma power below a certain reference value, and is placed in contact with the semiconductor layer. As long as the high power transparent conductive film is formed by sputtering with a plasma power larger than the reference value and is placed on or above the surface of the low power transparent conductive film, the thin film solar cell of this embodiment It can be.

  Moreover, the low-power transparent conductive films of the thin-film silicon solar cells 1000 and 2000 described above are formed on the surface of the already formed semiconductor layer (Si-based stacked body 120, Si-based stacked body 220) at the time of forming the film. It arrange | positions in the position which contact | connects, and is arrange | positioned in the position pinched | interposed by a semiconductor layer and a high power transparent conductive film. Furthermore, the low power transparent conductive film and the high power transparent conductive film are preferably made of the same material. The material of the low power transparent conductive film and the high power transparent conductive film is particularly selected from oxides. In the low-power transparent conductive film and the high-power transparent conductive film, “transparent” means that the semiconductor layer has transparency in at least any wavelength region having sensitivity, and “Conductivity” may be high in electrical resistance. In addition, the low-power transparent conductive film and the high-power transparent conductive film, for example, as a back electrode 230, have a function as a film of a light-transmitting conductor, for example, a Si-based laminate 220 during long-term operation. May have an effect of preventing the metal reflective electrode layer 236 from being affected.

[2 Thin-film solar cell manufacturing process]
Next, an example of the manufacturing process of the thin film solar cell provided in this embodiment will be described. Regarding the thin film silicon solar cell 1000, a case where the substrate 100 is a flexible substrate will be described as an example, and the thin film silicon solar cell 2000 will be described as another example.

  FIG. 3 is a flowchart showing a procedure for manufacturing some examples of the thin-film solar battery of this embodiment. FIG. 4 is a configuration diagram showing a schematic structure of a manufacturing apparatus that processes a part of the process for manufacturing the thin-film solar cell of an example of the present embodiment. Further, FIGS. 5 and 6 are configuration diagrams showing, in an enlarged manner, parts of the manufacturing apparatus in which the semiconductor layer forming process, the low power sputtering process, and the high power sputtering process are performed, respectively.

  FIG. 3 shows a method for manufacturing a thin-film silicon solar cell 1000 which is an example of the thin-film solar cell of the present embodiment. The manufacturing method includes a substrate-side electrode forming step S120, a semiconductor layer forming step S140, a low power sputtering step S160, and a high power sputtering step S180 in this order. Among these, the semiconductor layer forming step S140 is a step of forming a semiconductor layer for photoelectric conversion. The low power sputtering step S160 is a step of forming a low power transparent conductive film at a position in contact with the semiconductor layer. The high power sputtering step S180 is a step of forming the high power transparent conductive film on or above the surface of the low power transparent conductive film. The low power sputtering step S160 is a process of forming the low power transparent conductive film 132 with a small plasma power equal to or less than a certain reference power. On the other hand, the high power sputtering step S180 is a process for forming the high power transparent conductive film 134 with a plasma power larger than the power of the reference value.

[2-1 Substrate thin film solar cell manufacturing process]
When the thin-film silicon solar cell 1000 of FIG. 1 is manufactured, the first conductive type first non-single crystal silicon-based film forming step S142 of the semiconductor layer forming step S140 is a step of forming the n-type Si-based film 122. . Similarly, the second non-single-crystal silicon based film forming step S144 of the intrinsic semiconductor is a step of forming the i-type Si-based film 124, and the third non-single-crystal silicon based film forming step S146 is a p-type Si. This is a step of forming the system film 126. These steps S142 to S146 are executed in this order. The low power sputtering step S160 is performed after the third non-single crystal silicon film forming step S146.

[2-1-1 Manufacturing Process of Substrate Thin Film Solar Cell]
The exemplary structure and formation process of the thin-film silicon solar cell 1000 will be described in detail with reference to FIGS. As the substrate 100 when the thin film silicon solar cell 1000 is manufactured, a flexible substrate, for example, a resin substrate can be adopted. Most typically, the substrate 100 can be a polyimide film. An Ag / ZnO back electrode film is formed as the back electrode 110 on one surface of the substrate 100. In FIG. 1, the back electrode 110 is shown as a single layer. In the substrate-side electrode forming step S120 for forming the back electrode 110, for example, a DC sputtering method is employed. Then, before performing the semiconductor layer forming step S140, a step of forming the Si-based laminate 120, which is a Si film, heat treatment is performed in vacuum for degassing (not shown). The substrate 100 in the present embodiment is a long manufacturing substrate 100S in the course of manufacturing, and is wound up as needed to be rolled again.

  The manufacturing substrate 100S on which the back electrode 110 is formed and the heat treatment is completed is then mounted on a manufacturing apparatus 5000 shown in FIG. The manufacturing apparatus 5000 includes an unwinding chamber 502 and a winding chamber 504, a semiconductor layer forming chamber 510 disposed between the unwinding chamber 502 and the winding chamber 504, and a TCO chamber 520. The semiconductor layer forming chamber 510 has an N chamber 512, an I chamber 514, and a P chamber 516 in this order from the winding chamber 504 side. The production substrate 100 </ b> S wound on the roll is first loaded into the unwind chamber 502. At this time, most of the manufacturing substrate 100S is wound around the unwinding roll 502R. Similarly, the manufacturing substrate 100S that has been processed is finally accommodated in the winding chamber 504 and wound on the winding roll 504R. In general, the transfer of the manufacturing substrate 100S between the unwinding chamber 502 and the winding chamber 504 is intermittent. That is, the period during which the manufacturing substrate 100S is moved for transportation and the period during which the 100S is stationary for film formation can be repeatedly received in terms of time. In this way, the manufacturing substrate 100S is sequentially processed as needed from one end to the other end in the long longitudinal direction.

  The Si-based stacked body 120 is formed by plasma CVD using the semiconductor layer forming chamber 510. As shown in FIG. 11, a film formation process is performed in the N chamber 512, the I chamber 514, and the P chamber 516 in the order of the n-type Si-based film 122, the i-type Si-based film 124, and the p-type Si-based film 126. To do. Next, in the TCO chamber 520, the front electrode 130 is formed by a sputtering method.

  Each film formation chamber of the N chamber 512, the I chamber 514, and the P chamber 516 for forming the Si-based laminate 120 incorporates a film formation chamber 600 having a structure shown in FIG. The film formation chamber 600 includes an exhaust system (not shown) for exhausting to a high vacuum suitable for film formation, rather than the degree of vacuum inside the manufacturing apparatus 5000. The film forming chamber 600 includes a cathode electrode 602 and an anode electrode 604 inside. The cathode electrode 602 and the anode electrode 604 are disposed so as to form a pair of parallel plates. Further, high-frequency power can be applied between the cathode electrode 602 and the anode electrode 604. That is, for example, the high-frequency power source 612 is connected to the cathode electrode 602 via the matching unit 614, and the anode electrode 604 is grounded. The manufacturing substrate 100S is disposed in the gap between the cathode electrode 602 and the anode electrode 604, the film formation surface faces the cathode electrode 602, and the surface opposite to the film formation surface is in contact with the anode electrode 604. A large number of holes are provided on the surface of the cathode electrode 602 facing the manufacturing substrate 100S, and the source gas sent from the outside of the film formation chamber 600 to the inside of the cathode electrode 602 through an introduction pipe (not shown) Then, the light is emitted from the cathode electrode 602 toward the film formation surface of the manufacturing substrate 100S. With this structure, consideration is given to equalizing the gas flow reaching the film formation surface of the manufacturing substrate 100S. During the film forming process, the flow rate of the source gas is kept constant by a gas flow rate controller (not shown). The cathode electrode 602 is provided with a heater (not shown) so that the anode electrode 604 and the manufacturing substrate 100S can be heated to a predetermined temperature. The exhaust system (not shown) of the film forming chamber 600 is provided with a control device (not shown) that keeps the pressure (degree of vacuum) inside the film forming chamber 600 constant.

When performing the film forming step (S140) of the Si-based laminate 120, the internal pressure of the film forming chamber 600 is adjusted to a predetermined value while flowing a source gas such as SiH ₄ and hydrogen or a dilution gas at a constant flow rate. To do. At this time, the high frequency power source 612 outputs high frequency power to the cathode electrode 602 to excite the plasma. The matching unit 614 is adjusted so that the excitation of the plasma by this discharge is stabilized. For example, SiH ₄ and hydrogen are used as the source gas and dilution gas for forming the n-type and p-type non-single-crystal silicon-based films in the N chamber 512 and the P chamber 516, and impurity doping is performed there. It is assumed that phosphine gas or diborane gas is added. In each of the film formation chambers 600 in the N chamber 512, the I chamber 514, and the P chamber 516, Steps S142 to S146 are performed, respectively. When step S142 to step S146 are completed, an n-type Si-based film 122, an i-type Si-based film 124, and a p-type Si-based film 126 are formed. Thereafter, the part of the manufacturing substrate 100 </ b> S is sent to the TCO chamber 520.

In the TCO chamber 520, the front electrode 130 is formed by DC sputtering. The front electrode 130 is, for example, ITO. The TCO chamber 520 contains a film forming chamber 700 having a structure shown in FIG. Similarly to the film formation chamber 600 (FIG. 5), the film formation chamber 700 also maintains an exhaust system for exhausting to a high vacuum, a gas supply system for quantitatively supplying the sputtering gas, and the film formation chamber 700 at a constant pressure. A control system (both not shown) is provided. The film forming chamber 700 includes a cathode electrode 702 and an anode electrode 704 disposed so as to be parallel to each other. For example, a DC power source 712 is connected to the cathode electrode 702 in such a direction as to make the cathode electrode 702 a negative potential so that DC power can be applied between the cathode electrode 702 and the anode electrode 704. In the cathode electrode 702, a target 720 for the front electrode 130 is disposed on the surface facing the anode electrode 704. The manufacturing substrate 100S is disposed in the gap between the target 720 and the anode electrode 704 with the film formation surface facing the target 720 and the surface opposite to the film formation surface in contact with the anode electrode 704. A permanent magnet (not shown) for magnetron sputtering is disposed on the back surface of the cathode 702 as viewed from the target 720. The sputtering gas is, for example, Ar—O ₂ gas. The anode electrode 704 includes a heater (not shown), and can heat the anode electrode 704 and the manufacturing substrate 100S.

  In the film formation chamber 700, while supplying a sputtering gas at a constant flow rate for the film formation process (process S160 and process S180) of the low power transparent conductive film 132 and the high power transparent conductive film 134 that form the front electrode 130, Plasma is excited by a DC power source 712. The output of the DC power source 712 is controlled to be below a certain reference value in the low power sputtering step S160, and is controlled to exceed the reference value in the high power sputtering step S180. Both the low power sputtering step S160 and the high power sputtering step S180 are performed in the same film forming chamber 700. For such control, for example, a variable DC power supply capable of adjusting plasma power by changing voltage output or current output (hereinafter simply referred to as output power) is adopted as the DC power supply 712. It is also preferable that a controller 714 for controlling the DC power supply 712 is connected to the DC power supply 712. The part of the manufacturing substrate 100S on which the low-power transparent conductive film 132 and the high-power transparent conductive film 134 are formed is sent to the winding chamber 504 and wound on the winding roll 504R.

[2-2 Super straight thin film solar cell manufacturing process]
When the thin-film silicon solar cell 2000 of FIG. 2 is manufactured, the first non-single-crystal silicon-based film forming step S142 of the semiconductor layer forming step S140 is a step of forming the p-type Si-based film 222. Similarly, the formation process S144 of the second non-single crystal silicon film is a process of forming the i-type Si film 224, and the formation process S146 of the third non-single crystal silicon film is the n-type Si film 226. Is a step of forming. These steps S142 to S146 are executed in this order. The low power sputtering step S160 is performed after the third non-single crystal silicon film forming step S146.

[3 Experimental example]
Next, experimental examples based on samples in which the thin-film silicon solar cell 1000 of the present embodiment is actually manufactured will be described as several examples, and related comparative examples and preliminary experimental examples. The materials, amounts used, ratios, processing details, processing procedures, and the like shown in the following examples and the like can be changed as appropriate without departing from the spirit of the present invention. Therefore, the scope of the present invention is not limited to the following specific examples. Further, the drawings described so far are referred to as appropriate.

First, conditions and manufacturing steps common to the samples of Comparative Examples 1 to 3, Preliminary Experimental Examples 1 to 5, and Examples 1 to 3 will be described. In each sample, the structure of the thin film solar cell was substantially the same as that of the thin film silicon solar cell 1000 of FIG. Specifically, in the thin film solar cell of each example, the n-type Si-based film 122, the i-type Si-based film 124, and the p-type Si-based film 126 are located at a non-monocrystalline silicon layer (μc-Si). A crystalline silicon-based thin film solar cell (hereinafter referred to as “μc-Si solar cell”) was produced. That is, the μc-Si n-type Si-based film 122, i-type Si-based film 124, and p-type Si-based film 126 (hereinafter referred to as “n” respectively) are formed on the surface of the manufacturing substrate 100S on which the back electrode 110 is formed. Layer ”,“ i layer ”, and“ p layer ”). For this film formation, a plasma CVD method using the manufacturing apparatus 5000 shown in FIGS. 4 and 5 and the film formation chamber 600 incorporated therein is executed. The thicknesses of the n-layer, i-layer, and p-layer of each prepared sample were 30 nm, 2500 nm, and 30 nm (all converted values from the film formation rate). In Comparative Example 1 and Preliminary Experimental Examples 1 to 5, the front electrode 130 was formed with a constant output without changing the output power from the DC power supply 712 over time. In the samples of each example, the thickness of the front electrode 130 or the transparent conductive film corresponding to the front electrode 130 was unified to the same 70 nm. The ITO film was formed by the manufacturing apparatus 5000 and the film forming chamber 700 shown in FIGS. The conditions for forming the ITO film at that time were Ar-O ₂ (1.2%), gas flow rate 10 sccm, pressure 3 mtorr (about 0.4 Pa) in all examples, and the substrate temperature (anode electrode 704) at the time of film formation. The control temperature was 200 ° C. Hereinafter, conditions that are different among the samples of each example will be described.

[3-1 Comparative example (part)]
As Comparative Example 1, an ITO film (thickness: 70 nm) was laminated as a transparent conductive film corresponding to the front electrode 130 on the surface of the Si-based laminate 120 by a direct current sputtering method to prepare Sample # 1-1. In sample # 1-1, the output power of the DC power supply 712 was constant 140 W throughout. The conversion efficiency of Comparative Example 1 (Sample # 1-1) of the solar cell thus fabricated was measured and found to be 7.7%. The time required for forming the ITO film at a thickness of 70 nm at 140 W was 200 seconds.

[3-2 Preliminary experiment example]
Next, as Preliminary Experimental Examples 1 to 5, solar cells having the same structure as that of Comparative Example 1 were fabricated using several values of output power from the DC power supply 712. In Preliminary Experimental Examples 1 to 5, Samples # 2-1 to # 2-5 in which the output power of the DC power supply 712 was constant throughout the film formation of the transparent conductive ITO film corresponding to the front electrode 130. Was made. However, in samples # 2-1 to # 2-5, the outputs of the DC power supply 712 when forming the transparent conductive ITO film corresponding to the front electrode 130 are 35 W, 50 W, 70 W, 120 W and 140 W, respectively. And changed. Of these, 140 W sample # 2-5 was prepared under the same conditions as sample # 1-1 in Comparative Example 1 above. Then, the conversion efficiencies of Samples # 2-1 to # 2-5 prepared as Preliminary Experimental Examples 1 to 5 were measured and the results shown in Table 1 were obtained.

表１に示す予備実験例１〜５の結果から、ＩＴＯ膜を形成するスパッタリング時の出力パワーを、比較例１（サンプル＃１−１）の１４０Ｗから弱めることにより、最終的な薄膜太陽電池の変換効率が増加することを確認した。さらに、その出力パワーを７０Ｗ以下にすることにより、変換効率がほぼ飽和することを確認した。本願の発明者は、これらの実験事実、とりわけ表１に示す予備実験例１〜５（サンプル＃２−１〜＃２−５）の変換効率の値から、ＩＴＯ膜を低いプラズマ電力で成膜することが、薄膜太陽電池の変換効率を高める効果をもたらしていると判断した。なお、直流電源７１２の出力パワーを変化させた予備実験例１〜５（サンプル＃２−１〜＃２−５）において、成膜レートが直流電源７１２の出力パワーに比例していることも確認した。端的には、直流電源７１２の出力パワーを小さくし高い変換効率が得られるサンプルにおけるＩＴＯ膜の成膜時間は、それに応じて長くなった。なお、ＩＴＯ膜を３５Ｗにて７０ｎｍ厚に成膜するのに要する時間は８００秒であった。

From the results of Preliminary Experimental Examples 1 to 5 shown in Table 1, the final thin-film solar cell of the final thin-film solar cell can be obtained by weakening the output power during sputtering for forming the ITO film from 140 W of Comparative Example 1 (Sample # 1-1). It was confirmed that the conversion efficiency increased. Furthermore, it was confirmed that the conversion efficiency was almost saturated by setting the output power to 70 W or less. The inventor of the present application formed an ITO film with low plasma power based on these experimental facts, particularly the conversion efficiency values of Preliminary Experimental Examples 1 to 5 (Samples # 2-1 to # 2-5) shown in Table 1. It was determined that this has the effect of increasing the conversion efficiency of the thin film solar cell. In preliminary experiment examples 1 to 5 (samples # 2-1 to # 2-5) in which the output power of the DC power supply 712 was changed, it was also confirmed that the film formation rate was proportional to the output power of the DC power supply 712. did. In short, the film formation time of the ITO film in the sample in which the output power of the DC power supply 712 is reduced and high conversion efficiency is obtained has been increased accordingly. The time required for forming the ITO film at a thickness of 70 nm at 35 W was 800 seconds.

[3-3 Examples and Additional Comparative Examples]
Next, as Examples 1 to 3, samples # 3-3, # 3-5, and # 3-7 of thin film silicon solar cells having the structure of the thin film silicon solar cell 1000 were produced. The ITO film formed as the front electrode 130 is formed by changing the output power of the DC power source 712 during the film formation, and the two-stage film formation of the low power sputtering process S160 and the high power sputtering process S180 shown in FIG. It was. Specifically, in each of the samples of Examples 1 to 3, the low power transparent conductive film 132 was formed with a low power with the output power of the DC power supply 712 being 35 W as the low power sputtering step S160. Subsequently, switching to the high power sputtering step S180 was performed while continuing plasma excitation. In the high power sputtering step S180, the high power transparent conductive film 134 was formed at a high power with the same output power of 140W. Thus, the reference value of the output power in the low power sputtering step S160 was set to 35 W, and the output power of 140 W in the high power sputtering step S180 was set to be four times the reference value. FIG. 7 is a graph schematically illustrating how to switch the power at 35 W (low plasma power) and 140 W (high plasma power) employed in samples # 3-3, # 3-5, and # 3-7. is there. Thus, in Examples 1 to 3, the front electrode 130 in which the low power transparent conductive film 132 and the high power transparent conductive film 134 were formed in contact with each other as shown in FIG. 1 was formed.

  The inventors of the present application described the boundaries between the low-power transparent conductive film 132 and the high-power transparent conductive film 134 (FIG. 1) in Samples # 3-3, # 3-5, and # 3-7 produced as Examples 1-3. However, no clear boundary was found in attempts to confirm the above by several analytical means. However, in these samples, each layer of the low power transparent conductive film 132 and the high power transparent conductive film 134 as shown in FIG. 1 is formed as shown in FIG. The inventors of the present application are convinced that there is no particular change over time. One of the reasons is confirmed by a separate experiment that the film thicknesses of the low-power transparent conductive film 132 and the high-power transparent conductive film 134 increase according to the film-forming rates at output powers of 35 W and 140 W. Because.

Furthermore, in correspondence with Examples 1 and 2 (Samples # 3-3 and # 3-5), Comparative Examples 2 and 3 (Samples # 3-4 and # 3) in which the temporal order of low power and high power was reversed 3-6) was also produced. And conversion efficiency was measured similarly to the comparative example 1 and the preliminary experiment examples 1-5. Table 2 shows the conversion efficiency of each sample. The samples of Examples 1 to 3 are Sample # 3-3 (Example 1), # 3-5 (Example 2), Sample # 3-7 (Example 3), and Sample # 3-4 (Example 3). Comparative Example 2) and # 3-6 (Comparative Example 3) are comparative examples to be compared with Sample # 3-3 (Example 1) and Sample # 3-5 (Example 2), respectively.

  Table 2 also shows the power conditions when forming the ITO film of each sample, the film thickness of the ITO film formed with a low plasma power (35 W), and the film formation time of the ITO film. These film thicknesses are converted values from experiments for determining a separate film formation rate. The power conditions in Table 2 are delimited by slashes in the order of (former film formation conditions) / (subsequent film formation conditions) in the order of ITO film formation. For comparison, in Table 2, samples (samples # 3-1 and # 3-2) corresponding to the conditions of the comparative example 1 and the preliminary experimental example 5 described above are also shown again. As described above, the thickness of the ITO film formed in each sample is unified to be 70 nm in total. For this reason, the reciprocal of the film formation time in each sample is a relative index for an effective film formation rate.

  From the experimental examples shown in Table 2, the inventors of the present application have obtained the following knowledge. First, by comparing Sample # 3-1 (Comparative Example 1) and Sample # 3-2 (Preliminary Experimental Example 1), it was confirmed that the deposition rate of the ITO film was inversely proportional to the output of the DC power supply 712. . In addition, the lower the deposition rate, the higher the conversion efficiency. As described above, these are findings derived from Comparative Example 1 and Preliminary Experimental Examples 1 to 5. Next, according to the comparison between sample # 3-2 (preliminary experimental example 1) and sample # 3-3 (example 1), even if not all of the ITO film is formed with low plasma power, sample # 3 -2 (Preliminary Experimental Example 1), it was confirmed that it was possible to produce a thin film solar cell with high conversion efficiency. In Sample # 3-3 (Example 1), the film thickness of the low-power transparent conductive film 132 formed with a low plasma power of 35 W is about 2/3 of 70 nm, which is the film thickness of the front electrode 130.

  Here, the factor which obtained high conversion efficiency in sample # 3-3 (Example 1) was confirmed based on experiment. Specifically, in order to evaluate the influence that a low power transparent conductive film such as the low power transparent conductive film 132 is formed on a part of the film of the front electrode 130, sample # 3-3 (Example 1) is used. ) And Sample # 3-4 (Comparative Example 2). The ITO film in Sample # 3-4 (Comparative Example 2) is a position where the order of the low power transparent conductive film 132 and the high power transparent conductive film 134 of the front electrode 130 in Sample # 3-3 (Example 1) is just changed. It is formed to be in a relationship. As a result, in sample # 3-4 (comparative example 2), the conversion efficiency was somewhat lower than in sample # 3-3 (example 1). This experimental fact shows that the position of the ITO film formed with low plasma power affects the photoelectric conversion characteristics of the thin-film silicon solar cell 1000 in the front electrode 130 of Sample # 3-3 (Example 1). ing. If sample # 3-1 (Example 1) has improved conversion efficiency due to film properties such as the conductivity of the low power transparent conductive film 132 itself, it is formed with low plasma power. The relationship between the position of the ITO film and the conversion efficiency is unlikely to be clear. However, in the comparison between sample # 3-3 (Example 1) and sample # 3-4 (Comparative Example 2), a clear relationship was seen in the conversion efficiency, so the ITO film formed with low plasma power. Can be said to play an important role.

  At this time, the inventor of the present application converts the ITO film formed with low plasma power in contact with the Si-based laminate 120 in the experimental facts of Sample # 3-1 to Sample # 3-4. I noticed an increase in efficiency. Regarding the cause, the inventor of the present application considers that when the output power of the DC power supply 712 is reduced, the Si-based laminate 120 is less damaged by the sputtering process. The damage is, for example, ion bombardment with argon gas. Further, the low-power transparent conductive film 132 placed at a position in contact with the Si-based laminate 120 is used for sputtering for the high-power transparent conductive film 134 placed above the surface of the low-power transparent conductive film 132. The inventors of the present application speculated that it can be expected to play a role of protecting the Si-based laminate 120 from ion bombardment and the like.

  Therefore, in order to investigate how the effect of protecting the Si-based laminate 120 by the low-power transparent conductive film 132 has a relationship with the film thickness of the low-power transparent conductive film 132, and more time for the film formation process. In order to shorten the height and increase the effective film formation rate, the low power transparent conductive film 132 was made as thin as possible, and the possibility of increasing the film thickness of the high power transparent conductive film 134 was pursued. Specifically, in the sample # 3-3 (Example 1), the low power transparent conductive film 132 having a thickness of about 2/3 of the front electrode 130 and the high power transparent conductive film 134 of about 1/3 are employed. On the other hand, in Sample # 3-5 (Example 2), the film thickness of 132 is set to about 23 nm, which is about 1/3 of the front electrode 130, and the remaining 2/3 is made up of the high-power transparent conductive film 134. And tried to. As a result, it was confirmed that the sample # 3-5 (Example 2) still maintained the same high conversion efficiency as the sample # 3-3 (Example 1). Sample # 3-6 in which the order of the low-power transparent conductive film 132 and the high-power transparent conductive film 134 in Sample # 3-5 (Example 2) is reversed in order to confirm the knowledge of the inventor regarding damage. (Comparative Example 3) was produced. Also in this case, the phenomenon supporting the above knowledge that the conversion efficiency is lower than that of Sample # 3-5 (Example 2) was reproduced.

  Finally, Sample # 3-7 (Example 3) was produced in order to pursue the possibility of making the low power transparent conductive film 132 as thin as possible and increasing the thickness of the high power transparent conductive film 134. . And in sample # 3-7 (Example 3) which made the thickness of the low power transparent conductive film 132 6 nm, it confirmed that conversion efficiency was comparable with sample # 3-5 (Example 2). Thus, it was confirmed that the effect of protecting the Si-based laminate 120 by the low-power transparent conductive film 132 can be obtained if the low-power transparent conductive film 132 has a thickness of at least 6 nm.

  As described above, in the present embodiment, by providing the low power transparent conductive film 132 and the high power transparent conductive film 134, and by performing the low power sputtering step S160 and the high power sputtering step S180, It has been experimentally confirmed that a thin-film solar cell that realizes a short film forming time while maintaining high conversion characteristics is produced. Furthermore, it is confirmed that a sufficient effect can be expected even if the film thickness of the low power transparent conductive film 132 is less than 1/3 of the total film thickness of the low power transparent conductive film 132 and the high power transparent conductive film 134. did. Further, a particularly preferable condition for increasing the effective film formation rate of the front electrode 130 is that the thickness of the low power transparent conductive film 132 is 6 nm or more.

[4 Effects of low power sputtering in other types of thin film solar cells]
The same effects as those of the low power transparent conductive film 132 in the structure of the thin film silicon solar cell 1000 described above are also exhibited in the thin film silicon solar cell 2000 described with reference to FIG.

[4-1 Super straight type]
In the thin film silicon solar cell 2000 of FIG. 2, the low power transparent conductive film 232 is formed in contact with the surface of the Si-based laminate 220 formed on the substrate 200. Sputtering of the low power transparent conductive film 232 with low plasma power and the high power transparent conductive film 234 with higher plasma power reduces the effective film formation rate when forming the back electrode 230. It is possible to reduce damage to the Si-based laminate 220 while preventing the above.

[4-2 Other variations]
In the present embodiment, it is not excluded to add a member, a material, or a structural feature that is not described in any of the thin film silicon solar cells 1000 and 2000 described with reference to FIGS. 1 and 2. Should be noted. For example, the structure of the low power transparent conductive film 132 is useful also in a thin film solar cell having an integrated structure for series connection, for example. 1 and 2, the configuration having the single-junction silicon-based semiconductor stack 120 or 220 has been described. However, in a thin-film solar cell having a multi-junction silicon-based semiconductor stack, low-power transparent conductive By arranging the film in contact with the silicon-based semiconductor laminate, the same effects as in the present embodiment can be obtained.

  The embodiment of the present invention has been specifically described above. The above-described embodiments and examples are described for explaining the invention, and the scope of the invention of the present application should be determined based on the description of the claims. In addition, modifications that exist within the scope of the present invention including other combinations of the embodiments are also included in the scope of the claims.

  According to the present invention, it is possible to produce a high-performance thin film solar cell with high productivity.

1000 Thin-film silicon solar cell 100 Substrate 100S Manufacturing substrate 120 Silicon-based semiconductor laminate 110 Back electrode (substrate-side electrode)
122 n-type non-single crystal silicon film (n-type Si film, first non-single crystal silicon film)
124 i-type non-single crystal silicon film (i-type Si film, second non-single crystal silicon film)
126 p-type non-single crystal silicon film (p-type Si film, third non-single crystal silicon film)
130 Front electrode 132 Low power transparent conductive film 134 High power transparent conductive film 2000 Thin film silicon solar cell 200 Substrate 210 Front electrode (Substrate side electrode)
220 Silicon-based semiconductor laminate 222 p-type non-single-crystal silicon-based film (p-type Si-based film, first non-single-crystal silicon-based film)
224 i-type non-single crystal silicon film (i-type Si film, second non-single crystal silicon film)
226 n-type non-single crystal silicon film (n-type Si film, third non-single crystal silicon film)
230 Back electrode 232 Low power transparent conductive film 234 High power transparent conductive film 236 Metal reflective electrode layer 5000 Manufacturing apparatus 502 Unwinding chamber 504 Winding chamber 502R, 504R Roll 510 Semiconductor layer forming chamber 600, 700 Deposition chamber 602, 702 Cathode Electrode 604, 704 Anode electrode 612 High frequency power supply 614 Matching device 712 DC power supply 714 Controller 720 Target

Claims (12)

A substrate side electrode formed on the substrate;
A semiconductor layer for photoelectric conversion;
A low-power transparent conductive film formed by sputtering with a small plasma power below a certain reference value and placed in contact with the semiconductor layer;
A thin-film solar cell comprising: a high-power transparent conductive film formed by a sputtering method with a plasma power larger than the reference value, and placed above the surface of the low-power transparent conductive film in this order from the substrate. The semiconductor layer includes a first conductivity type first non-single crystal silicon film, an intrinsic semiconductor second non-single crystal silicon film, and a second conductivity type third non-single crystal silicon film. A silicon-based semiconductor laminate formed in this order from the side electrode,
The thin film solar cell according to claim 1, wherein the low-power transparent conductive film is in contact with the third non-single crystal silicon-based film. The first conductivity type and the second conductivity type are n-type and p-type, respectively;
The thin film solar cell according to claim 2, wherein light enters the silicon-based semiconductor laminate from the side of the third non-single-crystal silicon-based film through the high-power transparent conductive film and the low-power transparent conductive film. The substrate and the substrate side electrode have translucency,
The first conductivity type and the second conductivity type are p-type and n-type, respectively;
The thin film solar cell according to claim 2, wherein light enters the silicon-based semiconductor stacked body from the first non-single-crystal silicon-based film side through the substrate and the substrate-side electrode. 5. The thickness of the low-power transparent conductive film is 1/3 or less of the total thickness of the low-power transparent conductive film and the high-power transparent conductive film. The thin film solar cell described. The thin film solar cell according to any one of claims 1 to 4, wherein a thickness of the low power transparent conductive film is 6 nm or more. The thin-film solar cell according to any one of claims 1 to 4, wherein a value of plasma power when forming the high-power transparent conductive film is four times or more of the reference value. Forming a substrate-side electrode on the substrate;
A semiconductor layer forming step of forming a semiconductor layer for photoelectric conversion;
A low power sputtering step of forming a low power transparent conductive film in contact with the semiconductor layer with a small plasma power below a certain reference value;
A high power sputtering step of forming a high power transparent conductive film above the surface of the low power transparent conductive film with a plasma power greater than the reference value, in this order. The semiconductor layer forming step includes
Forming a first non-single-crystal silicon film of the first conductivity type;
Forming an intrinsic semiconductor second non-single-crystal silicon film;
Forming a third non-single crystal silicon-based film of the second conductivity type in this order,
The method for manufacturing a thin-film solar cell according to claim 8, wherein the low power sputtering step is performed after the step of forming the third non-single-crystal silicon-based film. The thin film solar cell according to claim 8 or 9, wherein a thickness of the low power transparent conductive film is 1/3 or less of a total thickness of the low power transparent conductive film and the high power transparent conductive film. Manufacturing method. The method for manufacturing a thin-film solar cell according to claim 8 or 9, wherein the low-power transparent conductive film has a thickness of 6 nm or more. The method of manufacturing a thin-film solar cell according to claim 8 or 9, wherein a value of plasma power in the high power sputtering step is four times or more of the reference value.

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