JP2010225884A - Method of manufacturing thin-film solar cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of manufacturing a thin-film solar cell for setting layers of compounds formed between a metal back electrode surface to which Mo is applied and a light absorption layer to an appropriate thickness and uniformly forming the layers of compounds in a CIGS-type solar cell. <P>SOLUTION: The method of manufacturing the thin-film solar cell comprises processes S01-S04 for forming the light absorption layer formed of Cu, In, Se and Ga on the surface of the metal back electrode layer installed on an electrode substrate, a process S05 for forming a buffer layer on the surface of the light absorption layer, a process S06 for forming a transparent electrode layer on the surface of the buffer layer, and a process S07 for applying laser beams to the light absorption layer from the side of the transparent electrode layer spatially selectively. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a method for manufacturing a thin film solar cell, and more particularly, to a method for manufacturing a chalcopyrite thin film solar cell using a CIGS layer as a light absorption layer.

  Chalcopyrite thin-film solar cells have been extensively developed in recent years because they have a wide variety of manufacturing methods and materials, and can cope with low cost and high performance. In addition, chalcopyrite thin-film solar cells are easier to increase in area compared to polycrystalline silicon solar cells, consume less energy during production, and can be mass-produced in large areas. It is progressing.

  As a method for producing a chalcopyrite thin film solar cell, a heat treatment method and a vacuum evaporation method are generally known, and these production methods form a light absorption layer. The vacuum evaporation method evaporates elements (In, Cu, Ga, etc.) in vacuum and deposits them on a heated substrate, and requires a large-scale facility. On the other hand, as a heat treatment method, a manufacturing method is known in which an element is supplied in a gaseous state to form a thin film layer, and annealing is performed in an appropriate temperature range in the presence of oxygen (Patent Document 1).

JP 2002-329877 A

The general structure of a chalcopyrite thin film solar cell has a laminated structure, and is composed of a glass substrate, a metal back electrode layer, a light absorption layer, a buffer layer, and a transparent electrode layer from the lower layer. In general, Mo is applied to the metal back electrode layer, and during heat treatment, light absorption composed of Mo, CIS (copper, indium, selenium), CIGS (copper, indium, gallium, selenium), and the like. A compound (such as MoSe ₂ ) with the element of the layer is formed at the interface between the light absorption layer and the metal back electrode layer.

  The formed compound layer is in ohmic contact with the light absorption layer and realizes a conductive state between the light absorption layer and the metal back electrode layer, and thus plays an important role in increasing the efficiency of the solar cell. A thick compound layer is not preferable because the electrical resistance increases to lower the performance of the solar cell and structurally lowers the adhesion between the glass substrate and the metal back electrode layer. In the prior art, baking was performed in a resistance heating furnace or heat treatment was performed using an oxygen beam as in Patent Document 1, but it was necessary to perform heat treatment for at least about one hour. In these methods, it takes time to raise and lower the temperature, the compound layer may be thick, and the performance of the solar cell may be deteriorated. Also, with conventional heat treatment methods, solar cells, particularly chalcopyrite thin film solar cells, often produce large-area substrates, and there is a risk that the compound layer will become non-uniform due to temperature unevenness during heat treatment. It was.

  In view of such a problem, the present invention appropriately provides a compound layer formed between a metal back electrode layer to which Mo is applied and a light absorption layer in a CIGS (copper, indium, gallium, selenium) type solar cell. An object of the present invention is to provide a method for manufacturing a thin-film solar cell having a uniform thickness and a uniform thickness.

  The present invention relates to a method for manufacturing a thin film solar cell. Such a manufacturing method includes a step of forming a light absorption layer made of Cu, In, Se, and Ga on a metal back electrode layer formed on an electrode substrate made of Mo, and a buffer layer is formed on the surface of the light absorption layer. A step, a step of forming a transparent electrode layer on the surface of the buffer layer, and a step of spatially selectively irradiating the light absorption layer from the transparent electrode layer side to the light absorption layer.

According to the above-described configuration, for example, by performing annealing with a laser beam of 0.1 mJ / cm ² or more, a local and rapid heat treatment is realized, and the thermal history to the light absorption layer is minimized and necessary. A compound layer (MoSe ₂ layer) between the minimum light absorption layer and the metal back electrode layer made of Mo can be formed. Further, after the buffer layer and the transparent electrode layer are formed, the compound layer can be formed while evaluating the battery performance by heat-treating the light absorption layer in a space selective manner, which is a feature of laser annealing. Furthermore, this configuration can prevent the light absorption layer from evaporating due to the heat treatment.

  In the above configuration, it is preferable that the laser beam pass through the buffer layer and the transparent electrode layer and exceed the band gap of the light absorption layer, for example, a wavelength of 750 to 850 nm. Spatial selective annealing of laser light can be realized. In addition, since the light absorption layer material of the chalcopyrite solar cell is a direct transition type semiconductor, the light absorption coefficient is large, and the effect is reduced with a laser having a wavelength of less than 750 nm, but the light absorption layer is as thin as 1 μm or less. In the case of a wide gap of 1.5 eV or more, a large effect can be expected even with a harmonic of a solid-state laser (for example, a wavelength of 532 nm).

  In the above structure, it is preferable that the laser light is uniformly irradiated while scanning the light absorption layer in a planar manner, and a uniform compound layer can be formed.

  According to the present invention, in a CIGS type solar cell, a thin film solar in which a compound layer formed between a metal back electrode layer to which Mo is applied and a light absorption layer has an appropriate thickness and is uniformly formed A method for manufacturing a battery can be provided.

It is the schematic which shows the structure of the thin film solar cell concerning one Embodiment of this invention. It is a manufacturing-process figure of the thin film solar cell concerning one Embodiment of this invention. It is the chart of the IV characteristic which compared the Example and comparative example concerning one Embodiment of this invention.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. Referring to FIG. 1, the structure of a thin-film solar cell according to an embodiment of the present invention includes an electrode substrate provided with a metal back electrode layer 2 constituting a positive electrode of a solar cell on a glass substrate 1, and a laser beam to be described later. The compound layer 3 formed by irradiation, the CIGS layer 4 serving as a light absorption layer, the buffer layer 5, and the transparent electrode layer 6 constituting the negative electrode of the solar cell are laminated.

As the glass substrate 1, for example, soda lime glass or non-alkali glass substrate is preferably used. As the metal back electrode layer 2, for example, a metal layer having high corrosion resistance such as Mo and W and having a high melting point can be applied. In this embodiment, a Mo back electrode of about 0.5 μm is provided on the glass substrate 1. It is supposed to be. The compound layer 3 is a compound layer (MoSe ₂ layer) of Mo of the metal back electrode layer 2 and Se of the CIGS layer 4. The compound layer 3 is in ohmic contact with the CIGS layer 4 which is a light absorption layer, and the CIGS layer 4 and the electrode substrate 2. A conduction state is realized.

The CIGS layer 4 is a chalcopyrite group I-III-VI group compound CuInGaSe ₂ (copper, indium, gallium, selenium) and is a p-type light absorption layer. CdS, ZnS, and InS can be applied to the buffer layer 5, and an InS layer is applied in this embodiment. This is because the InS layer is harmless for environmental hygiene and improves the characteristics of the chalcopyrite thin film solar cell. That is, the buffer layer 5 prevents carrier recombination at the interface between the CIGS layer 4 and the transparent electrode layer 6 and forms a pn junction. For the transparent electrode layer 6, for example, Al-added ZnO can be applied as a transparent and low-resistance n-type semiconductor thin film.

  With reference to FIG. 2, the manufacturing process of a thin film solar cell is demonstrated about one Example concerning one Embodiment. First, fine particles constituting the CIGS layer 4 are mixed with toluene as a solvent to form an ink (mixed liquid) (step S01). Fine particles include, for example, CIS particles having a composition molar ratio of Cu: In: Se = 21: 24: 55, and CGS particles having a composition molar ratio of Cu: Ga: Se = 18: 22: 60. The particle size can be 4 nm. In the present embodiment, three types of inks are prepared in which the mixing ratio of CIS particles to CGS particles is CIS: CGS = 4: 6, 6: 4, 8: 2 in terms of molecular weight ratio. Here, the fine particles constituting the CIS layer 4 and toluene are preferably mixed so that the weight ratio is 1: 3.

  Next, the ink is applied to the metal back electrode layer 2 on the glass substrate 1 by, for example, a spin coater method (step S02). As described above, the electrode substrate is formed by sputtering Mo on the glass substrate 1, and the thickness of the Mo layer can be 0.5 μm. As a condition of spin coating, for example, 1000 rpm × 30 seconds can be set. Through this process, the ink containing the elements constituting the CIGS layer 4 is uniformly applied to the metal back electrode layer 2.

  Next, the intermediate process product of the thin film solar cell to which the ink has been applied is dried by a hot plate or the like (step S03). Drying can be performed at 100 degreeC, for example.

  Next, the intermediate process product of the dried thin film solar cell is heated and baked (step S04). For heating and baking, for example, an infrared lamp heating furnace is used, and rapid thermal annealing can be performed by RTA (Rapid Thermal Annealing). In this example, the heating rate is 500 ° C./min, the holding time is 1 minute, and the holding temperature is 500 ° C. in a nitrogen atmosphere. Depending on the heating and firing, the compound layer 3 is interposed between the CIGS layer 4 and the metal back electrode layer 2. This is a condition that makes it difficult to form.

  As described above, there are three types of inks to be applied: CIS: CGS = 4: 6, 6: 4, and 8: 2 in terms of molecular weight ratio of CIS particles to CGS particles. In this embodiment, a series of steps from Step 02 to Step S04 is set as one cycle, and this cycle is first executed with CIS: CGS = 4: 6 ink, and then CIS: CGS = 6: 4 ink, CIS. This cycle is repeated in the order of: CGS = 8: 2 ink.

  In this example, in the cycle in which the ink of CIS: CGS = 4: 6 was applied, the film thickness of 0.6 μm after drying was 0.4 μm, and then the cycle in which the ink of CIS: CGS = 6: 4 was applied. Then, the film thickness was 0.8 μm, and the film thickness was 1.2 μm in the cycle in which the ink of CIS: CGS = 8: 2 was applied. Thus, in commercialization, the thickness required as a thin film solar cell can be realized by combining the steps S02 to S04 and executing the steps a plurality of times.

  In addition, the ink initially applied to the metal back electrode layer 2 has a molecular weight ratio of CIS: CGS = 4: 6, and the number of CGS particles is larger. And the composition molar ratio of CGS particle | grains is Cu: Ga: Se = 18: 22: 60, and there is much quantity of Se compared with the composition molar ratio Cu: In: Se = 21: 24: 55 of CIS particle | grains. It has become. Thus, the CGIS layer 4 close to the metal back electrode layer 2 is formed so that Se is in a rich state.

  Returning to the process diagram of FIG. 2 again, the buffer layer 5 is formed on the formed CIGS layer 4 of the intermediate process product of the heated and fired thin film solar cell (step S05). The buffer layer 5 can be an InS layer formed by immersion bath deposition, and has a thickness of 70 nm in this embodiment. Then, in the intermediate process product of the thin-film solar cell in which the buffer layer 5 is formed, the transparent electrode layer 6 is formed on the buffer layer 5 (step S06). The transparent electrode layer 6 can be formed of Al-doped ZnO as a transparent and low-resistance n-type semiconductor thin film by a sputtering method, and has a thickness of 1 μm in this embodiment. By performing the processes of step S01 to step S06, an intermediate process product of the thin film solar cell without the compound layer 3 of FIG. 1 is obtained.

  Next, laser light is irradiated from the transparent electrode layer 6 side as indicated by the arrow in FIG. 1 by a semiconductor laser, YAG laser or the like (step S07). The laser beam applied here is selected to pass through the buffer layer 5 and the transparent electrode layer 6 and have a wavelength with an energy exceeding the band gap of the CIGS layer 4.

  In general, a semiconductor transmits light having energy smaller than its own band gap and absorbs light having energy larger than its band gap. Therefore, light having energy smaller than the band gap of the buffer layer 5 passes through the transparent electrode layer 6 and the buffer layer 5 and enters the CIGS layer 4. And when this light has energy larger than the band gap of the CIGS layer 4, it is absorbed by the CIGS layer 4 and heats the CIGS layer 4. However, when the energy of light is very large compared to the band gap of the CIGS layer 4, the light is absorbed by the surface of the irradiated CIGS layer 4, and it becomes difficult to heat the CIGS layer 4 as a whole. Therefore, in this embodiment, a laser beam having a wavelength slightly larger than the band gap of the CIGS layer 4 is selected, and the CIGS layer 4 can be heated as a whole. Specifically, the ideal band gap of a compound solar cell is about 1.4 eV, generally 1.0 eV for a CIS type solar cell, and 1.2 eV for a highly efficient CIGS type solar cell. In this embodiment, a 1.56 eV, 795 nm semiconductor laser is used. However, the semiconductor laser is not limited as long as the above-described requirements are satisfied, and, for example, a double harmonic (532 nm) of a YAG laser can be applied.

The laser light transmitted through the buffer layer 5 and the transparent electrode layer 6 irradiates the CIGS layer 4 and heats the entire layer. Se contained in a large amount on the metal back electrode layer 2 side of the CIGS layer 4 is formed by segregating Mo of the metal back electrode layer 2 by heating the CIGS layer 4 to form a compound layer 3 of MoSe ₂ layers. To do. As described above, since the formation of the compound layer 3 is suppressed by performing a rapid heat treatment in the heating / firing process in step S04, a suitable thickness can be obtained by controlling the laser light irradiation in this step S07. The compound layer 3 can be formed. If the thickness of the compound layer 3 is too thick, the resistance increases. A suitable thickness was 0.05 to 0.2 μm, and in this example, a layer having a thickness of about 0.1 μm could be formed.

The laser light is uniformly irradiated while scanning the CIGS layer 4 in a plane so as to eliminate thermal unevenness and avoid local overheating. In this embodiment, irradiation is performed with an irradiation energy of 1 mJ / cm ² and an irradiation speed of 10 m / sec. The thickness of the compound layer 3 can be varied depending on the laser irradiation time and the type of laser, and by performing irradiation while scanning, a uniform compound layer can be formed on the entire surface even in a large-area solar cell. Can be formed. Furthermore, since irradiation with laser light enables rapid temperature increase and decrease as compared with the prior art, the CIGS layer 4 and the metal back electrode are controlled while performing performance evaluation during the processing step by controlling the irradiation conditions. Formation of the compound layer 3 between the layers 2 can be performed.

  Moreover, after forming the buffer layer 5 and the transparent electrode layer 6 like a present Example, the CIGS layer 4 can be prevented from evaporating by irradiating the CIGS layer 4 with a laser beam.

  FIG. 3 shows a chart comparing IV characteristics of an example of a CIGS type thin film solar cell to which the present invention is applied and a comparative example in which a light absorption layer is formed by a resistance heating furnace as a conventional technique. In the comparative example indicated by the broken line, heat treatment is performed at 500 ° C. for 90 minutes in an Se vapor atmosphere. Referring to FIG. 3, the area surrounded by the solid line, the I axis, and the V axis in the example is larger than the area surrounded by the broken line, the I axis, and the V axis in the comparative example. Especially, the current characteristics are improved.

  The embodiment of the present invention has been described above, but the present invention is not limited to such an embodiment, and can be modified and used without departing from the spirit of the present invention. In the above-described embodiment, the manufacturing method of the CIGS thin film solar cell has been described. However, the present invention can also be applied to, for example, other chalcopyrite thin film solar cells.

DESCRIPTION OF SYMBOLS 1 Glass substrate 2 Metal back electrode layer 3 Compound layer 4 CIGS layer 5 Buffer layer 6 Transparent electrode layer

Claims (3)

Forming a light absorption layer made of Cu, In, Se, Ga on the surface of the metal back electrode layer placed on the electrode substrate;
Forming a buffer layer on the surface of the light absorbing layer;
Forming a transparent electrode layer on the surface of the buffer layer;
A method of manufacturing a thin-film solar cell, comprising: a step of spatially selectively irradiating the light absorption layer with a laser beam from the transparent electrode layer side.   2. The method of manufacturing a thin-film solar cell according to claim 1, wherein the laser light has a wavelength that passes through the buffer layer and the transparent electrode layer and exceeds a band gap of the light absorption layer.   The method for manufacturing a thin film solar cell according to claim 1, wherein the laser light is uniformly irradiated while scanning the light absorption layer in a plane.

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