Patent Publication Number: US-2013228219-A1

Title: Solar cell, and method of manufacturing the same

Description:
Field of the Invention 
     The present invention relates to a solar cell and a method of manufacturing the same, and particularly relates to the structure of a multilayer electrode of the solar cell. 
     BACKGROUND OF THE INVENTION 
     Products requiring a large amount of electric power, e.g., all-electric houses and electric vehicles have been currently introduced to the market, and thus the demand for electricity has increased year by year. It has been, however, difficult to increase the number of thermal power plants and nuclear power plants in fear of carbon dioxide emission and radioactive contamination. For this reason, the widespread use of clean energy has been demanded. Particularly, photovoltaic power generation has received attention because of its infinite and pollution-free energy resource (sunlight). 
     Solar cells using Si crystals have been the most popular in photovoltaic power generation but are still expensive for 1W. Thus, inexpensive thin-film solar cells have been actively studied. Roll-to-roll processes using stainless steel or resin substrates have been examined to achieve lower cost. Japanese Patent No. 3093504 describes an example of a thin-film solar cell including a silicon layer having an amorphous phase. Referring to  FIG. 6 , the contents will be described below. 
     A multilayer lower electrode  102  including an Ag layer  103  and a ZnO transparent conductive layer  104  is stacked on a stainless steel (Steel Use Stainless) substrate  101  having an insulated surface (hereinafter, will be called a SUS substrate  101 ). Moreover, a power generation layer  105  including an n-type (or p-type) Si semiconductor layer  106 , an i-type semiconductor layer  107 , and a p-type (n-type) Si semiconductor layer  108  is formed thereon. Furthermore, an ITO layer  109  is formed on the p-type (n-type) Si semiconductor layer  108  to efficiently collect electric power, and then Ag electrodes  110  are formed thereon. 
     As illustrated in this example, typically, the multilayer lower electrode  102  disposed below a light entry face has a laminated structure including the ZnO transparent conductive layer  104  made of a transparent conductive material and the Ag layer  103  made of a metallic material. This configuration is expected to achieve satisfactory electrical conduction between a power generation layer and an electrode, suppress the influence of metallic impurities caused by the diffusion of a metallic material, e.g., the Ag electrode  103  into the power generation layer  105 , and improve a light confinement effect obtained by reflection on the interface between a power generation material and a transparent conductive material, the interface being caused by a refractive index difference between the power generation material and the transparent conductive material. 
     In the conventional thin-film solar cell using the multilayer electrode, however, the interface between the metallic material and the transparent conductive material may have exfoliation in long-term use at a high temperature and humidity, disadvantageously deteriorating solar cell characteristics. 
     To address this problem, a structure illustrated in  FIG. 7  of Japanese Patent Laid-Open No. 2002-151720 avoids exfoliation caused by long-term use. Specifically, in a multilayer lower electrode  111 , an Ag alloy layer  112  containing Ag and other metals is provided between an Ag layer  113  and a transparent conductive layer  114 , thereby improving adhesion between the Ag layer  113  and the transparent conductive layer  114 . 
     Japanese Patent Laid-Open No. 6-204533 describes a thin-film solar cell in which a mixed layer of an insulator and a metal is disposed between a substrate and a semiconductor layer. 
     Japanese Patent Laid-Open No. 9-87860 describes a thin-film solar cell that includes a silver thin film formed between an underlying layer containing silicon and an intermediate thin film containing silver, oxygen, and metallic elements constituting a metallic oxide having transparent conductivity. 
     Japanese Patent Laid-Open No. 2004-55745 describes a thin-film solar cell including an intermediate layer, a metallic layer, and a metallic oxide layer that are joined between a substrate and a semiconductor layer. 
     Japanese Patent Laid-Open No. 2011-82295 describes a thin-film solar cell including multiple layers stacked and joined with different coefficients of thermal expansion between a substrate and a semiconductor layer. 
     US2010/0071810 describes a thin film deposited on a substrate. The surface of the thin film is heated to 300° C. or higher in a short time, so that a temperature on a surface of the substrate opposite to the deposited thin film is suppressed to 150° C. or lower. 
     US2012/0060916 describes a thin-film solar cell including an Ag layer, an overcoat layer, and a transparent conductive film that are joined between a substrate and a semiconductor layer. 
     Disclosure of the Invention 
     In the method of Japanese Patent Laid-Open No. 2002-151720, however, the alloy layer is introduced into the conventional structure and thus the introduction of a new material may interfere with cost reduction that is an original object. Also in the techniques of Japanese Patent Laid-Open No. 6-204533, Japanese Patent Laid-Open No. 9-87860, Japanese Patent Laid-Open No. 2004-55745, Japanese Patent Laid-Open No. 2011-82295, US2010/0071810, and US2012/0060916, it is difficult to avoid exfoliation caused by long-term use. 
     The present invention has been devised to solve the problem. An object of the present invention is to provide a thin-film solar cell and a method of manufacturing the same, which prevent exfoliation caused by long-term use while avoiding cost increase in the thin-film solar cell including a multilayer electrode. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram illustrating the laminated structure of a solar cell according to a first embodiment of the present invention; 
         FIGS. 2(   a ) to  2 ( d ) are process drawings showing a manufacturing flow of the solar cell according to the first embodiment of the present invention; 
         FIGS. 3(   a ) to  3 ( e ) are process drawings showing a manufacturing flow of a solar cell according to a second embodiment of the present invention; 
         FIG. 4  is a schematic drawing illustrating the configuration of a solar cell according to a third embodiment of the present invention; 
         FIGS. 5(   a ) to  5 ( e ) are process drawings showing a manufacturing flow of the solar cell according to the third embodiment of the present invention; 
         FIG. 6  is a schematic diagram illustrating the laminated structure of a solar cell in Japanese Patent No. 3093504 and so on; and 
         FIG. 7  is a schematic diagram illustrating the laminated structure of a solar cell in Japanese Patent Laid-Open No. 2002-151720 and so on. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Embodiments of the present invention will be described below with reference to the accompanying drawings. 
     First Embodiment 
       FIGS. 1 and 2(   a ) to  2 ( d ) illustrate a first embodiment of the present invention. 
     As illustrated in  FIG. 1 , a solar cell having a substrate structure according to the present invention has a laminated structure in which a first electrode layer  202  serving as a multilayer electrode, a photoelectric conversion layer  206 , and an ITO layer  210  that is a transparent electrode layer serving as a second electrode layer are sequentially stacked on a glass substrate  201  having electrical insulation. 
     The first electrode layer  202  serving as an underlying electrode includes a Cr layer  203 , a mixed layer  204  of Cr and ZnO, and a ZnO layer  205  that are stacked in this order from the glass substrate  201 . 
     The photoelectric conversion layer  206  has a laminated structure including an n-type Si layer  207   b,  an i-type Si layer  208   b,  and a p-type Si layer  209   b  that are stacked in this order when viewed from the first electrode layer  202 . 
     The solar cell can be fabricated in the steps of  FIGS. 2(   a ) to  2 ( d ). 
     As illustrated in  FIG. 2(   a ), the Cr layer  203  having a thickness of about  500  nm is first deposited as a metallic material layer by sputtering on the heat-resistant glass substrate  201  having a thickness of about 400 μm to 1000 μm, and then the ZnO layer  205  having a thickness of about 100 nm is deposited thereon as a transparent conductive film material layer. 
     As illustrated in  FIG. 2(   b ), the photoelectric conversion layer  206  having an amorphous phase is then deposited on the ZnO layer  205  by sputtering. The photoelectric conversion layer  206  includes an n-type a-Si layer  207   a,  an i-type a-Si layer  208   a,  and a p-type a-Si layer  209   a  that are deposited in this order when viewed from the ZnO layer  205 . The n-type a-Si layer  207   a  is deposited by the sputtering using a sputtering target doped with P. The i-type a-Si layer  208   a  is deposited by a sputtering target having a low impurity density. The p-type a-Si layer  209   a  is deposited by a sputtering target doped with B. 
     The processed article in  FIG. 2(   b ) receives a quantity of heat from the surface of the p-type a-Si layer  209   a  by atmospheric pressure plasma  100  according to an atmospheric plasma technique. The quantity of heat is supplied to change the crystal of an amorphous phase, activate impurities such as B and P, and form the mixed layer  204  by thermal diffusion on a lower electrode. The quantity of heat is supplied to cause a-Si to change the layers, so that the photoelectric conversion layer  206  in  FIG. 2(   c ) includes the n-type Si layer  207   b,  the i-type Si layer  208   b,  and the p-type Si layer  209   b  that are stacked in this order when viewed from the ZnO film  205 . 
     The quantity of heat is supplied by the atmospheric pressure plasma technique to heat-treat the interface between the ZnO layer  205  and the Cr layer  203  to at least 300° C. 
     Finally, the layers are subjected to wet cleaning, and then as illustrated in  FIG. 2(   d ), the ITO layer  210 , a transparent conductive layer serving as a second electrode layer, is deposited to a thickness of about  100  nm by sputtering. In the wet cleaning, the layers are immersed into HF of 1% for about ten minutes to remove an oxide film (not shown) formed on the surface of a sample. 
     In the solar cell fabricated in this process, the mixed layer  204  tightly joins the Cr layer  203  that is a metallic material layer and a ZnO layer  205  that is a transparent conductive film material layer, thereby avoiding exfoliation in long-term use. 
     In the present embodiment, the flat and heat-resistant glass substrate  201  is a substrate material. The glass substrate  201  may be a glass substrate provided with a texture surface, a SUS substrate having an insulated surface, or a blue sheet glass that is less heat resistant and less expensive. 
     In the present embodiment, the metallic material is Cr. Cr may be replaced with one of W, Ag, Cu, Al, Mo, Au, Al, and Ti or an alloy containing W, Ag, Cu, Al, Mo, Au, Al, and Ti. 
     In the present embodiment, the photoelectric conversion layer  206  includes the n-type layer, the i-type layer, and the p-type layer that are deposited in this order when viewed from the ZnO layer  205 . The configuration is not particularly limited. The order of deposition may be the p-type layer, the i-type layer, and the n-type layer, or the n-type layer and the p-type layer and vice versa without the i-type layer. 
     In the present embodiment, the transparent conductive film material layer contains ZnO while the second electrode layer contains ITO. The transparent conductive film material layer and the second electrode layer are not particularly limited and thus may contain ZnO, ITO, and SnO 2  or a transparent conductive metal oxide material mainly composed of ZnO, ITO, and SnO 2 . 
     In the present embodiment, the mixed layer  204  is formed by the atmospheric pressure plasma technique. A method for metal diffusion is not limited to the atmospheric pressure plasma technique. Short-time processing can be achieved by at least one of atmospheric plasma, flash lamp annealing, and laser ablation, thereby advantageously reducing a stress caused by heat to the glass substrate  201 . 
     In the present embodiment, the mixed layer  204  formed by metal diffusion contains no metallic materials on the interface of the transparent conductive film material layer. The content of a metallic material gradually increases toward the metallic material layer, and only the metallic material remains on the interface of the metallic material layer. In this structure, the interfaces of the metallic material layer, the mixed layer, and the transparent conductive film material layer are eliminated, further suppressing exfoliation caused by interfaces during long-term use. 
     Second Embodiment 
       FIGS. 3(   a ) to  3 ( e ) illustrate a second embodiment of the present invention. 
     A manufacturing method according to the second embodiment additionally includes the step of forming a mixed layer by using an atmospheric pressure plasma technique after the deposition of a ZnO layer  205  having a thickness of about 100 nm. 
     Other steps and materials are similar to those of the first embodiment. The second embodiment can more sufficiently apply heat near the interface between a metallic material layer and a transparent conductive film material layer, allowing sufficient thermal diffusion of metals so as to reliably form the mixed layer. 
     In  FIG. 3(   a ), a Cr layer  203  having a thickness of about 500 nm is first deposited by sputtering as a metallic material layer on a heat-resistant glass substrate  201  having a thickness of about 400 μm to 1000 μm, and then a ZnO layer  205  having a thickness of about 100 nm is deposited thereon as a transparent conductive film material layer. 
     In  FIG. 3(   b ), a quantity of heat is supplied from the surface of the ZnO layer  205  by atmospheric pressure plasma  100  according to an atmospheric pressure plasma technique. The quantity of heat is supplied by the atmospheric pressure plasma technique to heat-treat the interface between the ZnO layer  205  and the Cr layer  203  to at least 300° C. Thus, as illustrated in  FIG. 3(   c ), the metal of the Cr layer  203  is diffused to the ZnO layer  205  so as to form a mixed layer  204  on the interface between the ZnO layer  205  and the Cr layer  203 . 
     In  FIG. 3(   d ), a processed article illustrated in  FIG. 3(   c ) further includes a photoelectric conversion layer  206  formed on the ZnO layer  205 . 
     Finally, the layers are subjected to wet cleaning, and then as illustrated in  FIG. 3(   e ), an ITO layer  210 , a transparent conductive layer serving as a second electrode layer, is deposited to a thickness of about 100 nm by sputtering. In the wet cleaning, the layers are immersed into HF of 1% for about ten minutes to remove an oxide film (not shown) formed on the surface of a sample. 
     Third Embodiment 
       FIGS. 4 and 5  illustrate a third embodiment of the present invention. 
     As illustrated in  FIG. 4 , a solar cell having a substrate structure according to the third embodiment includes a ZnO layer  302 , a photoelectric conversion layer  303 , and a first electrode layer  307  that are sequentially stacked on a glass substrate  301 . The ZnO layer  302  is a second electrode layer serving as a transparent electrode layer, and the first electrode layer  307  serves as a multilayer electrode. 
     The first electrode layer  307  includes a Cr layer  310 , a mixed layer  309  of Cr and ZnO, and a ZnO layer  308  that are stacked in this order when viewed from the glass substrate  301 . The photoelectric conversion layer  303  has a laminated structure including an n-type Si layer  306   b , an i-type Si layer  305   b,  and a p-type Si layer  304   b  that are stacked in this order from the first electrode layer  307 . 
     The solar cell can be fabricated in the steps of  FIGS. 5(   a ) to  5 ( e ). 
     First, in  FIG. 5(   a ), the ZnO layer  302  having a thickness of about 100 nm is deposited by sputtering on the heat-resistant glass substrate  301  having a thickness of about 400 μm to 1000 μm. The ZnO layer  302  is a transparent conductive layer serving as a second electrode layer. 
     Then, in  FIG. 5(   b ), the photoelectric conversion layer  303  having an amorphous phase is deposited by sputtering on the ZnO layer  302  serving as the second electrode layer. 
     The photoelectric conversion layer  303  includes a p-type a-Si layer  304   a,  an i-type a-Si layer  305   a,  and an n-type a-Si layer  306   a  that are deposited in this order when viewed from the ZnO layer  302  serving as the second electrode layer. 
     The layers are deposited by sputtering. The p-type a-Si layer  304   a  is deposited by a sputtering target doped with B, the i-type a-Si layer  305   a  is deposited by a sputtering target having a low impurity density, and the n-type a-Si layer  306   a  is deposited by a sputtering target doped with P. 
     A quantity of heat is supplied from the surface of the n-type a-Si layer  306   a  by atmospheric pressure plasma  100  according to an atmospheric pressure plasma technique. The quantity of heat is supplied to change the crystal of an amorphous phase and activate impurities such as B and P. 
     Thus, as illustrated in  FIG. 5(   c ), the quantity of heat supplied in  FIG. 5(   b ) causes a-Si to change the layers, so that the photoelectric conversion layer  303  includes an n-type Si layer  304   b,  an i-type Si layer  305   b , and a p-type Si layer  306   b  that are stacked in this order when viewed from the ZnO layer  302  serving as the second electrode layer. 
     In  FIG. 5(   d ), the layers are subjected to wet cleaning, and then the ZnO layer  308  serving as a transparent conductive film material layer is deposited to a thickness of about 100 nm. The Cr layer  310  serving as a metallic material layer is then deposited to a thickness of about 500 nm. 
     Furthermore, a quantity of heat is supplied from the surface of the Cr layer  310  by the atmospheric pressure plasma  100  according to the atmospheric pressure plasma technique. The quantity of heat is supplied to heat-treat the interface between the ZnO layer  308  and the Cr layer  310  to at least 300° C. Thus, as illustrated in  FIG. 4(   e ), the mixed layer  309  is formed on the interface between the Cr layer  310  and the ZnO layer  308  by thermal diffusion of Cr. 
     Finally, the layers are immersed into HF of 1% for about ten minutes to undergo wet cleaning. The wet cleaning is aimed at removing an oxide film (not shown) formed on the surface of a sample. 
     In the solar cell fabricated by this process, the mixed layer  309  tightly joins the metallic material layer and the transparent conductive film material layer, thereby avoiding exfoliation in long-term use. 
     In the present embodiment, the metallic material is Cr. Cr may be replaced with one of W, Ag, Cu, Al, Mo, Au, and Ti or an alloy containing W, Ag, Cu, Al, Mo, Au, and Ti. 
     In the present embodiment, the photoelectric conversion layer  303  includes the n-type layer, the i-type layer, and the p-type layer that are deposited in this order when viewed from the ZnO layer  302  serving as the second electrode layer. The configuration is not particularly limited. The order of deposition may be the p-type layer, the i-type layer, and the n-type layer, or the n-type layer and the p-type layer and vice versa without the i-type layer. 
     In the present embodiment, the transparent conductive film material layer and the second electrode layer contain ZnO. The transparent conductive film material layer and the second electrode layer are not particularly limited and thus may contain ZnO, ITO, and SnO 2  or a transparent conductive metal oxide material mainly composed of ZnO, ITO, and SnO 2 . 
     In the present embodiment, the mixed layer is formed by the atmospheric pressure plasma technique. A method for metal diffusion is not limited to the atmospheric pressure plasma technique. Short-time processing can be achieved by at least one of atmospheric pressure plasma, flash lamp annealing, and laser ablation, thereby advantageously reducing a stress caused by heat to the glass substrate  301 . 
     In the present embodiment, the mixed layer formed by metal diffusion contains no metallic materials on the interface of the transparent conductive film material layer. The content of the metallic material gradually increases toward the metallic material layer, and only the metallic material remains on the interface of the metallic material layer. In this structure, the interfaces of the metallic material layer, the mixed layer, and the transparent conductive film material layer are eliminated, further suppressing exfoliation caused by interfaces during long-term use. 
     In the structures of the mixed layers  204  and  309  according to the foregoing embodiments, the content of the metallic material (Cr) is zero on the interfaces of the transparent conductive film material layers  205  and  308  and gradually increases toward the metallic material layers  203  and  310 . Only the metallic material (Cr) remains on the interfaces of the metallic material layers  203  and  310 . The mixed layers  204  and  309  are expected to have substantially the same effect even in the case where the content of the metallic material (Cr) gradually increases toward the metallic material layers  203  and  310 , the content of the metallic material (Cr) is substantially zero on the interfaces of the transparent conductive film material layers  205  and  308 , and only the metallic material (Cr) remains on the interfaces of the metallic material layers  203  and  310 . 
     The present invention contributes to an improvement of reliability of solar cells and various facilities using the same.