Photovoltaic device
BACKGROUND OF THE INVENTION 
     1. Field of the invention 
     The present invention relates to photovoltaic devices and particularly to a photovoltaic device having a high photoelectric conversion efficiency, which can be manufactured at a low cost. 
     2. Description of the Prior Art 
     U.S. Pat. No. 4,281,208, for example, discloses a photovoltaic device having a laminated structure in which a light incident electrode, an amorphous semiconductor film including a photoactive layer for receiving light and generating carriers, and a back plate electrode are placed one upon another in this order on a transparent substrate of glass or the like. Such a photovoltaic device using an amorphous semiconductor film for a photoactive layer has a reduced manufacturing cost for a unit quantity of generated energy compared with a photovoltaic device using a single crystal wafer; however, it has a low photoelectric conversion efficiency. 
     In a photovoltaic device described by S. Nakano et al. in Technical Digest of International PVSEC-1 (1984), pp. 583-586, an amorphous semiconductor film including a photoactive layer is formed on a light acceptance electrode having a rough surface texture, whereby incident light is confined within the semiconductor film, which makes it possible to improve the photoelectric conversion efficiency. 
     In general, a light acceptance electrode comprises a single layer or multiple layers of transparent conductive oxide (TCO) such as tin indium oxide or tin oxide. Such a TCO electrode is formed on a transparent substrate of glass or the like, mainly by a thermal CVD process. On that occasion, the substrate needs to be maintained at a high temperature of about 500° C. in order to ensure a sufficient transparency for the TCO electrode and accordingly a large quantity of electric energy is required. As a result, the manufacturing cost of the TCO electrode is higher than the manufacturing cost for other films, which makes it difficult to reduce the manufacturing cost per a unit quantity of generated energy in a photovoltaic device. 
     SUMMARY OF THE INVENTION 
     In view of the above described prior art, an object of the present invention is to provide a photovoltaic device having a high photoelectric conversion efficiency and a reduced manufacturing cost. 
     According to an aspect of the present invention, a photovoltaic device comprises a photoactive layer for generating carriers upon receipt of light, and a window layer including at least silicon and hydrogen and located on the light incidence side of the photoactive layer, hydrogen concentration in the window layer being higher in the light incidence side portion than that in the portion facing the photoactive layer. 
     According to another aspect of the present invention, a photovoltaic device comprises an opaque conductive substrate having a flat surface, a photoactive layer formed on the opaque substrate to generate carriers upon receipt of light, and a window layer formed on the photoactive layer. The window layer includes a first sub-layer on its light incidence side and a second sub-layer on its photoactive layer side, the light incidence side surface of the first sub-layer being rougher than light incidence side surface of the second sub-layer. 
     In the photovoltaic device, incident light passes through the first sub-layer and the second sub-layer and reaches the photoactive layer. The photoactive layer absorbs the incident light and forms photo-generated carriers of electron-hole pairs. The electrons are collected on the conductive surface of the substrate. The holes are collected in the second sub-layer having a high conductivity. Thus, electrical energy may be taken out from the conductive surface of a substrate and from the second sub-layer. 
     On the other hand, light not absorbed in the photoactive layer is reflected on the reflective surface of the substrate and again reaches the photoactive layer where it is partially absorbed to generate carriers. Part of the reflected light from the substrate, not absorbed by the photoactive layer, is reflected in an irregular manner from the rough surface of the first sub-layer and again reaches the photoactive layer. Thus, most of the light once having entered the first sub-layer is confined until it is absorbed by the photoactive layer, whereby a high photoelectric conversion efficiency can be obtained. In addition, since the photovoltaic device of the invention does not require a conventional TCO electrode, the manufacturing cost can be reduced. 
     According to a further aspect of the present invention, a photovoltaic device comprises a transparent substrate having a flat surface, a window layer formed on the transparent substrate, and a photoactive layer formed on the window layer to generate carriers upon receipt of light. The window layer includes a first sub-layer on its substrate side and a second sub-layer on its photoactive layer side, the side of the first sub-layer facing the second sub-layer being rougher than the other surface thereof facing the substrate. A reflective electrode is formed on the photoactive layer. 
     A high photoelectric conversion efficiency can be obtained since the light incident on the transparent substrate is confined between the rough surface and the reflective metal electrode. In addition, the photovoltaic device does not require a conventional TCO electrode and thus the manufacturing cost can be reduced. 
     These objects and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectional view schematically illustrating a photovoltaic device according to an embodiment of the present invention. 
     FIG. 2 is a sectional view schematically showing a photovoltaic device according to another embodiment of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, a photovoltaic device according to an embodiment of the present invention comprises a substrate 1 having a light reflecting conductive surface. The substrate 1 may be formed of a light reflecting metal such as stainless steel or aluminum, or it may be formed by coating a light reflecting metal on a surface of an insulator substrate material of glass or other ceramics. A semiconductor film 2 formed on the substrate 1 is mainly composed of hydrogenated amorphous silicon (a-Si:H). The semiconductor film 2 includes an n type ohmic layer 2n for forming an ohmic contact with the conductive surface of the substrate 1. The semiconductor film 2 further includes a photoactive layer 2i which is non-doped or contains a conductivity type determining impurity of an extremely low concentration, and the photoactive layer 2i generates carriers of electron-hole pairs when it receives light. The semiconductor film 2 further includes a p type window layer 2p for admitting incidence of light onto the photoactive layer 2i. 
     The window layer 2p includes a first sub-layer 2 p1  on its light incidence side and a second sub-layer 2 p2  on its photoactive layer side. The second sub-layer 2 p2  contains hydrogen of about 15 at. %, while the first sub-layer 2 p1  contains hydrogen of about 20 at. % or more. The increase in the content of hydrogen causes the surface of the first sub-layer to become rough and have a textured structure. 
     For example, the surface of the first sub-layer 2 p1  of a-Si:H is almost flat with the content of hydrogen of less than about 15 at. %. When the content of hydrogen becomes 15 at. % or more, the surface of the first sub-layer 2 p1  is caused to have unevenness. The averaged periodicity of the unevenness on the first sub-layer 2 p1  is about 2000 to 3000 Å with the content of hydrogen of 20 at. % and 2500 to 6000 Å with the content of hydrogen of 25 at. % and it is 3000 Å to 1 μm with the content of hydrogen of 30 at. %. 
     The averaged periodicity of the unevenness on the first sub-layer of hydrogenated amorphous silicon carbide (a-SiC:H) is 2000 to 5000 Å with the content of hydrogen of 25 at. % and 3000 Å to 2 μm with the content of hydrogen of 30 at. %. Also, in the first sub-layer of hydrogenated amorphous silicon nitride (a-SiN:H), hydrogenated microcrystalline silicon (μc-Si:H) or hydrogenated microcrystalline silicon carbide (μc-SiC:H), the surface unevenness increases according to increase in the content of hydrogen. 
     On the other hand, the second sub-layer 2 p2  in contact with the photoactive layer 2i has a higher conductivity than that of the first sub-layer 2 p1 . The conductivity of the second sub-layer 2 p2  is almost equal to that of a conventional TCO electrode and it is desired to be 1×10 2  Ω -1  cm -1  or more. The second sub-layer 2 p2  having such a high conductivity may be formed by μc-SiC:H heavily doped with p type impurity. 
     The following table briefly indicates an example of compositions and manufacturing conditions of the semiconductor film 2. 
     
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              Manufacturing                                               
Composition   method      Reaction conditions                             
______________________________________                                    
1st sub-                                                                  
       a-SiC:H    Parallel flat                                           
                              SiH.sub.4 = 10 sccm, RF                     
layer             plate type  power 1 W B.sub.2 H.sub.6 /                 
                  glow discharge                                          
                              SiH.sub.4 = 0.3% Sub-                       
                              strate temperature                          
                              100° C. CH.sub.4 =                   
                              10 sccm, Pressure                           
                              1 Torr                                      
2nd sub-                                                                  
       μc-SiC:H                                                        
                  Electron    SiH.sub.4 = 2 sccm,                         
layer             cyclotron   Microwave power                             
                  resonance   20 W B.sub.2 H.sub.6 /                      
                  discharge   SiH.sub.4 = 0.3%, Sub-                      
                              strate temperature                          
                              250° C. Ar =                         
                              100 sccm, Pressure                          
                              5 m Torr                                    
                              H.sub.2 = 100 sccm                          
                              CH.sub.4 = 2 sccm                           
Photo- a-Si:H     Parallel flat                                           
                              SiH.sub.4 = 2 sccm, RF                      
active            plate type  power 2 W Substrate                         
layer             glow discharge                                          
                              temperature 200° C.                  
                              Pressure 100 m Torr                         
Ohmic  a-Si:H     Parallel flat                                           
                              SiH.sub.4 = 2 sccm, RF                      
layer             plate type  power 2 W PH.sub.3 /                        
                  glow discharge                                          
                              SiH.sub.4 = 1%, Sub-                        
                              strate temperature                          
                              200° C. H.sub.2 =                    
                              2 sccm, Pressure                            
                              100 m Torr                                  
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     The first sub-layer of a-SiC:H containing a high concentration of hydrogen can be also formed by microwave discharge in which current for a magnetic field in electron cyclotron resonance (ECR) discharge is set to 0. In this case, the first sub-layer is formed under reaction conditions of SiH 4  =10 sccm, B 2  H 6  /SiH 4  =0.3 %, CH 4  =10 sccm, microwave power of 1 W, a substrate temperature of 100° C. and a pressure of 50 mTorr. 
     The n type ohmic layer 2n, the photoactive layer 2i, the second sub-layer 2 p2  having the high conductivity, and the first sub-layer 2 p1  having a rough surface texture 2tex are deposited successively on the substrate 1 having a light reflecting conductive surface, and after that, a well-known transparent antireflective film 3 is deposited on the surface 2tex of the first sub-layer 2 p1 . The antireflective film 3 may be formed of SiO 2  or acrylic resin. 
     In the photovoltaic device in FIG. 1, light h ν falling from the air onto the antireflective film 3 is hardly reflected on its free surface 4. Most of the light entering the antireflective film 3 directly passes through the first sub-layer 2 p1  and the second sub-layer 2 p2  and reaches the photoactive layer 2i. The photoactive layer 2i absorbs the incident light and forms photo-generated carriers of electron-hole pairs. The electrons are collected on the conductive surface of the substrate 1 due to junction field formed by the window layer 2p, the photoactive layer 2i and the ohmic layer 2n, and the holes are collected in the second sub-layer 2 p2  having the high conductivity. Thus, electric energy is taken from the conductive surface of the substrate 1 and the second sub-layer 2 p2  to outside 
     On the other hand, the light not absorbed by the photoactive layer 2i and transmitted through the ohmic layer 2n is reflected on the reflective surface of the substrate 1 and reaches again the photoactive layer 2i, where it is absorbed to generate carriers. Part of reflected light from the substrate 1, not absorbed by the photoactive layer 2i is irregularly reflected on the rough surface 2tex and reaches again the photoactive layer 2i. 
     Thus, most of the light once having entered the semiconductor film 2 is confined within the semiconductor film 2 until it is absorbed by the photoactive layer 2i, whereby a high photoelectric conversion efficiency can be obtained. In addition, since the photovoltaic device of the invention does not require a conventional TCO electrode, the manufacturing cost can be reduced. 
     Referring to FIG. 2, a photovoltaic device according to another embodiment of the invention comprises a transparent insulator substrate 11 of glass or the like. A first sub-layer 12 p1  containing a large quantity of Si--H 2  bonding and having a rough surface texture 12tex is deposited on the substrate 11, and a second sub-layer 12 p2  having a high conductivity to function as a collector electrode is deposited on the rough surface 12tex. Further, a photoactive layer 12i and an ohmic layer 12n are deposited on the second sub-layer 12 p2  and finally a back plate electrode 13 of a metal of high reflectivity such as aluminum or silver is deposited. 
     Also in the photovoltaic device of FIG. 2, a high photoelectric conversion efficiency can be obtained since the light hν incident on the transparent substrate 11 is confined between the rough surface 12tex and the reflective metal electrode 13. In addition, the photovoltaic device of FIG. 2 does not require a conventional TCO electrode either and thus the manufacturing cost can be reduced. 
     Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims.