Abstract:
In a photovoltaic device comprising a thin intrinsic amorphous semiconductor film inserted in a junction portion of a crystalline semiconductor substrate and an amorphous semiconductor film which have conductive types reverse to each other, an optical band gap of the intrinsic amorphous semiconductor film is expanded on a side in contact with the amorphous semiconductor film.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a photovoltaic device, and more particularly to a photovoltaic device comprising a semiconductor junction of a crystalline semiconductor such as a single crystalline semiconductor or a polycrystalline semiconductor and an amorphous semiconductor.  
           [0003]    2. Related Art  
           [0004]    In recent years, there has vigorously been developed a photovoltaic device using a thin film semiconductor. The area of a photovoltaic device using a thin film semiconductor as a power generating layer can easily be increased and the basic requirements for the photovoltaic device such as a solar cell can be satisfied sufficiently.  
           [0005]    However, since the thin film semiconductor has a crystalline structure disordered, it has a large number of unbonded hands on a crystalline structure and is not a good material as compared with the crystalline semiconductor. For this reason, there is a problem in that a high photoelectric conversion efficiency cannot be obtained.  
           [0006]    In order to solve such a problem, there has been proposed a photovoltaic device in which a crystalline semiconductor substrate and an amorphous semiconductor film having conductive types reverse to each other are bonded. In such a photovoltaic device, since the crystalline semiconductor substrate is used as a power generating layer, a high photoelectric conversion efficiency can be obtained.  
           [0007]    Moreover, there has been proposed a photovoltaic device having such a structure that a thin intrinsic amorphous semiconductor film is inserted between a crystalline semiconductor substrate and an amorphous semiconductor film so as not to substantially contribute to power generation in order to reduce the recombination of photo-generated carriers in an interface state generated by a semiconductor junction.  
           [0008]    In recent years, however, a solar cell has been started to spread for personal houses and an expectation for a clean energy has been increased, and furthermore, it has been required that the conversion efficiency of the photovoltaic device can be more enhanced.  
         SUMMARY OF THE INVENTION  
         [0009]    It is an object of the present invention to provide a photovoltaic device capable of further reducing the recombination of photo-generated carriers in a semiconductor junction portion, thereby enhancing a photoelectric conversion efficiency.  
           [0010]    The present invention provides a photovoltaic device comprising a crystalline semiconductor substrate having one conductive type, an amorphous semiconductor film having a conductive type reverse to that of the crystalline semiconductor substrate, and a thin intrinsic amorphous semiconductor film inserted in a junction portion of the crystalline semiconductor substrate and the amorphous semiconductor film, wherein an optical band gap of the intrinsic amorphous semiconductor film is expanded on a side in contact with the amorphous semiconductor film.  
           [0011]    In the present invention, the optical band gap of the intrinsic amorphous semiconductor film is expanded toward the amorphous semiconductor film side. The optical band gap may be expanded continuously or a plurality of layers may be provided such that the band gap is expanded stepwise.  
           [0012]    In the case in which a plurality of layers are to be provided, different semiconductor materials may be provided. For example, an amorphous silicon (a-Si) layer may be provided on the crystalline semiconductor substrate side and an amorphous silicon carbide (a-SiC) layer, an amorphous silicon oxide (a-SiO) layer or an amorphous silicon nitride (a-SiN) layer may be provided on the amorphous semiconductor film side. In this case, it is preferable that the amorphous silicon layer has a thickness of 5 nm or less.  
           [0013]    In the case in which the amorphous silicon carbide layer, the amorphous silicon oxide layer or the amorphous silicon nitride layer is to be used in the intrinsic amorphous semiconductor film, the concentration of carbon, oxygen or nitrogen in these layers may be gradually increased apart from the substrate. By providing such a graded structure, it is possible to form such an optical band gap as to be gradually expanded apart from the substrate.  
           [0014]    When the amorphous silicon layer contains hydrogen, the optical band gap is increased. By forming the amorphous silicon layer as the intrinsic amorphous semiconductor film and gradually increasing the amount of hydrogen to be contained in the amorphous silicon layer to approach the amorphous semiconductor film side, therefore, the optical band gap may be expanded. Moreover, an amorphous silicon layer may be provided in a different amount of hydrogen to be contained. More specifically, an amorphous silicon layer having a relatively low amount of hydrogen to be contained may be provided on the crystalline semiconductor substrate side and an amorphous silicon layer having a relatively high amount of hydrogen to be contained may be provided on the amorphous semiconductor film side. In this case, it is preferable that the amorphous silicon layer provided on the crystalline semiconductor substrate side should have a thickness of 1 to 7 nm and more preferably 3 to 6 nm.  
           [0015]    In the present invention, the whole intrinsic amorphous semiconductor film has such a thickness as not to form a power generating layer, for example, 20 to 250 Å.  
           [0016]    The crystalline semiconductor substrate to be used in the present invention is a single crystalline semiconductor substrate or a polycrystalline semiconductor substrate, for example. Moreover, a microcrystalline semiconductor substrate or a thin film polycrystalline semiconductor substrate may be used. The amorphous semiconductor film to be used in the present invention also includes a microcrystalline semiconductors film such as a thin microcrystalline silicon film.  
           [0017]    In the photovoltaic device in which the crystalline semiconductor substrate and the amorphous semiconductor film having conductive types reverse to each other are bonded, the intrinsic amorphous semiconductor film inserted in a junction interface to reduce the recombination of photo-generated carriers in the interface state generated by a semiconductor junction produces greater advantages with a finer structure and also causes a current loss due to an increase in light absorption on the same film. In order to suppress the current loss, it is proposed that the thickness of the intrinsic amorphous semiconductor film is reduced. However, if the thickness is too reduced, a built-in potential is lowered in the photovoltaic device so that an open circuit voltage is dropped.  
           [0018]    In the present invention, the intrinsic amorphous semiconductor film having the optical band gap expanded on the side in contact with the amorphous semiconductor film is used as the intrinsic amorphous semiconductor film to be inserted in the junction interface of the crystalline semiconductor substrate and the amorphous semiconductor film.  
           [0019]    In other words, the intrinsic amorphous semiconductor layer of high quality is provided on the crystalline semiconductor substrate side, while the intrinsic amorphous semiconductor layer having a large optical band gap, that is, a wide gap is provided on the amorphous semiconductor film side. By providing the intrinsic amorphous semiconductor layer having the wide gap, thus, it is possible to decrease a current loss due to an increase in the light absorption and the built-in potential can also be prevented from being reduced. Accordingly, it is possible to obtain a photovoltaic device having a photoelectric conversion efficiency enhanced. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    [0020]FIG. 1 is a typical sectional view showing a photovoltaic device according to a first embodiment of the present invention,  
         [0021]    [0021]FIG. 2 is a typical sectional view showing a photovoltaic device according to a second embodiment of the present invention,  
         [0022]    [0022]FIG. 3 is a typical sectional view showing a photovoltaic device according to a conventional example for a comparison,  
         [0023]    [0023]FIG. 4 is a typical sectional view showing a photovoltaic device according to a third embodiment of the present invention,  
         [0024]    [0024]FIG. 5 is a typical sectional view showing a photovoltaic device according to a conventional example for a comparison,  
         [0025]    [0025]FIG. 6 is a chart showing a short circuit current obtained when the thickness of an amorphous silicon layer is changed in the first embodiment according to the present invention,  
         [0026]    [0026]FIG. 7 is a chart showing an open circuit voltage obtained when the thickness of the amorphous silicon layer is changed in the first embodiment according to the present invention,  
         [0027]    [0027]FIG. 8 is a chart showing an open circuit voltage obtained when the thickness of a first amorphous silicon layer is changed in the third embodiment according to the present invention,  
         [0028]    [0028]FIG. 9 is a chart showing a short circuit current obtained when the thickness of the first amorphous silicon layer is changed in the third embodiment according to the present invention, and  
         [0029]    [0029]FIG. 10 is a chart showing a maximum output obtained when the thickness of the first amorphous silicon layer is changed in the third embodiment according to the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0030]    (First Embodiment)  
         [0031]    [0031]FIG. 1 is a typical sectional view showing a photovoltaic device according to a first embodiment of the present invention. With reference to FIG. 1, an intrinsic amorphous silicon layer  12  is formed on the light incidence side of an n-type single crystalline silicon substrate  11 . An intrinsic amorphous silicon carbide layer  13  is formed on the intrinsic amorphous silicon layer  12 . The intrinsic amorphous semiconductor film according to the present invention is constituted by the intrinsic amorphous silicon layer  12  and the intrinsic amorphous silicon carbide layer  13 . Ap-type amorphous silicon film  14  is formed on the intrinsic amorphous silicon carbide layer  13 . A transparent electrode  15  is formed on the p-type amorphous silicon film  14  and a collector electrode  16  is provided on the transparent electrode  15 .  
         [0032]    An intrinsic amorphous silicon layer  17  is formed on the back face side of the n-type single crystalline silicon substrate  11  and an n-type amorphous silicon film  18  is formed on the intrinsic amorphous silicon layer  17 . A BSF (back surface field) region is formed by the intrinsic amorphous silicon film  17  and the n-type amorphous silicon film  18 . A back electrode  19  is formed on the n-type amorphous silicon film  18  and the collector electrode  16  is provided on the back electrode  19 .  
         [0033]    The conditions of the formation of the semiconductor film and the electrode which are to be provided on the light incidence side are as follows.  
         [0034]    The intrinsic amorphous silicon layer  12  is formed by a plasma CVD method on the condition of an SiH 4  gas flow rate of 60 sccm, an H 2  gas flow rate of 60 sccm, a pressure of 40 Pa, a substrate temperature of 170° C. and an RF power of 30 W. A thickness is 3 nm and an optical band gap is 1.54 eV.  
         [0035]    The intrinsic amorphous silicon carbide layer  13  is formed by the plasma CVD method on the condition of an SiH 4  gas flow rate of 40 sccm, a CH 4  gas flow rate of 40 sccm, a pressure of 40 Pa, a substrate temperature of 170° C. and an RF power of 30 W. A thickness is 5 nm and an optical band gap is 1.76 eV.  
         [0036]    The p-type amorphous silicon film 14 is formed by the plasma CVD method on the condition of an SiH 4  gas flow rate of 40 sccm, a B 2 H 6  gas (1% H 2  base) flow rate of 80 sccm, a pressure of 40 Pa, a substrate temperature of 170° C. and an RF power of 30 W. A thickness is 3 nm.  
         [0037]    The transparent electrode  15  is formed of indium tin oxide (ITO) by sputtering. The condition of the formation includes a substrate temperature of 180° C., an Ar gas flow rate of 70 sccm, an O 2  gas flow rate of 5 sccm, a pressure of 0.7 Pa, and an RF power of 800 W. A thickness is 100 nm. 5% O 2  (Ar base) is used as the O 2  gas.  
         [0038]    The semiconductor film and electrode on the back face side are formed on the following conditions.  
         [0039]    The intrinsic amorphous silicon layer  17  is formed by the plasma CVD method on the condition of an SiH 4  gas flow rate of 60 sccm, an H 2  gas flow rate of 60 sccm, a pressure of 40 Pa, a substrate temperature of 170° C. and an RF power of 30 W. A thickness is 15 nm and an optical band gap is 1.54 eV.  
         [0040]    The n-type amorphous silicon film  18  is formed by the plasma CVD method on the condition of an SiH 4  gas flow rate of 40 sccm, a PH 3  gas (2% H 2  base) flow rate of 20 sccm, a pressure of 30 Pa, a substrate temperature of 170° C. and an RF power of 30 W. A thickness is 22 nm.  
         [0041]    The back electrode  19  is formed in the same manner as the transparent electrode  15 .  
         [0042]    As a conventional example for a comparison, a photovoltaic device having a structure shown in FIG. 3 is fabricated. The photovoltaic device is formed in the same manner as that according to the first embodiment shown in FIG. 1 except that the intrinsic amorphous silicon carbide layer  13  is not formed. The intrinsic amorphous silicon layer  12  has a thickness of 8 nm.  
         [0043]    For the photovoltaic device according to the first embodiment thus fabricated and the photovoltaic device according to the conventional example, an open circuit voltage (Voc) and a short circuit current (Isc) are measured, which are shown in Table 1.  
                                         TABLE 1                                   Voc (V)   Isc (A)                                    Conventional example   0.675   3.87       Present invention   0.675   3.91                  
 
         [0044]    As is apparent from the Table 1, the photovoltaic device according to the first embodiment of the present invention has a short circuit current more enhanced and a conversion efficiency more increased than the photovoltaic device according to the conventional example.  
         [0045]    Next, the thickness of the whole intrinsic amorphous semiconductor film is set to be constant, that is, 7.5 nm and the thicknesses of the intrinsic amorphous silicon layer  12  and the intrinsic amorphous silicon carbide layer  13  are varied respectively to fabricate the photovoltaic device. An open circuit voltage and a short circuit current are measured. The result of the measurement is shown in FIGS. 6 and 7. The conditions of formation except for the thickness are the same as those in the above-mentioned embodiment.  
         [0046]    As is apparent from FIGS. 6 and 7, the short circuit current is increased in the present embodiment in the same manner as in the above-mentioned embodiment. In particular, excellent characteristics can be obtained by setting the thickness of the amorphous silicon layer to 5 nm or less. If the whole intrinsic amorphous semiconductor film is constituted by the amorphous silicon carbide layer, the film quality is more deteriorated than the amorphous silicon. Therefore, there is lessened the effect of reducing the recombination of the photo-generated carries in the interface state generated by the semiconductor junction of the crystalline semiconductor substrate andtheamorphous semiconductor film. Consequently, it is preferable that the amorphous silicon layer should have a thickness of at least several Å, for example, 1 nm or more.  
         [0047]    (Second Embodiment)  
         [0048]    [0048]FIG. 2 is a typical sectional view showing a photovoltaic device according to a second embodiment of the present invention.  
         [0049]    With reference to FIG. 2, an intrinsic amorphous silicon layer  22  is formed on the light incidence side of an n-type single crystalline silicon substrate  21  and an intrinsic amorphous silicon carbide layer  23  is formed on the intrinsic amorphous silicon layer  22 . The intrinsic amorphous semiconductor film according to the present invention is constituted by the intrinsic amorphous silicon layer  22  and the intrinsic amorphous silicon carbide layer  23 . A p-type amorphous silicon film  24  is formed on the intrinsic amorphous silicon carbide layer  23 , and a transparent electrode  25  and a collector electrode  26  are formed on the p-type amorphous silicon film  24 .  
         [0050]    An intrinsic amorphous silicon layer  27  is formed on the back face side of the n-type single crystalline silicon substrate  21  and an intrinsic amorphous silicon carbide layer  28  is formed on the intrinsic amorphous silicon layer  27 . The intrinsic amorphous semiconductor film according to the present invention is constituted by the intrinsic amorphous silicon layer  27  and the intrinsic amorphous silicon carbide layer  28 . An n-type amorphous silicon film  29  is formed on the intrinsic amorphous silicon carbide layer  28 . A back electrode  30  and a collector electrode  26  are formed on the n-type amorphous silicon film  29 .  
         [0051]    A BSF region is formed by the intrinsic amorphous silicon layer  27 , the intrinsic amorphous silicon carbide layer  28  and the n-type amorphous silicon film  29 .  
         [0052]    In the embodiment shown in FIG. 2, the semiconductor film and the electrode on the light incidence side are formed in the same manner as the semiconductor film and the electrode on the light incidence side shown in FIG. 1.  
         [0053]    The intrinsic amorphous silicon layer  27  on the back face side is formed by the same method and conditions as the formation of the intrinsic amorphous silicon layer  17  according to the embodiment shown in FIG. 1. A thickness is 5 nm.  
         [0054]    The intrinsic amorphous silicon carbide layer  28  is formed by the plasma CVD method on the condition of an SiH 4  gas flow rate of 40 sccm, a CH 4  gas flow rate of 40 sccm, a pressure of 40 Pa, a substrate temperature of 170° C. and an RF power of 30 W. A thickness is 10 nm and an optical band gap is 1.76 eV.  
         [0055]    The n-type amorphous silicon film  29  is formed by the same method and conditions as the formation of the n-type amorphous silicon film  18  according to the embodiment shown in FIG. 1. Moreover, the back electrode  30  is also formed by the same method and conditions as the formation of the back electrode  19  according to the embodiment shown in FIG. 1.  
         [0056]    Table 2 shows an open circuit voltage and a short circuit current in the photovoltaic device according to the second embodiment. The Table 2 also shows the open circuit voltage and the short circuit current in the photovoltaic device according to the conventional example shown in FIG. 3.  
                                         TABLE 2                                   Voc (V)   Isc (A)                                    Conventional example   0.675   3.87       Present invention   0.681   3.93                  
 
         [0057]    As shown in the Table 2, the photovoltaic device according to the second embodiment of the present invention has an open circuit voltage and a short circuit current more enhanced and a conversion efficiency more increased than the photovoltaic device according to the conventional example.  
         [0058]    (Third Embodiment)  
         [0059]    [0059]FIG. 4 is a typical sectional view showing a photovoltaic device according to a third embodiment of the present invention. With reference to FIG. 4, an intrinsic amorphous silicon layer  32  is formed on the light incidence side of an n-type single crystalline silicon substrate  31  and an intrinsic amorphous silicon layer  33  is formed on the intrinsic amorphous silicon layer  32 . The intrinsic amorphous silicon layer  33  is formed to have a relatively larger amount of hydrogen to be contained than the intrinsic amorphous silicon layer  32 . More specifically, the intrinsic amorphous silicon layer  33  is formed to have an optical band gap expanded with an increase in the amount of hydrogen to be contained. An intrinsic amorphous semiconductor film according to the present invention is constituted by the intrinsic amorphous silicon layer  32  and the intrinsic amorphous silicon layer  33 .  
         [0060]    A p-type amorphous silicon layer  34  is formed on the intrinsic amorphous silicon layer  33 . A transparent electrode  35  and a collector electrode  36  are formed on the p-type amorphous silicon layer  34 .  
         [0061]    An intrinsic amorphous silicon layer  37  is formed on the back face side of the n-type single crystalline silicon substrate  31  and an n-type amorphous silicon film  38  is formed on the intrinsic amorphous silicon layer  37 . A back electrode  39  and the collector electrode  36  are formed on the n-type amorphous silicon layer  38 .  
         [0062]    The semiconductor film and electrode on the light incidence side are formed in the following manner.  
         [0063]    The intrinsic amorphous silicon layer  32  is formed by a plasma CVD method on the condition of an SiH 4  gas flow rate of 10 sccm, a pressure of 53 Pa, a substrate temperature of 170° C. and an RF power of 10 W. A thickness is 4 nm and an optical band gap is 1.54 eV.  
         [0064]    The intrinsic amorphous silicon layer  33  is formed by the plasma CVD method on the condition of an SiH 4  gas flow rate of 10 sccm, an H 2  gas flow rate of 200 sccm, a pressure of 53 Pa, a substrate temperature of 170° C. and an RF power of 10 W. A thickness is 4 nm and an optical band gap is 1.63 eV.  
         [0065]    The p-type amorphous silicon layer  34  is formed by the plasma CVD method on the condition of an SiH 4  gas flow rate of 10 sccm, a B 2 H 6  gas (1% H 2  base) flow rate of 20 sccm, a pressure of 53 Pa, a substrate temperature of 170° C. and an RF power of 10 W. A thickness is 3 nm.  
         [0066]    The transparent electrode  35  is formed by the same method and conditions as the formation of the transparent electrode  15  according to the embodiment shown in FIG. 1.  
         [0067]    The semiconductor film and electrode on the back face side are formed in the following manner.  
         [0068]    The intrinsic amorphous silicon layer  37  is formed by the plasma CVD method on the condition of an SiH 4  gas flow rate of 20 sccm, a pressure of 27 Pa, a substrate temperature of 170° C. and an RF power of 20 W. A thickness is 15 nm and an optical band gap is 1.54 eV.  
         [0069]    The n-type amorphous silicon film 38 is formed by the plasma CVD method on the condition of an SiH 4  gas flow rate of 20 sccm, a PH 3  gas (2% H 2  base) flow rate of 10 sccm, a pressure of 27 Pa, a substrate temperature of 170° C. and an RF power of 20 W. A thickness is 22 nm.  
         [0070]    The back electrode  39  is formed by the same method and conditions as the formation of the back electrode  19  according to the embodiment shown in FIG. 1.  
         [0071]    As a photovoltaic device according to a conventional example for a comparison, a photovoltaic device shown in FIG. 5 is fabricated. As shown in FIG. 5, the photovoltaic device has the same structure as that of the embodiment shown in FIG. 4 except that the intrinsic amorphous silicon layer  33  is not formed. The intrinsic amorphous silicon layer  32  has a thickness of 8 nm.  
         [0072]    An open circuit voltage and a short circuit current in each of the photovoltaic device according to the third embodiment and the photovoltaic device shown in FIG. 5 are shown in Table 3.  
                                         TABLE 3                                   Voc (V)   Isc (A)                                    Conventional example   0.676   3.88       Present invention   0.710   3.83                  
 
         [0073]    As shown in the Table 3, the photovoltaic device according to the third embodiment of the present invention has the open circuit voltage more enhanced and a conversion efficiency more increased than the photovoltaic device according to the conventional example.  
         [0074]    Next, the thickness of the whole intrinsic amorphous semiconductor film is set to be constant, that is, 9 nm and the thicknesses of the first amorphous silicon layer  32  and the second amorphous silicon layer  33  are varied respectively to fabricate the photovoltaic device. Thus, photoelectric conversion characteristics are measured. The result of the measurement is shown in FIGS.  8  to  10 .  
         [0075]    As shown in FIGS.  8  to  10 , the photovoltaic device according to the present invention can have a higher photoelectric conversion efficiency than that of the photovoltaic device according to the conventional example in which the thickness of the first amorphous silicon layer is 9 nm. As is apparent from FIG. 10, moreover, the photoelectric conversion efficiency can particularly be increased with the first amorphous silicon layer having a thickness of 1 to 7 nm, and furthermore, can be more increased with a thickness of 3 to 6 nm.  
         [0076]    While the amorphous silicon carbide layer is used in the first and second embodiments, it has been confirmed that the same effects can be obtained by using an amorphous silicon oxide layer or an amorphous silicon nitride layer in place thereof.  
         [0077]    Although the amount of hydrogen to be contained is increased to expand the optical band gap of the amorphous silicon layer in the third embodiment, a raw material gas such as an SiH 4  gas may be diluted with a hydrogen gas to increase the amount of hydrogen to be contained, and furthermore, a thin film may be formed by using a He or Ar gas as a diluted gas for a method of expanding the optical band gap. Moreover, it is also possible to form a layer having a large optical band gap by repeating a film forming step using the SiH 4  gas and a plasma processing step using a He, Ar or Xe gas to form a film.  
         [0078]    Furthermore, the back face side according to the third embodiment shown in FIG. 3 may be set to be a BSF region having the intrinsic amorphous semiconductor film according to the present invention inserted between an n-type single crystalline silicon substrate and a p-type amorphous silicon film as in the second embodiment shown in FIG. 2.  
         [0079]    According to the present invention, it is possible to reduce the recombination of photo-generated carriers in a semiconductor junction portion and to enhance a photoelectric conversion efficiency.