Patent Publication Number: US-2002011590-A1

Title: Photovoltaic conversion device for thermophotovoltaic power generation apparatus

Description:
BACKGROUND OF THE INVENTION  
       [0001] 1. Field of the Invention  
       [0002] The present invention relates to a photovoltaic conversion device suitable for use in a thermophotovoltaic power generation apparatus that converts radiation, from a light emitter heated by a heat source, into electric power by means of the photovoltaic conversion device.  
       [0003] 2. Description of the Related Art  
       [0004] As a technology for obtaining electric energy directly from fossil fuel or combustible gas, thermophotovoltaic power generation (TPV power generation), that is, power generation by means of thermophotovoltaic energy conversion, has been attracting wide-spread attention. TPV power generation generates electric power by supplying combustion heat from a heat source to a light emitter (radiator, emitter), and by irradiating a photovoltaic conversion device (solar cell) with the light from the light emitter to obtain electric energy. Thus, a TPV power generation apparatus has no moving parts and, therefore, provides a system that is completely free of noise or vibration. As an energy source for the next generation, TPV power generation has distinct advantages in cleanliness and quietness.  
       [0005] A TPV power generation apparatus has been previously disclosed, for example, in Japanese Unexamined Patent Publication No. 63-316486, in which a thermophotovoltaic power generation apparatus comprising a light emitter fabricated from a porous solid, heating means for heating the light emitter by causing exhaust gas from combustion to flow through the light emitter, and a photovoltaic device for converting radiant energy, from the light emitter, into electric energy, is disclosed.  
       [0006] In TPV power generation, infrared radiation from an emitter at temperatures in the range 1000-1700° C. is utilized. In order to convert radiant energy radiated from the light emitter at wavelength of 1.4-1.7 μm into electric energy, it is necessary to employ a photovoltaic conversion device that is fabricated from material having a small band gap energy (Eg). Si (silicon), which is the most typical semiconductor material, cannot be used since it can convert radiation only at wavelength not greater than 1.1 μm into electric energy.  
       [0007] Thus, materials that have energy band gap energy (Eg) of 0.5-0.7 eV are suitable for a photovoltaic conversion device in a TPV thermophotovoltaic power generation apparatus. Typical materials include, for example, GaSb (gallium antimony, Eg=0.72 eV), InGaAs (indium gallium arsenide, Eg=0.60 eV), Ge (germanium, Eg=0.66 ev), and the like.  
       [0008] In order to increase the efficiency of energy conversion of TPV power generation and to reduce the number of expensive photovoltaic conversion devices used and thereby to reduce cost, a possible solution is to increase the intensity of the light emitted from by light emitter. If, for example, light intensity is increased by a factor of 100, the number of the photovoltaic conversion devices used can be reduced to {fraction (1/100)}, resulting in large reduction in cost as well as considerable improvement of energy conversion efficiency.  
       [0009] In this case, however, since the magnitude of generated electric current is increased, the area of the electrode on the front face side of the conventional photovoltaic conversion device needs to be increased in order to reduce resistive loss. An increase in the electrode area on the front face side of the photovoltaic conversion device, however, would reduce the amount of light incident on the photovoltaic conversion device, and this hinders full utilization of the increase of light intensity.  
       [0010] On the other hand, there is another electrode structure called a back-face electrode type which has no electrode on the front face side and which is utilized in a light-collecting type power generation system. The back-face electrode type is, however, a viable choice only for indirect transition type materials in which the diffusion distance of carriers is large, and in practice only for Si. An indirect transition type material that has small energy band gap includes Ge (germanium). In Ge, however, carrier life time is short compared to Si and recombination loss of carriers at the surface is large. Thus, a photovoltaic conversion device which uses Ge as the substrate material and employs a back-face electrode type as the electrode structure has not been put into practical use.  
       SUMMARY OF THE INVENTION  
       [0011] It is an object of the present invention to resolve above-described problem and to provide a photovoltaic conversion device which is suitable for use in TPV power generation and which has configuration that greatly reduces recombination loss of carriers at the surface and thus enables Ge to be used as the substrate material and back-face electrode type to be employed as the electrode structure.  
       [0012] To attain the above object, according to a first aspect of the present invention, a photovoltaic conversion device is provided which is suitable for use in a thermophotovoltaic power generation apparatus that converts radiation from a light emitter heated by a heat source into electric power by means of the photovoltaic conversion device, and which is comprised of a Ge substrate, a p-type semiconductor layer and an n-type semiconductor layer provided independently on the back-face side of the Ge substrate, positive and negative electrodes provided on the back-face side of the Ge substrate and connected, respectively, to the p-type and the n-type semiconductor layers, and a protective film provided on the front face side of the Ge substrate.  
       [0013] According to a second aspect of the present invention, a photovoltaic conversion device according to the above described first aspect is provided, wherein hydrogen or halogen is contained in the interface between the Ge substrate and the protective film.  
       [0014] According to a third aspect of the present invention, a photovoltaic conversion device according to the above described first aspect is provided, wherein a semiconductor layer with an impurity concentration higher than the Ge substrate is provided between the Ge substrate and the protective film.  
       [0015] Further, in order to attain the above object, according to a fourth aspect of the present invention, a photovoltaic conversion device is provided which is suitable for use in a thermophotovoltaic power generation apparatus that converts radiation from a light emitter heated by a heat source into electric power by means of the photovoltaic conversion device, and which is comprised of a Ge layer, a p-type semiconductor layer and a n-type semiconductor layer provided independently of each other on the back-face of the Ge layer, positive and negative electrodes provided on the back-face side of the Ge layer and connected to the p-type and the n-type semiconductor layers, respectively, an Si layer provided on the front face side of the Ge layer, and an SiO 2  film provided on the front face side of the Si layer.  
       [0016] According to a fifth aspect of the present invention, a photovoltaic conversion device, according to the above described fourth aspect, is provided wherein hydrogen or halogen is contained in the interface between the Si layer and the SiO 2  film.  
       [0017] According to a sixth aspect of the present invention, a photovoltaic conversion device according to the above described fourth aspect is provided, wherein a semiconductor layer with an impurity concentration higher than the Si layer is provided between the Si layer and the SiO 2  film, and a semiconductor layer with impurity concentration higher than the Ge layer is provided between the Ge layer and the Si layer.  
       [0018] According to a seventh aspect of the present invention, a photovoltaic conversion device according to the fourth aspect as described above is provided, wherein a mixed layer of Ge and Si is provided between the Ge layer and the Si layer. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0019] Further features and advantages of the present invention will be apparent from the following description with reference to the accompanying drawings, in which:  
     [0020]FIG. 1 is a cross sectional view showing a photovoltaic conversion device according to a first embodiment of the present invention;  
     [0021]FIG. 2 is a cross sectional view showing a photovoltaic conversion device according to a second embodiment of the present invention;  
     [0022]FIG. 3 is a cross sectional view showing a photovoltaic conversion device according to a third embodiment of the present invention;  
     [0023]FIG. 4 is a cross sectional view showing a photovoltaic conversion device according to a fourth embodiment of the present invention;  
     [0024]FIG. 5 is a cross sectional view showing a photovoltaic conversion device according to a fifth embodiment of the present invention;  
     [0025]FIG. 6 is a cross sectional view showing a photovoltaic conversion device according to a sixth embodiment of the present invention;  
     [0026]FIG. 7 is a cross sectional view showing a photovoltaic conversion device according to a seventh embodiment of the present invention;  
     [0027]FIG. 8 is a cross sectional view showing a photovoltaic conversion device according to an eighth embodiment of the present invention;  
     [0028]FIG. 9 is a cross sectional view showing a photovoltaic conversion device according to a ninth embodiment of the present invention;  
     [0029]FIG. 10 is a cross sectional view showing a photovoltaic conversion device according to a tenth embodiment of the present invention;  
     [0030]FIG. 11 is a cross sectional view showing a photovoltaic conversion device according to an eleventh embodiment of the present invention; and  
     [0031]FIG. 12 is a cross sectional view showing a photovoltaic conversion device according to a twelfth embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0032] The present invention will now be described with reference to drawings showing embodiments thereof.  
     [0033]FIG. 1 is a cross sectional view showing a photovoltaic conversion device according to a first embodiment of the present invention. In the photovoltaic conversion device, Ge (germanium, Eg=0.66 eV), a semiconductor material having small energy band gap (Eg), is used as a substrate  10 . On the back-face of the Ge substrate  10 , a p + -layer  20  as a p-type semiconductor layer and a n + -layer  22  as a n-type semiconductor layer are formed alternately and independently of each other to collect carriers. On the back-face side of the Ge substrate  10 , a positive electrode  24  connected to the p + -layer  20  and a negative electrode  26  connected to the n + -layer  22  are provided to realize a back-face electrode type structure. A protective film  30  is provided on the front face side of the Ge substrate  10  in order to decrease the number of surface defects and thereby to decrease annihilation of carriers at the surface. Similarly, a protective film  40  is provided on the back-face side of the Ge substrate  10  except where the semiconductor layers  20  and  22  are connected to electrodes  24  and  26 , respectively.  
     [0034] The structure of the photovoltaic conversion device shown in FIG. 1 will now be described more specifically. The Ge substrate  10  is, for example, 200 μm thick and forms p-type semiconductor with dopant concentration of 3×10 15 cm −3 . The p + -layer  20  has dopant concentration of 1×10 19 cm −3  and diffusion depth of 2 μm. Similarly, n + -layer  22  has dopant concentration of 1×10 19 cm −3  and diffusion depth of 2 μm. Material having good insulating and light transmission properties such as SiN, SiO 2 , TiO 2 , etc is suitable for use as the protective films  30  and  40 .  
     [0035] The process of fabrication of such a photovoltaic conversion device will next be described. First, protective films  30  and  40  are formed on front and back-faces of the Ge substrate  10 . Then, the portion of the protective film where p + -layer  20  and n + -layer  22  are to be formed is removed by photolithographic technique. Predetermined p + -layer  20  and n + -layer  22  are then formed by means of a thermal diffusion method, an ion implantation method, or the like. Finally, electrode patterns  24  and  26  are formed.  
     [0036] In the photovoltaic conversion device shown in FIG. 1, light incident from the front face side is absorbed by the Ge substrate  10  thereby to produce electrons and positive holes. The produced electrons diffuse toward the region of n + -layer  22 , and are collected by the negative electrode  26 , while the produced positive holes diffuse toward the region of p + -layer  20 , and are collected by the positive electrode  24 . Electrons and positive holes produced by absorption of light are separated in this manner to produce a photovoltaic electromotive force.  
     [0037] The photovoltaic conversion device having the construction as described above is suited to being used in TPV power generation that converts light from a heated light emitter into electric power, since light is absorbed by Ge with a small energy band gap. The electrode structure of a back-face electrode type adopted in this construction permits the area of the electrode to be increased, and thus permits the resistive loss to be kept small. Simultaneously with the adoption of the back-face electrode type as the electrode structure, the protective film  30  is provided on the front face side of the Ge substrate  10  so that number of surface defects is decreased, and capture of carriers by defects and resulting annihilation of carriers produced near the surface is also decreased. Therefore, with the construction as described above, when the intensity of light from the light emitter is increased in a TPV power generation system, it is possible to avoid an increase in resistive loss in the electrodes, and thereby to realize high conversion efficiency.  
     [0038] As discussed above, the number of photovoltaic conversion devices used can be greatly reduced by increasing the light intensity in TPV power generation system, so that a highly efficient system can be realized at low cost. In such a system, the photovoltaic conversion device of the present invention can take advantage of the small energy band gap of Ge to convert infra red radiation from a light emitter efficiently into electric energy.  
     [0039]FIG. 2 is a cross sectional view showing a photovoltaic conversion device according to a second embodiment of the present invention. In FIG. 2, the same elements as in FIG. 1 are denoted by the same reference numerals, and explanations thereof are not repeated. Construction as shown in FIG. 2 differs from that in FIG. 1 in that, in the construction in FIG. 2 as compared to that in FIG. 1, hydrogen or halogen is contained in the interface  32  between the Ge substrate  10  and the protective film  30 . Similarly, hydrogen or halogen is also contained in the interface  12  between the Ge substrate  10  and the protective film  40 .  
     [0040] A fabrication method for fabricating the photovoltaic conversion device of FIG. 2 will next be described. In a first fabrication method, SiNx (silicon nitride) films are first formed as the protective films  30  and  40  on both faces of Ge substrate  10  by plasma CVD (chemical vapor deposition) method. When forming the SiNx films, hydrogen gas is mixed to form an SiNx: H film containing hydrogen. In subsequent heat treatment, hydrogen atoms are moved to the interface to be combined with the dangling bonds on the surface of the Ge substrate so as to decrease the number of electric defects.  
     [0041] In a second fabrication method, after the protective films are formed, heat treatment is performed in a hydrogen atmosphere to cause hydrogen atoms to be diffused to the interface. The hydrogen atoms are combined, as in the first fabrication method, with the dangling bonds on the surface of the Ge substrate so as to decrease the number of electric defects. In the case of halogen atoms, a process similar to that the first and the second fabrication methods in the case of hydrogen may be employed to distribute halogen atoms in the interface.  
     [0042] With the photovoltaic conversion device constructed as shown in FIG. 2, the number of dangling bonds is decreased as hydrogen or halogen atoms are combined with the dangling bonds. Thus, number of electric defects that capture carriers and deteriorate the performance is decreased and thus, in turn, reduces the recombination loss of carriers, resulting in improved performance, that is, improved photovoltaic conversion efficiency. Thus, efficiency of the TPV power generation apparatus is improved, leading to increased power production.  
     [0043]FIG. 3 is a cross sectional view showing a photovoltaic conversion device according to a third embodiment of the present invention. In FIG. 3, the same elements as in FIG. 1 are denoted by the same reference numerals, and an explanation thereof is not repeated. The construction shown in FIG. 3 differs from that in FIG. 1 in that, in the construction in FIG. 3 as compared to that in FIG. 1, a semiconductor layer  50  is formed on the front face of the Ge substrate  10 . Thus, a semiconductor layer  50  with impurity concentration higher than the Ge substrate  10  is provided between the Ge substrate  10  and the protective film  30 .  
     [0044] The semiconductor layer  50  is formed as p + -layer having dopant concentration of 1×10 18 cm −3  and diffusion depth of 2 μm. A fabrication method for fabricating the photovoltaic conversion device of FIG. 3 will next be described. First, protective films  30  and  40  are formed on the front and back-faces of the Ge substrate  10 . Then, the portion of the protective films where the p + -layer  20  and n + -layer  22  are to be formed is removed by photolithographic technique. Predetermined p + -layer  20  and n + -layer  22  are then formed by means of a thermal diffusion method, an ion implantation method, or the like. Then, the protective film  30  on the front face side is removed, and a semiconductor layer  50  is formed on the front face side of the Ge substrate  10 . The protective film  30  is again formed on the front face side. Finally, electrode patterns  24  and  26  are formed.  
     [0045] In the photovoltaic conversion device having the construction as shown in FIG. 3, the semiconductor layer (p + -layer)  50  is a region of higher energy level and therefore greatly reduces the proportion of the carriers (electrons) produced near the surface which move toward the surface where many defects are present. Thus, the number of carriers (electrons) that move toward surface defects and are annihilated there is decreased, leading to a reduction in the recombination loss and an improvement in the performance (photovoltaic conversion efficiency). Therefore, efficiency of the TPV power generation apparatus is improved, leading to an increased power production.  
     [0046]FIG. 4 is a cross sectional view showing a photovoltaic conversion device according to a fourth embodiment of the present invention. Again, in the photovoltaic conversion device, a layer of Ge which is semiconductor material having small energy band gap is used as the Ge substrate  10 . On the back-face of the Ge substrate  10 , a p + -layer  20  as a p-type semiconductor layer and a n + -layer  22  as a n-type semiconductor layer are formed alternately and independently of each other to collect carriers. On the back-face side of the Ge substrate  10 , a positive electrode  24  connected to the p + -layer  20  and a negative electrode  26  connected to the n + -layer  22  are provided and thereby realize back-face type electrode structure. A Si layer  60  is provided on the front face side of the Ge substrate  10 , and a SiO 2  film  70  is provided on the front face side of the Si layer  60 . A protective film  40  is provided on the back-face side of the Ge substrate  10  except where the semiconductor layers  20  and  22  are connected to electrodes  24  and  26 , respectively.  
     [0047] The structure of the photovoltaic conversion device shown in FIG. 4 will now be described more specifically. The Ge substrate  10  is, for example, 200 μm thick and forms p-type semiconductor with dopant concentration of 3×10 15 cm −3  . The p + -layer  20  has dopant concentration of 1×10 19 cm −3  and diffusion depth of 2 μm. Similarly, n + -layer  22  has dopant concentration of 1×10 19 cm −3  and diffusion depth of 2 μm. The Si layer  60  is 5 μm thick and forms p-type semiconductor with dopant concentration of 1×10 15 cm −3 . The SiO 2  film  70  is 110 μnm thick.  
     [0048] A process of fabricating such a photovoltaic conversion device of FIG. 4 will next be described. First, the Si layer  60  is formed on the surface of the Ge substrate  10  by plasma CVD method or the like. Then, the SiO 2  film  70  is formed on the front face side of the Si layer  60 . The protective film  40  is then formed on the back-face side of the Ge substrate  10 . Next, the portion of the protective film where p + -layer  20  and n + -layer  22  are to be formed is removed by photolithographic technique. Then, predetermined p + -layer  20  and n + -layer  22  are formed by means of thermal diffusion method, ion implantation method, or the like. Finally, electrode patterns  24  and  26  are formed.  
     [0049] With the photovoltaic conversion device constructed as shown in FIG. 4, a protective film (SiO 2  film)  70  having fewer interface defects is formed on the front face of Si layer  60 . Therefore, recombination loss on the front face side can be reduced to a greater extent than when the protective film is formed directly on the Ge surface. The number of defects present on the Ge surface can be thus decreased using Si layer  60  and SiO 2  film  70 . In this manner, this construction can take advantage of the small energy band gap of Ge to convert infra red radiation emitted by a light emitter efficiently into electricity.  
     [0050]FIG. 5 is a cross sectional view showing a photovoltaic conversion device according to a fifth embodiment of the present invention. In FIG. 5, the same elements as in FIG. 4 are denoted by the same reference numerals, and explanations thereof are not repeated. The construction shown in FIG. 5 differs from that in FIG. 4 in that, in the construction in FIG. 5 as compared to that in FIG. 4, hydrogen or halogen is contained in the interface  72  between the Si layer  60  and the SiO 2  film  70 . Similarly, hydrogen or halogen is contained also in the interface  62  between the Ge substrate  10  and the Si layer  60 , and in the interface  12  between the Ge substrate  10  and the protective film  40 . The same fabrication method as described with reference to FIG. 2 may be employed to cause hydrogen or halogen to be present in this manner, and the presence of hydrogen or halogen provides the same operational effect as that described with reference to FIG. 2.  
     [0051]FIG. 6 is a cross sectional view showing a photovoltaic conversion device according to a sixth embodiment of the present invention. In FIG. 6, the same elements as in FIG. 4 are denoted by the same reference numerals, and explanations thereof are not repeated. The construction as shown in FIG. 6 differs from that in FIG. 4 in that, in the construction in FIG. 6 as compared to that in FIG. 4, a semiconductor layer  80  is formed on the front face of the Si layer  60 , and that a semiconductor layer  50  is formed on the front face of the Ge substrate  10 . Thus, as shown in FIG. 6, a semiconductor layer  80  having an impurity concentration higher than that of the Si layer  60  is provided between the Si layer  60  and the SiO 2  film  70 , and a semiconductor layer  50  having impurity concentration higher than that of the Ge substrate  10  is provided between the Ge substrate  10  and the Si layer  60 . These semiconductor layers  50  and  80  may be formed using the same fabrication method as described with reference to FIG. 3, and the presence of these layers provides the same operational effect as described with reference to FIG. 3.  
     [0052]FIG. 7 is a cross sectional view showing a photovoltaic conversion device according to a seventh embodiment of the present invention. In FIG. 7, the same elements as in FIG. 4 are denoted by the same reference numerals, and explanations thereof are not repeated. The construction as shown in FIG. 7 differs from that in FIG. 4 in that, in the construction in FIG. 7 as compared to that in FIG. 4, a mixed layer of Ge and Si, that is, an intermediate layer  90 , is provided between the Ge substrate  10  and the Si layer  60 .  
     [0053] A fabrication method for fabricating the photovoltaic conversion device of FIG. 7 will next be described. First, a mixed layer  90  of Si and Ge is formed on the surface of the Ge substrate  10  using a plasma CVD method or the like. When forming this layer  90 , the mixing ratio of Si and Ge is varied continuously in the intermediate layer  90  by adjusting the proportion of raw material gases for Si and Ge so as to cause Ge to be more abundant near the Ge substrate and Si to be more abundant near the front face. Then, the Si layer  60  is formed on the surface of the intermediate layer by plasma CVD method or the like. A protective film  40  is then formed on the back-face of the Ge substrate  10 . Next, the portion of the protective film where p + -layer  20  and n + -layer  22  are to be formed is removed by photolithographic technique. Then, predetermined p + -layer  20  and n + -layer  22  are formed by means of a thermal diffusion method, an ion implantation method, or the like. Finally, electrode patterns  24  and  26  are formed.  
     [0054] With the photovoltaic conversion device having the construction as shown in FIG. 7, since the intermediate layer  90  is provided in which mixing ratio of Si and Ge varies continuously, the energy band gap for the region between the Ge substrate  10  and the Si layer  60  varies continuously so that discontinuity of band (knotch or gap) formed in the hetero junction is relaxed to a great extent. Since the discontinuity of band that forms a barrier for the movement of carriers produced in the Si layer  60  toward the Ge substrate  10  is relaxed, recombination loss of carriers is reduced, which results in improvement of the performance (photovoltaic conversion efficiency). Therefore, efficiency of the TPV power generation apparatus is improved, leading to increased power production.  
     [0055] Above described embodiments are based on the Ge substrate. On the other hand, Si substrates are widely used in semiconductor devices, and Si is abundant as a natural resource, and less expensive than Ge. However, as discussed before, Si cannot convert infra red radiation emitted by a light emitter in a TPV system efficiently into electric energy. A photovoltaic conversion device using Si substrate will be described in the following.  
     [0056]FIG. 8 is a cross sectional view showing a photovoltaic conversion device according to a eighth embodiment of the present invention. In the photovoltaic conversion device shown in FIG. 8, a Ge layer  110  is formed on the back-face side of a Si substrate  160  that is a Si layer. On the back-face side of the Ge layer  110 , a p + -layer  20  as a p-type semiconductor layer and a n + -layer  22  as a n-type semiconductor layer are formed alternately and independently of each other to collect carriers. On the back-face side of the Ge layer  110 , a positive electrode  24  connected to the p + -layer  20  and a negative electrode  26  connected to the n + -layer  22  are provided to realize a back-face electrode type structure. A SiO 2  film  70  is provided on the front face side of the Si substrate  160 . A protective film  40  is provided on the back-face side of the Ge layer  110  except where the semiconductor layers  20  and  22  are connected, respectively, to electrodes  24  and  26 .  
     [0057] The structure of the photovoltaic conversion device shown in FIG. 8 will now be described more specifically, The Si substrate  160  is, for example, 200 μm thick and forms p-type semiconductor with dopant concentration of 1×10 15 cm −3 . The Ge layer  110  is 10 μm thick and forms p-type semiconductor with dopant concentration of 3×10 15 cm −3 . The p + -layer  20  has dopant concentration of 1×10 19 cm −3  and diffusion depth of 2 μm. Similarly, n + -layer  22  has dopant concentration of 1×10 19 cm −3  and diffusion depth of 2 μm. The SiO 2  film  70  is 110 μm thick.  
     [0058] A fabrication method for fabricating the photovoltaic conversion device of FIG. 8 will next be described. First, the SiO 2  film  70  is formed on the front face of the Si substrate  160 . Then, the Ge layer  110  is formed on the back-face side of the Si substrate  160  using a plasma CVD method or the like. The protective film  40  is then formed on the back-face of Ge layer  110 . Then, the portion of the protective film where p + -layer  20  and n + -layer  22  are to be formed is removed by means of a photolithographic technique. Then, predetermined p + -layer  20  and n + -layer  22  are formed by means of a thermal diffusion method, an ion implantation method, or the like. Finally, electrode patterns  24  and  26  are formed.  
     [0059] In the photovoltaic conversion device shown in FIG. 8, the Ge layer  110  is provided on the back-face side of the Si substrate  160  so that infrared radiation can be converted efficiently to electricity. By fabricating the back-face electrode type photovoltaic conversion device so as to include a Ge layer on the back-face side of the Si substrate, it is possible to realize a photovoltaic conversion device, suitable for use in TPV power generation, at low cost.  
     [0060]FIG. 9 is a cross sectional view showing a photovoltaic conversion device according to a ninth embodiment of the present invention. In FIG. 9, the same elements as in FIG. 8 are denoted by the same reference numerals, and explanations thereof are not repeated. The construction as shown in FIG. 9 differs from that in FIG. 8 in that, in the construction in FIG. 9 as compared to that in FIG. 8, hydrogen or halogen is contained in the interface  72  between the Si substrate  160  and the SiO 2  film  70 . Similarly, hydrogen or halogen is contained in the interface  162  between the Ge layer  110  and the Si substrate  160  and in the interface  112  between the Ge layer  110  and the protective film  40 . The same fabrication method as described with reference to FIG. 2 may be employed to cause hydrogen or halogen to be present in this manner, and the presence of hydrogen or halogen provides the same operational effect as described with reference to FIG. 2.  
     [0061]FIG. 10 is a cross sectional view showing a photovoltaic conversion device according to a tenth embodiment of the present invention. In FIG. 10, the same elements as in FIG. 8 are denoted by the same reference numerals, and explanations thereof are not repeated. The construction as shown in FIG. 10 differs from that in FIG. 8 in that, in the construction in FIG. 10 as compared to that in FIG. 8, a semiconductor layer  80  is formed on the front face of Si substrate  160  and that a semiconductor layer  50  is formed on the front face of the Ge layer  110 . Thus, a semiconductor layer  80  that has an impurity concentration higher than Si substrate is provided between the Si substrate  160  and the SiO 2  film  70 , and a semiconductor layer  50  that has an impurity concentration higher than the Ge layer  110  is provided between the Ge layer  110  and the Si substrate  160 . These semiconductor layers  50  and  80  may be formed using the same fabrication method as described with reference to FIG. 3, and the presence of these layers provides the same operational effect as described with reference to FIG. 3.  
     [0062]FIG. 11 is a cross sectional view showing a photovoltaic conversion device according to an eleventh embodiment of the present invention. In FIG. 11, the same elements as in FIG. 8 are denoted by the same reference numerals, and explanations thereof are not repeated. The construction as shown in FIG. 11 differs from that in FIG. 8 in that, in the construction in FIG. 11 as compared to that in FIG. 8, an intermediate layer  90 , that is, a mixed layer of Ge and Si, is provided between the Ge layer  110  and the Si substrate  160 . Such an intermediate layer  90  is formed by the same fabrication method as described with reference to FIG. 7, and its presence provides the same operational effect as described with reference to FIG. 7.  
     [0063]FIG. 12 is a cross sectional view showing a photovoltaic conversion device according to a twelfth embodiment of the present invention. This construction shows the most excellent balance between cost and performance. On the front face side of the Si substrate  160 , a p + -semiconductor layer  80  and an SiO 2  film  70  are formed. On the back-face side, an Si-Ge intermediate layer  90  is provided, followed by the formation of a Ge-p + -semiconductor layer  50  and the Ge layer  110 . Then, p + -layer  20  and n + -layer  22  for collecting carriers as well as electrode patterns  24  and  26 , are provided.  
     [0064] As described before, the intermediate layer  90  serves to improve the mobility of carriers. The p + -semiconductor layers  80  and  50  that are provided on the front face side of the Si substrate  160  and on the front face side of the Ge layer  110 , respectively, serve to prevent carriers from diffusing toward the surface where many defects are present, and being annihilated there, as described before. Hydrogen or halogen is contained in each of the interfaces  112 ,  92 ,  162 , and  72 , and serves to decrease the number of defects in the interfaces, as described before, thereby leading to a reduction in the recombination loss.  
     [0065] By adopting the construction as described above, a back-face electrode type photovoltaic conversion device can be formed. With such a photovoltaic conversion device, energy conversion efficiency in a TPV power generation apparatus using a light emitter with high photon flux can be improved at low cost.  
     [0066] As has been discussed, according to the present invention, a photovoltaic conversion device is provided which uses Ge that is suited to the TPV power generation application as main material, and employs back-face electrode type as the electrode structure, and which has the device construction that enables the recombination loss of carriers at the surface to be reduced greatly.  
     [0067] The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiment is therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.