Patent Publication Number: US-2011061716-A1

Title: Photovoltaic device and method for manufacturing the same

Description:
CROSS REFERENCE TO RELATED PATENT APPLICATIONS 
     The present application claims priority under 35 U.S.C. §119 to Korean Patent Application Serial Number 10-2009-0085718 filed on Sep. 11, 2009, the entirety of which is hereby incorporated by reference. 
     FIELD OF THE INVENTION 
     This embodiment relates to a photovoltaic device and a method for manufacturing the same. 
     BACKGROUND OF THE INVENTION 
     Recently, because of high oil prices and the global warming phenomenon based on a large amount of CO2 emissions, energy is becoming the most important issue in determining the future life of mankind. Even though many technologies using renewable energy sources including wind force, bio-fuels, hydrogen/fuel cells and the like have been developed, a photovoltaic device using sunlight is in the spotlight. This is because solar energy, the origin of all energies, is an almost infinite clean energy source. 
     The sunlight incident on the surface of the earth has an electric power of 120,000 TW. Thus, theoretically, a photovoltaic device having a photoelectric conversion efficiency of 10% and covering only 0.16% of the land surface of the earth is capable of generating 20 TW of electric power, which is twice as much as the amount of energy globally consumed during one year. 
     Practically, the world photovoltaic market has grown by almost a 40% annual growth rate for the last ten years. Now, a bulk-type silicon photovoltaic device occupies 90% of the photovoltaic device market share. The bulk-type silicon photovoltaic device includes a single-crystalline silicon photovoltaic device and a multi-crystalline or a poly-crystalline silicon photovoltaic device and the like. However, productivity of a solar-grade silicon wafer which is the main material of the photovoltaic device is not able to fill the explosive demand thereof, so the solar-grade silicon wafer is globally in short supply. Therefore, this shortage of the solar-grade silicon wafer is a huge threatening factor in reducing the manufacturing cost of a photovoltaic device. 
     Contrary to this, a thin-film silicon photovoltaic device including a light absorbing layer based on a hydrogenated amorphous silicon (a-Si:H) allows a reduction of thickness of a silicon layer equal to or less than 1/100 as large as that of a silicon wafer of the bulk-type silicon photovoltaic device. Also, it makes possible to manufacture a large area photovoltaic device at a lower cost. 
     Meanwhile, a single-junction thin-film silicon photovoltaic device is limited in its achievable performance. Accordingly, a double junction thin-film silicon photovoltaic device or a triple junction thin-film silicon photovoltaic device having a plurality of stacked unit cells has been developed, pursuing high stabilized efficiency. 
     The double junction or the triple junction thin-film silicon photovoltaic device is referred to as a tandem-type photovoltaic device. The open circuit voltage of the tandem-type photovoltaic device corresponds to a sum of each unit cell&#39;s open circuit voltage. Short circuit current is determined by a minimum value among the short circuit currents of the unit cells. 
     Regarding the tandem-type photovoltaic device, research is being devoted to an intermediate reflector which is capable of improving efficiency by enhancing internal light reflection between the unit cells. 
     SUMMARY OF THE INVENTION 
     One aspect of this invention is a method for manufacturing a photovoltaic device. The method comprising: forming a first electrode on a substrate; forming a first unit cell on the first electrode, the first unit cell comprising an intrinsic semiconductor layer; forming an intermediate reflector on the first unit cell, the intermediate reflector comprises a plurality of sub-layers stacked alternately by modulating the applied voltages in accordance with time, the applied voltages exciting plasma and having mutually different frequencies; forming a second unit cell on the intermediate reflector, the second unit cell comprising an intrinsic semiconductor layer; and forming a second electrode on the second unit cell. 
     Another aspect of this invention is a photovoltaic device. The device comprises: a substrate; a first electrode placed on the substrate; a first unit cell placed on the first electrode and comprising an intrinsic semiconductor layer; an intermediate reflector placed on the first unit cell, and comprising a plurality of sub-layers stacked alternately and having different crystal volume fractions from each other by modulating the applied voltages in accordance with time, the applied voltages exciting plasma and having mutually different frequencies; a second unit cell placed on the intermediate reflector and comprising an intrinsic semiconductor layer; and a second electrode placed on the second unit cell. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1   a  to  1   g  show a method for manufacturing a photovoltaic device according to an embodiment of the present invention. 
         FIG. 2  shows a plasma-enhanced chemical vapor deposition apparatus for forming an intermediate reflector in accordance with the embodiment of the present invention. 
         FIGS. 3 and 4  show frequency variations of a first power source and seqond power source which are supplied to a reaction chamber so as to form the intermediate reflector in accordance with the embodiment of the present invention. 
         FIG. 5  shows the intermediate reflector included in the embodiment of the present invention. 
         FIG. 6  shows a photovoltaic device according to another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A method for manufacturing a photovoltaic device according to an embodiment of the present invention will be described with reference to the drawings.  FIGS. 1   a  to  1   g  show a method for manufacturing a photovoltaic device according to an embodiment of the present invention. 
     As shown in  FIG. 1   a , a substrate  100  is provided. The substrate  100  may include an insulating transparent substrate and insulating opaque substrate. The insulating transparent substrate may be included in a p-i-n type photovoltaic device. The insulating opaque substrate may be included in an n-i-p type photovoltaic device. The p-i-n type photovoltaic device and n-i-p type photovoltaic device will be described later in detail. 
     As shown in  FIG. 1   b , a first electrode  210  is formed on the substrate  100 . In the embodiment of the present invention, the first electrode  210  can be formed by a chemical vapor deposition (CVD) method and composed of transparent conductive oxide (TCO) such as Tin dioxide (SnO 2 ) or Zinc Oxide (ZnO). 
     As shown in  FIG. 1   c , a laser beam is irradiated onto the first electrode  210  or substrate  100  so that the first electrode  210  is scribed. As a result, a first separation groove  220  is formed on the first electrode  210 . That is, since the first separation groove  220  penetrates the first electrode  210 , the first electrodes  210  adjacent thereto are prevented from being short-circuited therebetween. 
     As shown in  FIG. 1   d , a first unit cell  230  is stacked on the first electrode  210  by a CVD method. Here, the first unit cell  230  includes a p-type semiconductor layer, an intrinsic semiconductor layer, and an n-type semiconductor layer. When source gas including silicon, such as SiH 4 , and doping gas including group 3 elements, such as B 2 H 6 , are injected together into a reaction chamber in order to form the p-type semiconductor layer, the p-type semiconductor layer is formed by a CVD method. After that, the intrinsic semiconductor layer is formed on the p-type semiconductor layer by the CVD method after source gas including silicon is introduced into the reaction chamber. Doping gas including group 5 elements, such as PH 3 , and source gas including silicon are injected together, and then the n-type semiconductor layer is stacked on the intrinsic semiconductor layer by the CVD method. As a result, the p-type semiconductor layer, intrinsic semiconductor layer, and n-type semiconductor layer are sequentially stacked on the first electrode  210 . 
     In an embodiment of the present invention, the p-type semiconductor layer, intrinsic semiconductor layer and n-type semiconductor layer may be sequentially stacked. Otherwise, the n-type semiconductor layer, intrinsic semiconductor layer and p-type semiconductor layer are sequentially stacked. 
     As shown in  FIG. 1   e , an intermediate reflector  235  is formed on the n-type semiconductor layer or p-type semiconductor layer of the first unit cell  230  through a plasma-enhanced chemical vapor deposition method. Non-silicon based source gas, n-type doping gas and source gas, including silicon, are introduced into the reaction chamber in order to form the intermediate reflector  235 . The non-silicon based source gas includes oxygen source gas, carbon source gas, or nitrogen source gas. 
     As described in more detail below, a voltage alternately changing between a first frequency f 1  and second frequency f 2  is supplied to the reaction chamber so as to form the intermediate reflector  235 . Here, a power source having the first frequency f 1  and a power source having the second frequency t 2  may supply a voltage alternately. Otherwise, a voltage changing between the first frequency f 1  and second frequency t 2  may be supplied by one power source. A first voltage having the first frequency f 1  is continuously supplied, and second voltage having the second frequency f 2  higher than the first frequency f 1  is alternately supplied. As a result, the intermediate reflector  235  according to the embodiment of the present invention has a multilayer structure and includes a hydrogenated n-type nano-crystalline silicon oxide (n-nc-SiO:H), hydrogenated n-type nano-crystalline silicon carbide (n-nc-SiC:H), or hydrogenated n-type nano-crystalline silicon nitride (n-nc-SiN:H). The intermediate reflector  235  will be described later in more detail. 
     As shown in  FIG. 1   f , a second unit cell  240  including the p-type semiconductor layer, intrinsic semiconductor layer, and n-type semiconductor layer is formed on the intermediate reflector  235 . If the first unit cell  230  includes the n-type semiconductor layer, intrinsic semiconductor layer, and p-type semiconductor layer which are stacked in the order listed, the second unit cell  240  also includes the n-type semiconductor layer, intrinsic semiconductor layer, and p-type semiconductor layer which are stacked in the order listed. 
     As shown in  FIG. 1   g , after a second separation groove  260  penetrating the first unit cell  230 , intermediate reflector  235  and second unit cell  240  is formed, a second electrode  250  is formed on the second unit cell  240  such that the second separation groove  260  is filled. 
     The embodiment of the present invention shown in  FIGS. 1   a  to  1   g  may include a double junction photovoltaic device composed of two unit cells or a triple junction photovoltaic device composed of three unit cells. 
     Next, a method for forming the intermediate reflector  235  will be described in detail with reference to the drawings.  FIG. 2  shows a plasma-enhanced chemical vapor deposition apparatus for forming an intermediate reflector according to an embodiment of the present invention. As shown in  FIG. 2 , the substrate  100  on which the first electrode  210  and first unit cell  230  are formed is placed on a plate  300  functioning as an electrode. 
     The first unit cell  230  may include the p-type semiconductor layer, intrinsic semiconductor layer, and n-type semiconductor layer. Here, the n-type semiconductor layer may include a hydrogenated n-type nano-crystalline silicon (n-nc-Si:H), and the source gas for forming the n-type nano-crystalline silicon may include silane (SiH 4 ), hydrogen (H 2 ) or phosphine (PH 3 ). 
     After the n-type semiconductor layer including a hydrogenated n-type nano-crystalline silicon is formed, the non-silicon based source gas, such as oxygen source gas, carbon source gas, or nitrogen source gas, is introduced into the reaction chamber  310  in a state where the flow rate, substrate temperature, and process pressure of the source gas introduced into the reaction chamber  310  are maintained. 
     Here, since the non-silicon based source gas is introduced into the reaction chamber  310  by keeping the flow rate, substrate temperature, and process pressure of the source gas in the reaction chamber  310 , the n-type semiconductor layer of the first unit cell  230  and the intermediate reflector  235  can be formed in the same reaction chamber  310 . The method of forming the n-type semiconductor layer and intermediate reflector  235  in the same reaction chamber  310  can be applied not only to the p-i-n type photovoltaic device according to the embodiment of the present invention but also to the n-i-p type photovoltaic device. 
     As shown in  FIG. 2 , the source gases such as hydrogen (H 2 ), silane (SiH 4 ), or phosphine (PH 3 ) are introduced into the reaction chamber  310  through mass flow controllers MFC 1 , MFC 2 , and MFC 3  and an electrode  340  having nozzles formed therein. The non-silicon based source gas is introduced into the reaction chamber  310  through the mass flow controller (MFC 4 ) and nozzle of the electrode  340 . When the non-silicon based source gas is oxygen source gas, the oxygen source gas may include oxygen or carbon dioxide. When the non-silicon based source gas is carbon source gas, the carbon source gas may include CH 4 , C 2 H 4 , or C 2 H 2 . When the non-silicon based source gas is nitrogen source gas, the nitrogen source gas may include NH 4 , N 2 O, or NO. Here, an angle valve  330  is controlled to maintain the pressure of the reaction chamber  310  constant. When the pressure of the reaction chamber  310  is maintained constant, production of the silicon powder due to turbulence generation in the reaction chamber  310  is prevented and the deposition condition is maintained constant. The hydrogen is introduced in order to dilute the silane and the reduces Staebler-Wronski effect. 
     When the non-silicon based source gas is introduced with the source gases and when a first power source E 1  and second power source E 2  supply a first voltage and second voltage respectively, an electrical potential difference between the electrode  340  and plate  300  makes the gases in the reaction chamber  310  change into a plasma state and then be deposited on the hydrogenated n-type nano-crystalline silicon of the first unit cell  230 . As a result, an intermediate reflector  235  is formed. 
     When oxygen source gas is introduced, the intermediate reflector  235  includes a hydrogenated n-type nano-crystalline silicon oxide (n-nc-SiO:H). When carbon source gas is introduced, the intermediate reflector  235  includes a hydrogenated n-type nano-crystalline silicon carbide (n-nc-SiC:H). When nitrogen source gas is introduced, the intermediate reflector  235  includes a hydrogenated n-type nano-crystalline silicon nitride (n-nc-SiN:H). As such, since the intermediate reflector  235  includes the hydrogenated n-type nano-crystalline silicon based material similar to the hydrogenated n-type nano-crystalline silicon of a unit cell closest to the light incident side, the intermediate reflector  235  can be easily joined with the unit cell which is closest to the light incident side. 
       FIGS. 3 and 4  show frequency variations of the first power source E 1  and second power source E 2  which are supplied to a reaction chamber  310  so as to form the intermediate reflector in accordance with the embodiment of the present invention. In an embodiment of the present invention, the flow rates of hydrogen, silane and non-silicon based source gas which are introduced into the reaction chamber are constant in accordance with the elapsed deposition time T. 
     As shown in  FIG. 3 , the first power source E 1  and second power source E 2  respectively supply the first voltage having the first frequency f 1  and the second voltage having the second frequency f 2  in an alternating manner. During one cycle, derived from a sum of a duration time t 1  for supplying the first voltage and a duration time t 2  for supplying the second voltage, a ratio of duration time t 1  for supplying the first voltage having the first frequency f 1  to duration time t 2  for supplying the second voltage having the first frequency  12  is constant in accordance with the elapsed time. As a result, the intermediate reflector  235  includes at least one pair of a first sub-layer and second sub-layer, wherein the thickness ratio between the first sub-layer and second sub-layer in each of the pairs is constant. 
     As shown in  FIG. 4 , the first power source E 1  continuously supplies a voltage having the first frequency f 1  in accordance with the deposition time T. The second power source E 2  discontinuously supplies a voltage having the second frequency  12 . That is, the second power source E 2  repeatedly supplies and stops supplying the voltage. Here, a ratio of a duration time t 2  for supplying the second voltage having the second frequency f 2  to a duration time for discontinuing the supply of the second voltage having the second frequency f 2 , i.e., the duration time t 1  for supplying only the first voltage, is constant in each cycle. As a result, the intermediate reflector  235  includes at least one pair of a first sub-layer and second sub-layer, wherein the thickness ratio between the first sub-layer and second sub-layer in each of the pairs is constant. The first sub-layer and second sub-layer of the intermediate reflector  235  will be described later in detail. 
     As shown in  FIGS. 3 and 4 , when the first voltage and second voltage, which have mutually different frequencies, are supplied, as shown in  FIG. 5 , the intermediate reflector  235  including a plurality of sub-layers  235   a  and  235   b  is formed on the n-type semiconductor layer of the first unit cell  230 . As such, since the flow rate A of hydrogen and the flow rate B of silane remain constant in accordance with the elapsed deposition time T, the hydrogen dilution ratio, i.e., a ratio of the flow rate of hydrogen to the flow rate of silane, is constant. 
     The sub-layers  235   a  and  235   b  of the intermediate reflector  235  are composed of a hydrogenated n-type nano-crystalline silicon based sub-layer  235   b  including crystalline silicon grains and a hydrogenated n-type nano-crystalline silicon based sub-layer  235   a . The hydrogenated n-type nano-crystalline silicon based material included in the plurality of sub-layers  235   a  and  235   b  is produced during a phase transition from an amorphous silicon based material to a crystalline silicon based material. Hereinafter, the hydrogenated n-type nano-crystalline silicon based sub-layer is referred to as the first sub-layer  235   a , and the hydrogenated n-type nano-crystalline silicon based sub-layer including crystalline silicon grains is referred to as the second sub-layer  235   b.    
     While crystallinity and deposition rate decrease as the frequency of the voltage supplied to the reaction chamber decreases, the crystallinity and deposition rate increase as the frequency of the voltage supplied to the reaction chamber increases. As a result, as shown in  FIG. 3  to  FIG. 4 , the first sub-layer  235   a , i.e., the hydrogenated n-type nano-crystalline silicon based sub-layer, is formed during the supply of a voltage having the first frequency f 1 , and the second sub-layer  235   b , i.e., the hydrogenated n-type nano-crystalline silicon based sub-layer including the crystalline silicon grains, is formed during the supply of a voltage having the second frequency t 2 , wherein f 2  is a higher frequency than the first frequency f 1 . 
     The crystalline silicon grains of the second sub-layer  235   a  change a crystal volume fraction of the second sub-layer  235   b , and the non-silicon based source gas changes a refractive index thereof. That is, the crystal volume fraction of the first sub-layer  235   a  formed at the duration time of supplying a voltage having the first frequency f 1  is less than that of the second sub-layer  235   b  formed at the duration time of supplying a voltage having the second frequency f 2 , wherein f 2  is a higher frequency than the first frequency f 1 . The crystal volume fraction is a ratio of a volume occupied by crystal to the unit volume. 
     As a result, when a voltage having the first frequency f 1  and a voltage having the second frequency f 2  are supplied in an alternating manner as shown in  FIG. 3 , or when the voltage having the first frequency f 1  is continuously supplied and the voltage having the second frequency  12  is repeatedly supplied and discontinued as shown in  FIG. 4 , the first sub-layer  235   a  and second sub-layer  235   b  include a hydrogenated n-type nano-crystalline silicon oxide (n-nc-SiO:H), and the second sub-layer  235   b  includes the crystalline silicon grains surrounded by a hydrogenated n-type nano-crystalline silicon oxide. 
     When the non-silicon based source gas, such as carbon source gas, is supplied, the first sub-layer  235   a  and second sub-layer  235   b  include a hydrogenated n-type nano-crystalline silicon carbide (n-nc-SiC:H), and the second sub-layer  235   b  includes the crystalline silicon grains surrounded by a hydrogenated n-type nano-crystalline silicon carbide. When the non-silicon based source gas, such as nitrogen source gas, is supplied, the first sub-layer  235   a  and second sub-layer  235   b  include a hydrogenated n-type nano-crystalline silicon nitride (n-nc-SiN:H), and the second sub-layer  235   b  includes the crystalline silicon grains surrounded by a hydrogenated n-type nano-crystalline silicon nitride. 
     As such, since the sub-layers  235   a  and  235   b  having the mutually different crystal volume fractions or mutually different refractive indexes are alternatively stacked, and each sub-layer  235   a  and  235   b  functions as a waveguide, it is possible to maximize the reflection of light by the intermediate reflector  235 . Here, the second sub-layer  235   b  has a crystal volume fraction greater than that of the first sub-layer  235   a . Simply put, the second sub-layer  235   b  having the crystalline silicon grains has a vertical electrical conductivity greater than that of the first sub-layer  235   a . Accordingly, the intermediate reflector  235  allows an electric current to easily flow between the first unit cell  230  and the second unit cell  240 . 
     The refractive index of the second sub-layer  235   b  including the crystalline silicon grains is greater than that of the first sub-layer  235   a . Therefore, since the first sub-layer  235   a , having a refractive index lower than that of the second sub-layer  235   b , matches the refractive index with the unit cell closest to the light incident side, the first sub-layer  235   a  increases the reflection of light having a short wavelength which has high energy density, for example, light with a wavelength from 500 nm to 700 nm. 
     The diameter of the crystalline silicon grains of the second sub-layer  235   b  may be greater than or equal to 3 nm and less than or equal to 10 nm. Forming of the crystalline silicon grains having a diameter less than 3 nm decreases the vertical electrical conductivity. When the diameter of the crystalline silicon grains is greater than 10 nm, grain boundary surrounding the crystalline silicon grains has an excessively increased volume. Therefore, carrier recombination also increases and so efficiency may be decreased. 
     Meanwhile, as mentioned above, the hydrogen dilution ratio and pressure inside the chamber  310  are constant in the embodiments of the present invention. The flow rates of the hydrogen, silane and non-silicon based source gas which are supplied to the chamber  310  are constant. As a result, a possibility occurring of the turbulences of the hydrogen, silane and non-silicon based source gas in the chamber  310  is reduced, so that the film quality of the intermediate reflector  235  is improved. 
     Meanwhile, as described above, the plasma-enhanced chemical vapor deposition method is used instead of the photo-CVD in the embodiments of the present invention. Regarding the photo-CVD, not only it is not appropriate for manufacturing of the large area photovoltaic device, but also the UV light penetrating through a quartz window of the photo-CVD device decreases since a thin film is deposited on the quartz window as the deposition progresses. Since the deposition rate thereof gradually decreases, the thicknesses of the first sub-layer  235   a  and second sub-layer  235   b  gradually decrease. On the other hand, such weaknesses of the photo-CVD may be overcome by the plasma-enhanced chemical vapor deposition method. 
     In the plasma-enhanced chemical vapor deposition method used in the embodiment of the present invention, frequencies of voltages supplied from the first power source E 1  and second power source E 2  may be equal to or more than 13.56 MHz. When the frequency of the voltage is equal to or more than 13.56 MHz, the deposition rate of the intermediate reflector  235  is increased. When the second frequency  12  is equal to or more than 27.12 MHz, the deposition rate increases and the crystalline silicon grains can be easily formed. 
     In an embodiments of the present invention, the thickness of the intermediate reflector  235  may be greater than or equal to 30 nm and less than or equal to 200 nm. When the thickness of the intermediate reflector  235  is greater than or equal to 30 nm, the refractive index match between the unit cell closest to the light incident side and the intermediate reflector  235  is obtained and the internal reflection can easily occur. When the thickness of the intermediate reflector  235  is less than or equal to 200 nm, the excessive light absorption by the intermediate reflector  235  itself caused by the thickness increase thereof is prevented. 
     The thicknesses of the first sub-layer  235   a  and second sub-layer  235   b  may be greater than or equal to 10 nm and less than or equal to 50 nm. When the thicknesses of the first sub-layer  235   a  and second sub-layer  235   b  are greater than or equal to 10 nm, the refractive index is matched and the crystalline silicon grains can be sufficiently formed. Further, when the thickness of the first sub-layer  235   a  or second sub-layer  235   b  is greater than 50 nm, the number of sub-layers included in the intermediate reflector  235  may decrease due to the large thickness. As a result, the internal reflection by the intermediate reflector  235  may be decreased. Therefore, when the thicknesses of the first sub-layer  235   a  and second sub-layer  235   b  are less than or equal to 50 nm, the appropriate number of sub-layers may be included in the intermediate reflector  235  and so the light can be easily reflected. 
     As mentioned above, the number of the sub-layers included in the intermediate reflector  235  can be greater than or equal to three in that the thickness of the intermediate reflector  235  is greater than or equal to 30 nm and less than or equal to 200 nm and the thicknesses of the first sub-layer  235   a  and second sub-layer  235   b  are greater than or equal to 10 nm and less than or equal to 50 nm. 
     Meanwhile, the refractive index of the intermediate reflector  235  including the first sub-layer  235   a  and second sub-layer  235   b  may be greater than or equal to 1.7 and less than or equal to 2.2. When the refractive index of the intermediate reflector  235  is greater than or equal to 1.7, the vertical electrical conductivity of the intermediate reflector  235  is increased and a fill factor (FF) of a multiple junction photovoltaic device is improved. As a result, the efficiency is increased. When the refractive index of the intermediate reflector  235  is less than or equal to 2.2, light of a wavelength from 500 nm to 700 nm is easily reflected and the short circuit current of the first unit cell  230  increases. As a result, the efficiency is increased. 
     The average content of the non-silicon based element contained in the intermediate reflector  235  from the non-silicon based source gas may be greater than or equal to 10 atomic % and less than or equal to 30 atomic %. In the embodiment of the present invention, the non-silicon based source gas may be oxygen, carbon, or nitrogen. When the average content of the non-silicon based element is greater than or equal to 10 atomic %, the refractive index match between the unit cell closest to the light incident side and the intermediate reflector  235  is achieved and the internal reflection can easily occur. Further, when the average content of the non-silicon based element is unnecessarily large, the vertical electrical conductivity of the sub-layers may deteriorate since the crystal volume fraction thereof decreases. Therefore, in the embodiment of the present invention, when the average content of the non-silicon based element is less than or equal to 30 atomic %, the electrical conductivity is improved since the average crystal volume fraction of the intermediate reflector  235  is appropriately maintained and it prevents intermediate reflector  235  from getting amorphous. 
     The average hydrogen content of the intermediate reflector  235  may be greater than or equal to 10 atomic % and less than or equal to 25 atomic %. When the average hydrogen content of the intermediate reflector  235  is greater than or equal to 10 atomic %, the film quality of the intermediate reflector  235  is improved since the dangling bonds are passivated. When the average hydrogen content in the intermediate reflector  235  is unnecessarily large, the electrical conductivity of the intermediate reflector  235  decreases since the crystal volume fraction thereof becomes small. Therefore, when the average hydrogen content contained in the intermediate reflector  235  is less than or equal to 25 atomic %, the vertical electrical conductivity increases since it prevents the intermediate reflector  235  from getting amorphous caused by the decrease of the crystal volume fraction. 
     The average crystal volume fraction of the intermediate reflector  235  can be greater than or equal to 4% and less than or equal to 30%. When the average crystal volume fraction of the intermediate reflector  235  is greater than or equal to 4%, the tunnel junction property improves. When the average crystal volume fraction of the intermediate reflector  235  is less than 30%, degradation of the refractive index matching property is prevented since the content of the non-silicon based material is maintained. 
     Since the intermediate reflector  235  according to the embodiment of the present invention includes an n-type nano-crystalline silicon having a good vertical electrical conductivity, it may be substituted for an n-type semiconductor layer of the unit cell of the side from which light is incident. For example, the photovoltaic device according to the embodiment of the present invention includes a first unit cell including a p-type semiconductor layer and an intrinsic semiconductor layer, the intermediate reflector  235 , and a second unit cell including a p-type semiconductor layer, an intrinsic semiconductor layer, and an n-type semiconductor layer. When the intermediate reflector  235  is substituted for the n-type semiconductor layer of the unit cell of the side from which light is incident, it can reduce the manufacturing time and cost of the photovoltaic device. 
     In the case of the p-i-n type photovoltaic device on which light is incident through the first unit cell  230 , the intermediate reflector  235  may replace the n-type semiconductor layer of the first unit cell  230 . Regarding the n-i-p type photovoltaic device on which light is incident through the second unit cell  240 , the intermediate reflector  235  may replace the n-type semiconductor layer of the second unit cell  240 . 
     Although the p-i-n type photovoltaic device on which light is incident in the direction from the first unit cell  230  formed on the substrate  100  to the second unit cell  240  has been described in the embodiment of the present invention, the present invention may be applied to an n-i-p type photovoltaic device on which light is incident from the opposite side to the substrate  100 , that is, in the direction from the second unit cell  240  to the first unit cell  230 . 
     As shown in  FIG. 6 , regarding the n-i-p type photovoltaic device, light is incident from the opposite side of the substrate  100 , and the first unit cell  230 ′ having an n-type semiconductor layer  230   n ′, an intrinsic semiconductor layer  230   i ′, and a p-type semiconductor layer  230   p ′ sequentially stacked therein is formed on the first electrode  210 . The intermediate reflector  235 ′ is formed on the first unit cell  230 ′. The second unit cell  240 ′ having an n-type semiconductor layer  240   n ′, an intrinsic semiconductor layer  240   i ′, and a p-type semiconductor layer  240   p ′ sequentially stacked therein is formed on the intermediate reflector  235 ′. The second electrode  250  is formed on the second unit cell  240 ′. 
     The intermediate reflector  235 ′ is required to form a refractive index matching with the second unit cell  240 ′ of the side from which light is incident. The intermediate reflector  235 ′ contacts with the n-type semiconductor layer of the second unit cell  240 ′. Therefore, after forming the p-type semiconductor layer of the first unit cell  230 ′, the intermediate reflector  235 ′ including n-type nano-crystalline silicon based material is formed. Here, the intermediate reflector  235  includes a plurality of sub-layers in accordance with the frequency of the applied voltage. 
     Meanwhile, the photovoltaic device according to the embodiments of the present invention includes the intermediate reflector  235  so as to improve the efficiency of a tandem structure including a plurality of the unit cells. It is possible to provide even better efficiency by controlling the electric currents of the plurality of the unit cells in addition to introducing the intermediate reflector  235 . 
     In general, the operating temperature of the photovoltaic device is an important factor in designing current matching among the plurality of the unit cells of the photovoltaic device having a tandem structure. For example, a photovoltaic device installed in a region having high temperature or strong ultraviolet radiation is designed such that short circuit current of the photovoltaic device is determined by the short circuit current of the unit cell which is closest to the light incident side among the unit cells of the photovoltaic device. This is because the photovoltaic device having its short circuit current determined by the short circuit current of the unit cell which is closest to the light incident side has a low temperature coefficient (i.e., an efficiency degradation rate of the photovoltaic device according to temperature rise by 1° C.). That is, the temperature rise of the photovoltaic device has small influence on the efficiency degradation thereof. 
     On the other hand, a photovoltaic device installed in a region having low temperature or small amount of ultraviolet radiation is designed such that short circuit current of the photovoltaic device is determined by the short circuit current of the unit cell which is farthest from the light incident side among the unit cells of the photovoltaic device. Even though the photovoltaic device having its short circuit current determined by the short circuit current of the unit cell which is farthest from the light incident side has a high temperature coefficient (i.e., an efficiency degradation rate of the photovoltaic device according to a temperature rise by 1° C.), it has low degradation ratio. Since the photovoltaic device installed in a low temperature region is relatively less affected by the temperature coefficient, the photovoltaic device is designed such that the short circuit current of the photovoltaic device is determined by the short circuit current of the unit cell which is farthest from the light incident side. 
     A rated output (efficiency) of the photovoltaic device designed in this manner is measured indoors under standard test conditions (hereinafter, referred to as STC). The set of STC consists of the followings. 
     AM 1.5 (AIR MASS 1.5) 
     Irradiance: 1000 W·m 2    
     Photovoltaic cell Temperature: 25° C. 
     However, when a photovoltaic device is installed outdoors, it happens that the temperature of the photovoltaic device is higher than 25° C. In this case, due to the temperature coefficient of the photovoltaic device, the efficiency of the photovoltaic device becomes lower than the rated efficiency of the photovoltaic device measured under the STC. That is, when the photovoltaic device is operating, most of light energy absorbed by the photovoltaic device is converted into heat energy. An actual operating temperature of the photovoltaic device hereby easily becomes higher than 25° C., i.e., the photovoltaic cell temperature under the STC. Accordingly, the temperature coefficient of the photovoltaic device causes the efficiency of the photovoltaic device to be lower than the rated efficiency of the photovoltaic device measured under the STC. 
     Because of such problems, when current matching design in the photovoltaic device having a tandem structure is performed on the basis of 25° C., i.e., the temperature of the photovoltaic device according to the STC without considering the actual operating temperature thereof in the external environment, the photovoltaic device may not achieve a desired efficiency. 
     Accordingly, current matching design of the photovoltaic device according to the embodiment of the present invention is performed under a nominal operating cell temperature obtained in a standard reference environment which is similar to the actual condition under which the photovoltaic device is installed. The standard reference environment includes the followings. 
     Tilt angle of photovoltaic device: 45° from the horizon 
     Total irradiance: 800 W·m 2    
     Circumstance temperature: 20° C. 
     Wind speed: 1 m·s −1    
     Electric load: none (open state) 
     The nominal operating cell temperature corresponds to a temperature at which the photovoltaic device mounted on an open rack operates under the standard reference environment. The photovoltaic device is used in a variety of actual environments. Therefore, when designing the current matching of the photovoltaic device having a tandem structure that is performed under nominal operating cell temperature measured in the standard reference environment which is similar to the condition under the photovoltaic device is actually installed, it is possible to manufacture the photovoltaic device suitable for the actual installation environment. By controlling the thicknesses and optical band gaps of the i-type photoelectric conversion layers of the first unit cell  230 ′ and second unit cell  240 ′ such that the short circuit currents of the first unit cell  230 ′ and the second unit cell  240 ′ are controlled, the efficiency of the photovoltaic device may be enhanced. 
     For this reason, in the embodiment of the present invention, when the nominal operating cell temperature of the photovoltaic device is equal to or more than 35 degrees Celsius, the thickness and optical band gap of the i-type photoelectric conversion layer of one unit cell which is closest to the light incident side between the first unit cell  230 ′ and second unit cell  240 ′ is set such that the short circuit current of the one unit cell is equal to or less than that of the other unit cell. As a result, the short circuit current of the photovoltaic device according to the embodiment of the present invention is determined by the short circuit current of the unit cell which is closest to the light incident side. 
     As such, when the short circuit current of the unit cell which is closest to the light incident side is equal to or less than that of the other unit cell, the temperature coefficient becomes smaller. Therefore, although the actual temperature of the photovoltaic device becomes higher, electricity generation performance is decreased due to decreased efficiency. For example, when the photovoltaic device designed for making the short circuit current of one unit cell which is closest to the light incident side to be equal to or less than the short circuit current of the other unit cell is installed in a region having high temperature or strong ultraviolet rays of sunlight, including intensive short wavelength rays in a blue-color range, the temperature coefficient is small. Therefore, although the actual temperature of the photovoltaic device becomes higher, the electricity generation performance decreases due to decreased efficiency. 
     Contrary to this, when the nominal operating cell temperature of the photovoltaic device is less than and not equal to 35 degrees Celsius, the thicknesses and optical band gap of the i-type photoelectric conversion layer of one unit cell which is farthest from the light incident side between the first unit cell  230 ′ and second unit cell  240 ′ is set such that the short circuit current of the other unit cell which is closest to the light incident side is equal to or less than that of the one unit cell. In other words, when the nominal operating cell temperature of the photovoltaic device is less than and not equal to 35 degrees Celsius, the thickness and optical band gap of the i-type photoelectric conversion layer of one unit cell which is closest to the light incident side between the first unit cell  230 ′ and second unit cell  240 ′ is determined such that the short circuit current of the other unit cell is equal to or more than that of the one unit cell. 
     A resulting short circuit current of the photovoltaic device according to the embodiment of the present invention is hereby determined by the short circuit current of the unit cell which is farthest from the light incident side between the first unit cell and second unit cell. In this case, even though temperature coefficient of the photovoltaic device is high, degradation ratio of the photovoltaic device is reduced. Since the actual operating temperature of the photovoltaic device is relatively low, the electricity generation performance may be improved in that the performance improvement due to the low degradation ratio may overtake the performance deterioration due to the high temperature coefficient. Particularly, because the degradation rate in fill factor is small, the photovoltaic device has an excellent outdoor electricity generation performance in an environment having a circumference temperature lower than 25° C., i.e., the STC. 
     As described in the embodiment, regarding the photovoltaic device of which current matching design is performed under the nominal operating cell temperature, the short circuit current of the photovoltaic device can be measured under the STC. 
     The foregoing embodiments and advantages are merely exemplary and are not to be construed as limiting the present invention. The present teaching can be readily applied to other types of apparatuses. The description of the foregoing embodiments is intended to be illustrative, and not to limit the scope of the claims. Many alternatives, modifications, and variations will be apparent to those skilled in the art. In the claims, means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents but also equivalent structures.