Patent Publication Number: US-2017373213-A1

Title: Photovoltaic devices with improved n-type partner and methods for making the same

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention relates to photovoltaic devices. More particularly, the present invention relates to photovoltaic devices with a chalcogen absorber layer. 
     BACKGROUND OF THE INVENTION 
     Solar panels employ photovoltaic cells to generate current flow. When a photon hits a photovoltaic cell, the photon may be transmitted through, reflected off, or absorbed by the photovoltaic cell if the photon energy is higher than the material band gap value. This generates an electron-hole pair and sometimes heat, depending on the band structure. 
     A photovoltaic cell can be described in terms of its open circuit voltage (V oc ), short circuit current (J sc ) and fill factor (FF). Fill factor is the ratio of the maximum power point (P m ) divided by the open circuit voltage (V oc ) and short circuit current (J sc ): FF=P m /V oc J sc . The fill factor is directly affected by the values of the cell&#39;s series and shunt resistance. Increasing the shunt resistance (R sh ) and decreasing the series resistance (R s ) will lead to a higher fill factor, thus resulting in greater efficiency, and pushing the cells output power closer towards its theoretical maximum. 
     There are many different materials used to fabricate photovoltaic cells such as CIGS (copper indium gallium selenide), CZTS (copper zinc tin sulfide), or organic polymers. Elemental selenium is the first semiconductor material to be used in a photovoltaic device by Charles Fritts in 1873. However, the initial efficiency was below 1%. Over the years, the best Se cell reported to date has only reached an efficiency up to 5.1% with the structure of: Glass/TiO 2 /Se/Au. 
     SUMMARY OF THE INVENTION 
     In one aspect, the present invention provides a photovoltaic device. The photovoltaic device includes: a transparent substrate; a transparent conductive electrode layer disposed on the transparent substrate; an n-type layer of a compound having the formula Zn 1-x Mg x O, wherein 0&lt;x≦1, disposed on the transparent conductive electrode layer; a chalcogen absorber layer disposed on the n-type layer; and a conductive layer disposed on the interlayer. 
     In another aspect, the present invention provides a method for fabricating a photovoltaic device. The method includes the steps of: forming a transparent conductive electrode layer on a transparent substrate; forming an n-type layer of a compound having the formula Zn 1-x Mg x O, wherein 0≦x≦1, on the transparent conductive electrode layer; forming a chalcogen absorber layer on the n-type layer; forming a conductive layer on the chalcogen absorber layer; and annealing at a temperature, pressure, and length of time sufficient to form the structure of the photovoltaic device. 
     In another aspect, the present invention further provides another photovoltaic device. This photovoltaic device includes: a transparent superstrate; a conductive layer disposed on the transparent superstrate; a chalcogen absorber layer disposed on the conductive layer; an n-type layer of a compound having the formula Zn 1-x Mg x O, wherein 0≦x≦1, disposed on the chalcogen absorber layer; and a transparent conductive electrode layer disposed on the n-type layer. 
     The present invention also provides another method for fabricating a photovoltaic device. This method includes the steps of: forming a conductive layer on a transparent superstrate; forming a chalcogen absorber layer on the conductive layer; forming an n-type layer of a compound having the formula Zn 1-x Mg x O, wherein 0≦x≦1, on the chalcogen absorber layer; forming a transparent conductive electrode layer on the n-type layer; and annealing at a temperature, pressure, and length of time sufficient to form the structure of the photovoltaic device. 
     A more complete understanding of the present invention, as well as further features and advantages of the present invention, will be shown through the following detailed description and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  is a cross-sectional diagram illustrating a transparent conductive electrode layer formed on a substrate according to an embodiment of the present invention. 
         FIG. 2  is a cross-sectional diagram illustrating an n-type layer formed on the transparent conductive electrode layer according to an embodiment of the present invention. 
         FIG. 3  is a cross-sectional diagram illustrating a chalcogen absorber layer formed on the n-type layer according to an embodiment of the present invention. 
         FIG. 4  is a cross-sectional diagram illustrating an optional p-type molybdenum trioxide (MoO 3 ) interlayer formed on the chalcogen absorber layer according to an embodiment of the present invention. 
         FIG. 5  is a cross-sectional diagram illustrating a conductive metal layer formed on the p-type interlayer according to an embodiment of the present invention. 
         FIG. 6  is a cross-sectional diagram illustrating a photovoltaic device in a superstrate configuration formed according to a method of the present invention. 
         FIG. 7( a )  is a diagram illustrating a photovoltaic device having a substrate configuration according to an embodiment of the present invention. 
         FIG. 7( b )  is a diagram illustrating a photovoltaic device having a superstrate configuration according to an embodiment of the present invention. 
         FIG. 8  is a diagram illustrating the characteristics of different photovoltaic cells with differing n-type partner compositions without aging. 
         FIG. 9  is a diagram illustrating the characteristics of different photovoltaic cells with differing n-type partner compositions with aging. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Some preferred embodiments will be described in more detail with reference to the accompanying drawings, in which the preferred embodiments of the present invention have been illustrated. However, the present invention can be implemented in various manners, and thus should not be construed to be limited to the embodiments disclosed herein. On the contrary, those embodiments are provided for the thorough and complete understanding of the present invention, and to completely convey the scope of the present invention to those skilled in the art. 
     A p-n junction is the boundary or interface between two types of semiconductor material, the n-type and p-type. The “p,” or positive side, contains an excess of electron holes, while the “n,” or negative side, contains an excess of electrons. The p-n junctions is an elementary part of photovoltaic cells since the junction is the active site where the electronic action of the device takes place. 
     Referring to  FIGS. 1-5 , an exemplary methodology for fabricating a photovoltaic device with an improved n-type partner is shown. The method includes the steps of: forming a transparent conductive electrode layer  104  on a transparent substrate  102 ; forming an n-type layer  106  of a compound having the formula Zn 1-x Mg x O, wherein 0&lt;x≦1, on the transparent conductive electrode layer  104 ; forming a chalcogen absorber layer  108  on the n-type layer  106 ; forming a conductive layer  112  on the chalcogen absorber layer; and annealing at a temperature, pressure, and length of time sufficient to form the structure of the photovoltaic device. Furthermore, an optional tellurium adhesion layer (not shown) may be deposited on the improved n-type layer  106  before deposition of the chalcogen absorber layer  108  and an optional p-type molybdenum trioxide (MoO 3 ) interlayer  110  can be formed after deposition of the chalcogen absorber layer  108 . 
     To begin the process, as shown in  FIG. 1 , a substrate  102  is provided. Suitable substrate materials include, but are not limited to, glass, plastic, ceramic and metal foil (e.g., aluminum, copper, etc.) substrates. As will be described in detail below, it has been found that employing a reflective back contact on the substrate  102  aids in increasing the efficiency of the device. A reflective back contact can be created by forming the back contact, in the manner described below, on a planar substrate (glass or metal foil substrate) or on a polished substrate. Thus, it may be desirable at this stage to polish the substrate, especially in the case of a plastic or ceramic substrate. Polishing of the substrate  102  may be carried out using any mechanical or chemical mechanical process known in the art. 
     A transparent conductive electrode layer  104  is then formed on the substrate. During operation, the transparent conductive electrode layer  104  is used as an electrode for low resistance electrical contacts without blocking light. According to an exemplary embodiment of the present invention, the transparent conductive electrode  104  is formed from a transparent conductive material, such as fluorine doped tin oxide (FTO), indium doped tin oxide (ITO), aluminum doped zinc oxide (ZnO:Al), or fluorine doped tin dioxide (SnO 2 :F). The techniques for forming a transparent conductive electrode from these materials would be apparent to one of skill in the art and thus are not described further herein. 
     In  FIG. 2 , according to an exemplary embodiment of the present invention, an improved n-type layer  106  is formed on the transparent conductive electrode layer  104 . An n-type layer is usually a material such as titanium dioxide or zinc dioxide. However, in various embodiments of the present invention, a Zn 1-x Mg x O compound is used as the improved n-type layer  106 . In embodiments of the present invention, the Zn 1-x Mg x O n-type layer  106  was deposited via sputtering using a 75 watt ZnO gun and a 100 watt MgO gun. The guns were located at a distance of 22.5 centimeters above the substrate and oxygen flow rate was controlled at 1.5 sccm (standard cubic centimeters per minute) for a total deposition time of 9400 seconds. While these were the conditions used for various embodiments of the present invention, the same deposition can be achieved using suitable techniques that would be apparent to one of skill in the art. The improved n-type layer  106  can have a thickness ranging from about 2 nm to about 200 nm with a preferred thickness of about 30 nm to about 60 nm. During operation, the improved n-type layer  106  serves as the electron selective layer to collect electrons. 
     In  FIG. 3 , according to an exemplary embodiment of the present invention, a chalcogen absorber layer  108  is formed on the improved n-type layer  106 . The chalcogen absorber layer can be any chalcogen such as sulfur, selenium, tellurium or any combination thereof. In various embodiments of the present invention, highly pure selenium (99.999%) is the preferred chalcogen used. The chalcogen absorber layer  108  can be deposited through vacuum evaporation, chemical bath deposition, electrochemical deposition, atomic layer deposition, successive ionic layer absorption and reaction (SILAR), chemical vapor deposition, sputtering, spin coating, doctor blading, physical vapor deposition or any other suitable technique that would be apparent to one of skill in the art. The chalcogen absorber layer  108  has a thickness from about 25 nm to about 200 nm with a preferred thickness of about 80 nm to about 120 nm. 
     Optionally, a tellurium adhesion layer (not shown) may be deposited on the improved n-type layer  106  before deposition of the chalcogen absorber layer  108 . The thickness of the tellurium adhesion layer is very small, for example, about 1 nm and improves the adhesion between the improved n-type layer  106  and the chalcogen absorber layer  108 . 
     In  FIG. 4 , according to an exemplary embodiment of the present invention, an optional p-type molybdenum trioxide (MoO 3 ) interlayer  110  is formed on the chalcogen absorber layer  108 . The p-type molybdenum trioxide (MoO 3 ) interlayer  110  can be deposited through vacuum evaporation, chemical bath deposition, electrochemical deposition, atomic layer deposition, successive ionic layer absorption and reaction (SILAR), chemical vapor deposition, sputtering, spin coating, doctor blading, physical vapor deposition or any other suitable technique that would be apparent to one of skill in the art. The thickness of the p-type molybdenum trioxide (MoO 3 ) layer  110  is from about 2 nm to about 200 nm with a preferred thickness of about 20 nm to about 60 nm and an optimal thickness of about 20 nm. The optional p-type molybdenum trioxide (MoO 3 ) interlayer  110  increases the work function of the conductive layer  112 . The work function of a metal is the minimum energy needed to remove an electron from a solid to a point in the vacuum immediately outside the solid surface. Here, the p-type molybdenum trioxide (MoO 3 ) interlayer has a work function of ˜5.3 eV. In photovoltaic cells, increasing the work function of the conductive layer correlates positively to an increase in open circuit voltage (V oc ) and short circuit current (J sc ). 
     In  FIG. 5 , according to an exemplary embodiment of the present invention, a conductive layer  112  is deposited on the p-type interlayer  110 . The conductive layer  112  can be: (1) carbon materials such as graphite, graphene, nanotubes; (2) metals and their alloys such as gold, silver, copper, platinum, palladium; Zn, Ni, Co, Mo, Fe V, Cr, Sn, W, Mo, Ti, Mg; and (3) conductive oxides such as fluoride doped tin oxide (FTO), indium doped tin oxide (ITO) and aluminum doped zinc oxide (ZnO:Al). The conductive layer  112  can be deposited through vacuum evaporation, chemical bath deposition, electrochemical deposition, atomic layer deposition, successive ionic layer absorption and reaction (SILAR), chemical vapor deposition, sputtering, spin coating, doctor blading, physical vapor deposition or any other suitable technique that would be apparent to one of skill in the art. The thickness of the conductive layer  112  is preferably from about 2 nm to about 200 nm. The device is annealed at a temperature, pressure, and length of time sufficient to form the structure of the photovoltaic device. 
     In the alternative, if the p-type interlayer  110  is not being used, then the conductive layer  112  would assume the role of the p-type layer with regard to the p-n heterojunction and the conductive layer would be directly deposited onto the chalcogen absorber layer  108 . 
     In  FIG. 6 , another embodiment of the present invention is shown where a superstrate is used instead of a substrate. The method for fabricating the superstrate configuration of the present invention includes: forming a conductive layer  212  on a transparent superstrate  202 ; forming an optional p-type molybdenum trioxide (MoO 3 ) interlayer  210  on the conductive layer  212 ; forming a chalcogen absorber layer  208  on the optional p-type molybdenum trioxide (MoO 3 ) interlayer  210 ; forming an improved n-type layer  206  on the chalcogen absorber layer  208 ; forming a transparent conductive electrode layer  204  on the improved n-type layer  206 ; and annealing at a temperature, pressure, and length of time sufficient to form the structure of the photovoltaic device. 
     Furthermore, the optional tellurium adhesion layer (not shown) can be deposited on the optional p-type molybdenum trioxide (MoO 3 ) layer  210  before deposition of the chalcogen absorber layer  208 . If the optional p-type molybdenum trioxide (MoO 3 ) layer  210  is not being used, then the optional tellurium adhesion layer (not shown) can be deposited on the conductive layer  212 . If neither optional layer is being used then the chalcogen absorber layer  208  would be deposited on the conductive layer  212  and the improved n-type layer would be deposited onto the chalcogen absorber layer  208 . 
     Suitable superstrate materials include, but are not limited to, glass, plastic, ceramic and metal foil (e.g., aluminum, copper, etc.) superstrates. 
     The conductive layer  212  can be: (1) carbon materials such as graphite, graphene, nanotubes; (2) metals and their alloys such as gold, silver, copper, platinum, palladium; Zn, Ni, Co, Mo, Fe V, Cr, Sn, W, Mo, Ti, Mg; and (3) conductive oxides such as fluoride doped tin oxide (FTO), indium doped tin oxide (ITO) and aluminum doped zinc oxide (ZnO:Al). The conductive layer  112  can be deposited through vacuum evaporation, chemical bath deposition, electrochemical deposition, atomic layer deposition, successive ionic layer absorption and reaction (SILAR), chemical vapor deposition, sputtering, spin coating, doctor blading, physical vapor deposition or any other suitable technique that would be apparent to one of skill in the art. The thickness of the conductive layer  212  is preferably from about 2 nm to about 200 nm. The device is annealed at a temperature, pressure, and length of time sufficient to form the structure of the photovoltaic device. 
     The optional p-type molybdenum trioxide (MoO 3 ) interlayer  210  can be deposited through vacuum evaporation, chemical bath deposition, electrochemical deposition, atomic layer deposition, successive ionic layer absorption and reaction (SILAR), chemical vapor deposition, sputtering, spin coating, doctor blading, physical vapor deposition or any other suitable technique that would be apparent to one of skill in the art. The thickness of the p-type molybdenum trioxide (MoO 3 ) layer  210  is from about 2 nm to about 200 nm with a preferred thickness of about 20 nm to about 60 nm and an optimal thickness of about 20 nm. If optional p-type molybdenum trioxide (MoO 3 ) interlayer  210  is not being used, then the conductive layer  212  would take the role as the p-type layer for the p-n heterojunction and the conductive layer  212  would be deposited directly onto the chalcogen absorber layer  208 . 
     The chalcogen absorber layer can be any chalcogen such as sulfur, selenium, tellurium or any combination thereof. In various embodiments of the present invention, highly pure selenium (99.999%) is the preferred chalcogen used. The chalcogen absorber layer  208  can be deposited through vacuum evaporation, chemical bath deposition, electrochemical deposition, atomic layer deposition, successive ionic layer absorption and reaction (SILAR), chemical vapor deposition, sputtering, spin coating, doctor blading, physical vapor deposition or any other suitable technique that would be apparent to one of skill in the art. The chalcogen absorber layer  208  has a thickness from about 25 nm to about 200 nm with a preferred thickness of about 80 nm to about 120 nm. 
     An n-type layer is usually a material such as titanium dioxide or zinc dioxide. However, in various embodiments of the present invention, a Zn 1-x Mg x O compound is used as an improved n-type layer  206 . In embodiments of the present invention, the Zn 1-x Mg x O n-type layer  206  was deposited via sputtering using a 75 watt ZnO gun and a 100 watt MgO gun. The guns were located at a distance of 22.5 centimeters above the substrate and oxygen flow rate was controlled at 1.5 sccm (standard cubic centimeters per minute) for a total deposition time of 9400 seconds. While these were the conditions used for various embodiments of the present invention, the same deposition can be achieved using suitable techniques that would be apparent to one of skill in the art. The improved n-type layer  206  can have a thickness ranging from about 2 nm to about 200 nm with a preferred thickness of about 30 nm to about 60 nm. During operation, the improved n-type layer  206  serves as the electron selective layer to collect electrons. 
     During operation, the transparent conductive electrode layer  204  is used as an electrode for low resistance electrical contacts without blocking light. According to an exemplary embodiment of the present invention, the transparent conductive electrode  104  is formed from a transparent conductive material, such as fluorine doped tin oxide (FTO), indium doped tin oxide (ITO), aluminum doped zinc dioxide (ZnO 2 :Al), or fluorine doped tin dioxide (SnO 2 :F). The techniques for forming a transparent conductive electrode from these materials would be apparent to one of skill in the art and thus are not described further herein. 
     Referring now to  FIG. 7( a ) , a diagram illustrating a photovoltaic device having a substrate configuration according to an embodiment of the present invention is shown. The photovoltaic device includes: a transparent substrate  302 ; a transparent conductive electrode layer  304  disposed on the transparent substrate  302 ; an n-type layer  306  of a compound having the formula Zn 1-x Mg x O, wherein 0&lt;x≦1, disposed on the transparent conductive electrode layer  304 ; a chalcogen absorber layer  308  disposed on the n-type layer  306 ; and a conductive layer  312  disposed on the chalcogen absorber layer  308 . Furthermore, an optional tellurium adhesion layer (not shown) may be disposed on the n-type layer  306  before the chalcogen absorber layer  308  and an optional p-type molybdenum trioxide (MoO 3 ) interlayer  310  can be disposed on the chalcogen absorber layer  308 . 
     In  FIG. 7( b ) , a diagram illustrating a photovoltaic device having a superstrate configuration according to an embodiment of the present invention is shown. The photovoltaic device includes: a transparent superstrate  402 ; a conductive layer  412  disposed on the transparent superstrate  402 ; a chalcogen absorber layer  408  disposed on the conductive layer  412 ; an n-type layer  406  of a compound having the formula Zn 1-x Mg x O, wherein 0&lt;x≦1, disposed on the chalcogen absorber layer  408 ; and a transparent conductive electrode layer  404  disposed on the n-type layer  406 . Furthermore, an optional tellurium adhesion layer (not shown) may be disposed on the conductive layer  412  before the chalcogen absorber layer  408  and an optional p-type molybdenum trioxide (MoO 3 ) interlayer  410  can be disposed on the conductive layer  412 . 
     Referring to  FIG. 8 , a diagram illustrating the characteristics of different photovoltaic cells with differing n-type partner compositions without aging is shown. Each individual square in the diagram shown in  FIG. 8  represents a photovoltaic cell fabricated using the method provided in  FIGS. 1-5 . However, the deposition of the improved n-type partner varies from cell to cell. The cells closer to the Y-Axis have lower concentrations of Zn 1-x Mg x O and the cells farther from the Y-Axis have higher concentrations of Zn 1-x Mg x O. Furthermore, cells that are closer to the X-Axis have higher concentrations of ZnO whereas cells farther from the X-Axis have higher concentrations of MgO. The number of located at the top of each cell is for identification of the cell. 
     In Table 1 below, the three-device average production of each location is described in terms of efficiency (Eff), fill factor (FF), and open circuit voltage (V oc ). The production of a reference cell is also included having an n-type partner only consisting of ZnO. 
     
       
         
           
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Location 
                 Efficiency (%) 
                 Fill Factor (%) 
                 V oc  (mV) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 ZnO Only N- 
                 4.83 
                 50.65 
                 789.18 
               
               
                 Type Reference 
                   
                   
                   
               
               
                 13 
                 1.77 
                 25.25 
                 942.64 
               
               
                 25 
                 3.05 
                 33.70 
                 951.62 
               
               
                 33 
                 4.87 
                 46.60 
                 866.93 
               
               
                 36 
                 4.57 
                 44.49 
                 929.20 
               
               
                 43 
                 4.04 
                 54.82 
                 816.60 
               
               
                 45 
                 5.60 
                 53.85 
                 852.97 
               
               
                 46 
                 5.77 
                 53.57 
                 883.90 
               
               
                 55 
                 5.73 
                 57.13 
                 802.33 
               
               
                 63 
                 5.34 
                 57.28 
                 787.77 
               
               
                   
               
            
           
         
       
     
     In Table 1, the locations most distant from the Y-Axis and between the MgO and ZnO guns produced the highest open circuit voltage (V oc ) and efficiency (Eff), namely locations 36 and 46. The Zn 1-x Mg x O n-type partner produces much higher open circuit voltage (V oc ) as compared to traditional photovoltaic devices that use highly pure chalcogen absorber layer while preserving the photovoltaic cell efficiency. 
     In  FIG. 9 , a diagram illustrating the characteristics of different photovoltaic cells with differing n-type partner compositions with aging is shown. Each individual square in the diagram shown in  FIG. 9  represents a photovoltaic cell fabricated using the method provided in  FIGS. 1-5 . However, the deposition of the improved n-type partner varies from cell to cell. The cells closer to the Y-Axis have lower concentrations of Zn 1-x Mg x O and the cells farther from the Y-Axis have higher concentrations of Zn 1-x Mg x O. Furthermore, cells that are closer to the X-Axis have higher concentrations of ZnO whereas cells farther from the X-Axis have higher concentrations of MgO. 
     The devices were aged at least one month after the initial measurement and show a further increase in efficiency (Eff) and fill factor (FF). The number of located at the top of each cell is for identification of the cell. In Table 2 below, the aged three-device average production of each location is described in terms of efficiency (Eff), fill factor (FF), and open circuit voltage (V oc ). 
     
       
         
           
               
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                 Location 
                 Efficiency (%) 
                 Fill Factor (%) 
                 V oc  (mV) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 33 
                 6.10 
                 51.58 
                 852.60 
               
               
                 36 
                 4.99 
                 40.80 
                 947.90 
               
               
                 42 
                 5.03 
                 45.19 
                 742.83 
               
               
                 43 
                 6.21 
                 56.78 
                 791.70 
               
               
                 45 
                 6.53 
                 54.44 
                 850.64 
               
               
                 46 
                 7.05 
                 57.44 
                 859.98 
               
               
                 51 
                 4.43 
                 45.19 
                 742.83 
               
               
                 55 
                 6.53 
                 56.39 
                 814.50 
               
               
                 63 
                 6.26 
                 59.07 
                 745.12 
               
               
                   
               
            
           
         
       
     
     In Table 2, the aged photovoltaic cells produced higher efficiency while preserving increased open circuit voltage (V oc ). Similar to Table 1, the locations most distant from the Y-Axis and between the MgO and ZnO guns produced the highest open circuit voltage (V oc ) and efficiency (Eff), namely locations 36 and 46. The aged photovoltaic devices with a Zn 1-x Mg x O n-type partner produce much higher open circuit voltage (V oc ) as compared to traditional photovoltaic devices that use highly pure chalcogen absorber layer while preserving the photovoltaic cell efficiency. 
     Although illustrative embodiments of the present invention have been described herein, it is to be understood that the invention is not limited to those precise embodiments, and that various other changes and modifications may be made by one skilled in the art without departing from the scope of the invention.