Abstract:
A photovoltaic device that includes a reflective stack. The reflective stack is formed from a transparent material between two metal layers. The reflective stack is located within the photovoltaic device to partially reflect wavelengths of radiation that do not substantially contribute to the photovoltaic effect.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority to U.S. Provisional Application No. 61/539,293, filed on Sep. 26, 2011, the disclosure of which is incorporated by reference in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    Disclosed embodiments relate to the field of photovoltaic power generation systems, and more particularly to a photovoltaic device and manufacturing method thereof. 
       BACKGROUND 
       [0003]    A photovoltaic device, such as a photovoltaic module or cell, converts sun radiation directly into electrical current by the photovoltaic effect. For most photovoltaic devices, only a portion of the spectrum of sun radiation is utilized to generate electrical current. The remaining portions of the spectrum of sun radiation are typically absorbed by, and heat, the photovoltaic devices. A rise in temperature of a photovoltaic device generally decreases the efficiency with which the device generates electrical current. Accordingly, a photovoltaic device with improved efficiency is desirable. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0004]      FIG. 1  is a cross-sectional view of a structure in accordance with a disclosed embodiment. 
           [0005]      FIG. 2  is the cross-sectional view of the structure of  FIG. 1  illustrating radiation being reflected in accordance with a disclosed embodiment. 
           [0006]      FIG. 3  is a cross-sectional view of another structure in accordance with a disclosed embodiment. 
           [0007]      FIG. 4  is a cross-sectional view of another structure in accordance with a disclosed embodiment. 
           [0008]      FIG. 5  is a diagram illustrating the formation of a structure in accordance with a disclosed embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0009]    In the following detailed description, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration specific embodiments that may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to make and use them, and it is to be understood that structural, logical, or procedural changes may be made to the specific embodiments disclosed without departing from the spirit and scope of the invention. 
         [0010]      FIG. 1  is a cross-sectional view of a substrate structure  100  used for photovoltaic devices, such as photovoltaic modules or cells, in accordance with a disclosed embodiment. The structure  100  comprises multiple sequential layers of various materials deposited on a front support  110 . In one exemplary embodiment, layers of the structure  100  may include reflective stack layers  130 , one or more transparent conductive oxide (TCO) layers  150 , optionally, one or more buffer layers  160 , at least one semiconductor window layer  170 , at least one semiconductor absorber layer  180 , a back contact layer  190 , and a back support layer  200 . The front support layer  110  is made of an insulative material that is transparent or translucent to radiation, such as soda lime glass, low iron glass, solar float glass or other suitable glass. The back support layer  200  may be formed of similar materials as the front support layer  110 . The TCO layer(s)  150  may be doped tin oxide, cadmium tin oxide, tin oxide, indium oxide, zinc oxide, other transparent conductive oxides, or a combination thereof. 
         [0011]    The buffer layer(s)  160  may be tin oxide, zinc tin oxide, zinc magnesium oxide, zinc sulfur oxide, other transparent conductive oxides, or a combination thereof. The absorber layer  180  may generate photo carriers upon absorption of solar radiation and may be made of amorphous silicon, copper indium gallium diselenide, cadmium telluride or any other suitable radiation absorbing material. In one embodiment, the window layer  170  may mitigate the internal loss of photo carriers (e.g., electrons and holes) in the structure  100 . The window layer  170  is a semiconductor material, such as cadium sulfide, zinc sulfide, cadium zinc sulfide, zinc magnesium oxide or any other suitable photovoltaic semiconductor material. The back contact layer  190  may be one or more metal layers and may be formed of molybdenum, aluminum, chromium, iron, nickel, titanium, vanadium, manganese, cobalt, zinc, ruthenium, tungsten, silver, gold, copper, mercury tellurium, titanium disilicide, titanium silicide, molybdenum nitride, titanium nitride, tungsten nitride, platinum, or similar materials. 
         [0012]    It should be noted that structure  100  is not intended to be considered a limitation on the types of photovoltaic devices to which the present disclosure may be applied, but rather a convenient representation for the following description. In addition, each of the layers  110 ,  130 ,  150 ,  160 ,  170 ,  180 ,  190 ,  200  may include one or more layers or films, one or more different types of materials and/or same material types with differing compositions. And although the layers  130 ,  150 ,  160 ,  170 ,  180 ,  190 ,  200  are shown as being formed on the front support layer  110 , structure  100  can also be built up from back support layer  200  using various material layers known in the art. The layers can also have differing thicknesses and other dimensions. Other materials may be optionally included in the structure  100  beyond what is mentioned to further improve performance. 
         [0013]    The reflective stack  130  reflects undesired wavelengths of solar radiation away from the structure  100 . Solar radiation has significant power in the spectral range of 300-2500 nm that may be used to generate current. Most photovoltaic devices, however, are unable to use this entire spectral range to generate significant amounts of current and instead only rely on specific wavelength bands to generate current. For example, photovoltaic devices that use cadmium telluride in the absorber layer  180  may rely on wavelengths of radiation between 300 nm and 850 nm as useful wavelengths of radiation to generate current. The remaining portions of the solar radiation with significant power (i.e. radiation with wavelengths between 850 nm and 2500 nm) are absorbed by the layers in the photovoltaic device and heat the device. The portions of solar radiation that generate little or no current and heat the device, thereby lowering the operating efficiency of the device, are considered to be non-useful wavelengths of radiation. The TCO layer  150 , the buffer layer  160 , the window layer  170 , the absorber layer  180 , and the back-contact layer  190  may all absorb these non-useful wavelengths of radiation. Other layers within a photovoltaic device may also absorb these non-useful wavelengths of radiation. As another example, photovoltaic devices that use cadium sulfide and copper indium gallium diselenide in the absorber layer  180  may only use wavelengths of radiation between 300 nm and 1100 nm to generate current. The remaining non-useful portions of the solar radiation with significant power (i.e. radiation with wavelengths between 1100 nm and 2500 nm) may be absorbed by the layers in the photovoltaic device and heat the device. 
         [0014]    These non-useful wavelengths, when absorbed undesirably, heat the photovoltaic device making it less efficient. For example, in some photovoltaic devices, each time the temperature of the device raises a single degree Celsius, the device generates 0.25% less power. 
         [0015]    Referring again to  FIG. 1 , the reflective stack  130  reflects undesired wavelengths of solar radiation away from the structure  100 . In this exemplary embodiment, the reflective stack  130  comprises a first metal layer  132 , a second metal layer  136 , and a transparent material layer  134  that is located between the first and second metal layers  132  and  136 . Each of layers  132 ,  134 , and  136  may pass wavelengths of radiation useful for generating electrical energy with a photovoltaic device. The first and second metal layers  132  and  136  are metal materials such as molybdenum, tantalum, zirconium, tungsten, vanadium, titanium, chromium, copper, cobalt, aluminum, silver, niobium, their alloys, or any other suitable metallic material. In one embodiment, the first and second metal layers  132  and  136  may be of the same material. In another embodiment, the first and second metal layers  132  and  136  may be of different materials. Furthermore, the processing technique used to form subsequent layers in the structure  100  may limit the choice of metal materials. For example, high temperature processing of the structure  100  may preclude the use of silver or aluminum. The transparent material layer  134  may be a dielectric such as silicon dioxide, titanium dioxide, zirconium oxide, aluminum oxide, a combination of these materials, or any other suitable dielectric. The transparent material layer  134  may also be a semi-conductive material, such as doped tin dioxide, zinc oxide, silicon dioxide, or any other suitable semi-conductive material. 
         [0016]    The reflective stack  130  reduces the intensity of non-useful radiation that is typically absorbed by the structure  100  by reflecting portions of the non-useful radiation. Reducing the intensity of the non-useful radiation reduces the heat generated by the absorption of this non-useful radiation. By reducing the amount of heat generated by absorption of radiation, the structure  100  may operate at a lower temperature and, thus, more efficiently. The intensity of radiation transmitted through the TCO layer  150 , buffer layer  160 , window layer  170 , and the absorber layer  180  and the amount absorbed by these layers equals the total intensity of the radiation minus the combined intensity of the reflections caused by the reflective stack  130 . 
         [0017]    The reflective stack  130  further increases the efficiency of the device by reducing reflections of the useful wavelengths of radiation. In this way, all of the intensity of the useful wavelengths of radiation may be used by the structure  100  to generate current. In particular, the reflective stack  130  operates to eliminate reflections for a wavelength of radiation in the center of the useful range of radiation. For example, in one embodiment, the structure  100  may have a semiconductor layer that includes cadmium telluride where the center of the useful range of radiation is 650 nm. In this embodiment, the reflective stack  130  produces no reflection of radiation having a wavelength of 650 nm. The reflective stack  130  also reduces undesired reflections for the remaining useful wavelengths of radiation. However, the amount of reflection reduction for a particular wavelength is reduced the farther that the particular wavelength is from the center wavelength. For example, if the center wavelength is 650 nm, then the reflection reduction for wavelengths of 725 nm is less than the reflection reduction for wavelengths of 675 nm. As a result, wavelengths farther from the center wavelength have reduced intensities as compared to wavelengths closer to the center wavelength. Details with respect to how the reflective stack  130  reflects the non-useful wavelengths of radiation and reduces reflections of the useful wavelengths of radiation are described below with regard to  FIG. 2 . 
         [0018]      FIG. 2  illustrates useful radiation  220  and non-useful radiation  240  entering the structure  100 , passing through the reflective stack  130 , and transmitting into the TCO layer  150 , the buffer layer  160 , the window layer  170 , and the absorber layer  180  according to an exemplary embodiment.  FIG. 2  further illustrates that a portion of the useful radiation  220  is reflected when the useful radiation  220  transitions through first metal layer  132 , creating first useful radiation reflection  222 . As a result of the first useful radiation reflection  222 , the intensity of the useful radiation  220  that is transmitted into the transparent material layer  134  is decreased as compared to the intensity of the useful radiation  220  transmitted to the first metal layer  132 . The intensity or absolute amplitude of the first useful radiation reflection  222  is determined by the thickness of the first metal layer  132 . In this embodiment, the thickness of the first metal layer  132  is between 10 to 100 angstroms thick. It should be noted that as the thickness of the first metal layer  132  increases, the intensity of the first useful radiation reflection  222  increases. 
         [0019]    A second portion of the useful radiation  220  is reflected when the useful radiation  220  transitions through the second metal layer  136 , creating second useful radiation reflection  224 . The intensity of the second useful radiation reflection  224  is determined by the thickness of the second metal layer  136 . In this embodiment, the thickness of the second metal layer  136  is between 10 to 100 angstroms thick. Similar to the first metal layer  132 , as the thickness of the second metal layer  136  increases the intensity of the second useful radiation reflection  224  increases. 
         [0020]    In order to reduce the undesired reflection of the useful radiation  220 , the reflective stack  130  causes the first and second useful radiation reflections  222  and  224  to destructively interfere. When the first and second useful radiations reflections  222  and  224  completely destructively interfere, the combined intensities of the reflections  222  and  224  equals zero. As noted earlier, the intensity of transmitted radiation equals the intensity of the radiation minus the combined intensity of any reflections and minus the intensity of light that was absorbed in the reflective stack layers  130 . Because the combined intensities of the first and second useful reflections  222  and  224  equal zero, the intensity of the useful radiation  220  is maximized as it is transmitted through the reflective stack  130 . 
         [0021]    To cause complete destructive interference between the first and second useful radiations reflections  222  and  224 , the intensities of the first and second useful radiation reflections  222  and  224  need to be the same. As noted above, the intensities of the first and second useful radiation reflections  222  and  224  are controlled by the thickness of the first and second metal layers  132  and  136  respectively. To match the intensities, the second metal layer  136  needs to be thicker than the thickness of the first metal layer  132  because a greater percentage of the intensity of the useful radiation  220  is required to create a second useful radiation reflection  224  with an intensity equal to the intensity of the first useful radiation reflection  222 . This is necessary because the intensity of the useful radiation  220  is less when the second useful radiation reflection  224  is created than when the first useful radiation reflection  222  is created. It should be noted that the embodiment is not limited to just one set of thicknesses for the first and second metal layers  132  and  136  that may be used to match the intensities of the first and second useful radiation reflections  222  and  224 . Rather, more than one set of thicknesses for the first and second metal layers  132  and  136  may be used to match the intensities of the first and second useful radiation reflections  222  and  224 . 
         [0022]    In addition to matching the intensities of the first and second useful radiation reflections  222  and  224 , to cause complete destructive interference between the first and second useful radiations reflections  222  and  224  the phases of the first and second useful radiation reflections  222  and  224  need to be offset by 180 degrees. An offset of 180 degrees occurs when λ(M−½)=2*N*D, where λ equals the wavelength of the radiation, N equals the refractive index of the transparent material  134 , D equals the thickness of the transparent material, and M is an integer. Accordingly, the phases of the first and second useful radiation reflections  222  and  224  may be offset by adjusting the thickness (D) of the transparent material  134 . For example, in the embodiment depicted in  FIG. 2 , if the transparent material  134  has an index of refraction (N) of 2.5 and the integer M=1, the thickness (D) of the transparent material layer  134  is ¼ of 650 nm divided by 2.5, or approximately 65 nm. It should be appreciated that other thicknesses of the transparent material layer  134  may also be used to achieve a 180-degree phase shift. 
         [0023]    In this embodiment, the thickness of the layers  132 ,  134 , and  136  are selected to cause complete destructive interference of wavelengths of 650 nm. Wavelengths of useful radiation  220  other than 650 nm will also experience destructive interference of reflections; however, they do not experience complete destructive interference because the reflections of these wavelengths are not exactly 180 degrees out-of-phase. As a result, some of the intensity of these wavelengths is reflected and not transmitted to the remaining layers of the structure  100 . However, because some destructive interference occurs between the reflections at other wavelengths, the transmitted intensity is greater than if no destructive interference had occurred. Furthermore, due to the existence of other reflective interfaces in the structure  100 , the thickness of the layers in the reflective stack  130  may be further optimized to minimize reflections for other wavelengths of radiation within the useful range of radiation. It should be understood that in practice, complete destructive interference may not occur even at the wavelength for which the device is optimally designed because the thicknesses of the layers in the reflective stack  130  may not have the exact thickness required to produce complete destructive interference. However, the concepts and theories explained herein may be used to reduce reflections of useful radiation in a reflective stack. 
         [0024]    Referring again to  FIG. 2 , reflection of the unused wavelengths of radiation is now described. Similarly to the useful radiation  220 , a portion of the non-useful radiation  240  is reflected when the non-useful radiation  240  transitions through the first metal layer  132 , creating first non-useful radiation reflection  242 . As a result of the first non-useful radiation reflection  242 , the intensity of the non-useful radiation  240  decreases. A second portion of the non-useful radiation  240  is reflected when the non-useful radiation  240  transitions through the second metal layer  136 , creating a second non-useful radiation reflection  244 . Thus, the intensity of the non-useful radiation  240  further decreases. In the instance of the non-useful radiation  240 , the transparent material layer  134  causes the first and second non-useful radiation reflections  242  and  244  to have similar phases leading to the first and second non-useful radiation reflections  242  and  244  constructively interfering. This constructive interference produces greater reflections of the non-useful radiation  240  thereby lowering the intensity of the non-useful radiation  240  that may be absorbed by the structure  100 . 
         [0025]      FIG. 3  illustrates a structure  300  according to another exemplary embodiment. Structure  300  includes the layers described above with respect to  FIG. 1  and further includes a barrier layer  320  and a second buffer layer  340 . The barrier layer  320  is located between the front support  110  and reflective stack  130 . The barrier layer  320  may be silicon oxide, silicon aluminum oxide, tin oxide, other suitable material, or a combination thereof. The barrier layer  320  reduces the likelihood of ions and impurities from the front support  110  diffusing into the first metal layer  132  during processing of the structure  300 , which could lead to separation between layers, sensitivity to moisture, and reduction in the optical properties of the structure  300 . 
         [0026]    The second buffer layer  340  is located between the TCO layer  150  and the reflective stack  130 . In various embodiments, the second buffer layer  340  may be formed from tin oxide, zinc tin oxide, zinc magnesium oxide, zinc sulfur oxide, other TCO, other similar materials, or a combination thereof. In other embodiments, the second buffer layer  340  may be formed from a suitable dielectric material such as silicon oxide or silicon aluminum oxide. The second buffer layer  340  reduces the number of ions that may diffuse into the reflective stack  130  from other layers during processing of the structure  300 , which could lead to separation between layers, sensitivity to moisture, and reduction in the optical properties of the structure  300 . The second buffer layer  340  reduces the number of ions that may diffuse from the substrate  110  or the reflective stack  130  into the semiconductor layers during processing of the structure  300  and improve adhesion between layers  130  and  150 . 
         [0027]    Furthermore, in the  FIG. 3  embodiment, the transparent material layer  134  may be tin oxide, zinc tin oxide, zinc magnesium oxide, zinc sulfur oxide, other TCO, other semi-conductive materials, or a combination thereof as long as the transparent material layer  134  is partially conductive. If the transparent material layer  134  and second buffer layer  340  are partially conductive, the first and second metal layers  132  and  136  act as conductors in parallel with the TCO layer  150  to carry current generated by the structure  300  laterally to the edge of the structure  300 . For example, in one embodiment, the first and second metal layers  132  and  136  and the TCO layer  150  may each have a sheet resistance of 20 ohms per square. If the transparent material  134  and the second buffer layer  340  have resistivity less than one mega ohm per cm then the first and second metal layers  132  and  136  are electrically connected in parallel with the TCO layer  150 . With the TCO layer  150  and the first and second metal layers  132  and  136  connected in parallel, their combined parallel resistance is equal to the total sheet resistance for the conducting lateral current. Because the first and second metal layers  132  and  136  and the TCO layer  150  each have a sheet resistance of 20 ohms per square, the total sheet resistance for conducting lateral current would be approximately 6.66 ohms per square. An ideal sheet resistance for a TCO layer is 6 ohms per square. 
         [0028]    With the first and second metal layers  132  and  136  electrically connected in parallel with the TCO layer  150 , the resistivity of the TCO layer  150  may be higher than if the TCO layer  150  was solely responsible for conducting current laterally to the edge of the structure  300 . As a result, the thickness of the TCO layer  150  may be reduced. The reduction in the thickness of the TCO layer  150  may be more than the thickness of the additional layers, resulting in the overall reduction of the thickness of the structure  300 . Furthermore, the cost of the material for the TCO layer  150  may be higher than the costs for the additional layers. As a result, the overall costs of materials for the structure  300  may also be reduced even though additional costs are incurred for the additional layers. Additionally, the reduction in the thickness of the TCO layer  150  may further reduce absorption of the non-useful radiation  240  and thereby increase the overall efficiency of the structure  300  because the TCO layer  150  absorbs much of the non-useful radiation  240 . Additionally, the reduction in the thickness of the TCO layer  150  may further reduce absorption of the useful radiation  220  in the TCO layer  150  and thereby increasing the overall efficiency of the structure  300  because more light will be transmitted into the absorber layer  180 . 
         [0029]      FIG. 4  illustrates another structure  400  according to an exemplary embodiment. The structure  400  includes the layers described with respect to  FIG. 3  and further includes reflective stack  430  including a third metal layer  438  and a second transparent material layer  437 . The second transparent material layer  437  is located above the second metal layer  136  and the third metal layer  438  is located between the second buffer layer  340  and the second metal layer  136 . 
         [0030]    The third metal layer  438  is a metal material such as molybdenum, tantalum, zirconium, tungsten, vanadium, titanium, chromium, copper, cobalt, aluminum, silver, niobium, their alloys, or any other suitable metallic material. Furthermore, the third metal layer  438  may be of the same material as the first and second metal layers  132  and  136  or it may be different. The second transparent material layer  437  may be a dielectric such as silicon dioxide, titanium dioxide, zirconium oxide, aluminum oxide, a combination of these materials, or any other suitable dielectric. Alternatively, second transparent material layer  437  may also be a semi-conductive material, such as doped tin dioxide, zinc oxide, silicon dioxide, or any other suitable semi-conductive material. Furthermore, the second transparent material layer  437  may be the same material as the transparent material layer  134  or it may be different. 
         [0031]    The third metal layer  438  and the second transparent material layer  437  are used to further increase the transmission of useful radiation  220  and the reflection of non-useful radiation  240 . Furthermore, the thicknesses of the third metal layer  438  and the second transparent material layer  437  may be adjusted according to the thickness of the first and second metal layers  132  and  136 , the transparent material layer  134 , and other reflective surfaces in the structure  400  to increase the intensity of transmission of a wider band of useful radiation  220  and reflections of a wider band of non-useful radiation  240 . It should be noted that the particular configurations of the reflective stack of the embodiments may be determined by one of ordinary skill in the art manually or using, for example, available software programs for calculating the reflection, absorption, and transmission of light at wavelengths within a range of interest based on the properties of the particular materials being used, such as the wavelength dependent refractive index of the material and the material&#39;s absorption coefficient. It should also be noted that additional metal and transparent material layers may be used to optimize the performance of a reflective stack. Furthermore, it should be understood that an optimal design may be dependent on the application and other manufacturing considerations and constraints. 
         [0032]      FIG. 5  illustrates a sputter system  500  that is one apparatus that may be used to form the various layers of a reflective stack  130  or  430  according to an exemplary embodiment. The sputter system  500  is a DC sputtering system that includes a chamber  510  and a pulsed DC power supply  560  with a pulse of any suitable length, such as 4 microseconds. The power output of the source may range from about 3 kW (˜1.4 W/cm 2 ) to about 9 kW (˜4.2 W/cm 2 ). The target voltage may range from about 300 volts to about 420 volts. 
         [0033]    Within the chamber  510 , a structure  570  (e.g. the front support  110 ) upon which the reflective stack  130  is formed is mounted on a plate or holder  580  or positioned in any other suitable manner. A metal/alloy/compound target  540  is held within a distance of 50 mm to 500 mm of the structure  570  by a grounded fixture  530 . The target  540  may be a ceramic target or a metallic target and may be prepared by casting, sintering, or various thermal spray methods. The chamber  510  is filled with an ambient gas such as helium, neon, argon, krypton, xenon, or other suitable gasses at a pressure ranging from about 2.0 mTorr to about 8.0 mTorr. During the sputtering process, particles  550  from the target  540  are deposited onto the structure  570  to form a reflective stack. The sputtering process may also be used to form other layers in a photovoltaic device. 
         [0034]    In another embodiment, the sputter system  500  may be a RF sputtering system or a matching circuit AC sputtering system. Furthermore, the various layers of reflective stack  130  or  430  may be formed through physical deposition, chemical deposition, or any other deposition method. 
         [0035]    While embodiments have been described in detail, it should be readily understood that the invention is not limited to the disclosed embodiments. Rather the embodiments can be modified to incorporate any number of variations, alterations, substitutions, or equivalent arrangements not heretofore described without departing from the spirit and scope of the invention.