Abstract:
Photovoltaic devices with a zinc oxide layer replacing all or part of at least one of a window layer and a buffer layer, and methods of making the devices.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority to U.S. Provisional Application No. 61/790,000, filed on Mar. 15, 2013, which is hereby fully incorporated by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to the field of photovoltaic devices, including photovoltaic cells and photovoltaic modules containing a plurality of photovoltaic cells. More particularly, the invention relates to the use of a zinc oxide layer within a photovoltaic device. 
       BACKGROUND OF THE INVENTION 
       [0003]    Photovoltaic (PV) devices are PV cells or PV modules containing a plurality of PV cells, or any device that converts photo-radiation or light into electricity. Generally, a thin film PV device includes two conductive electrodes sandwiching a series of semiconductor layers. A buffer layer may be provided on one of the conductive electrodes to provide a smooth surface upon which the semiconductor layers can be formed. The semiconductor layers include an n-type window layer in close proximity to a p-type absorber layer to form a p-n junction. During operation, light passes through the window layer, and is absorbed by the absorber layer. The absorber layer produces photo-generated electron-hole pairs, the movement of which, promoted by an electric field generated at the p-n junction, produces electric current that can be output to other electrical devices through the two electrodes. 
         [0004]    Since an electric field, formed by the p-n junction, is required to provide electric current, the window layer should be sufficiently thick to maintain the p-n junction with the nearby absorber layer. Unfortunately, the window layer, e.g. CdS, as well as the underlying buffer layer, if provided, absorb a portion of the light before it reaches the absorber layer, thus reducing the number of photo-generated electron-hole pairs (i.e., carriers) that are available to produce electricity and reducing the short circuit current density (a measure of the maximum current available from a solar cell per unit area). It would be desirable to provide a PV device structure that allows more light to reach the absorber layer. It would also be desirable to reduce the cost of device fabrication. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]      FIGS. 1 ,  1 A and  1 B are photovoltaic devices with a zinc oxide layer in accordance with disclosed embodiments; 
           [0006]      FIG. 2  is a photovoltaic device with a zinc oxide layer in accordance with a disclosed embodiment; 
           [0007]      FIG. 3  is a photovoltaic device with a zinc oxide layer in accordance with a disclosed embodiment; and 
           [0008]      FIG. 4  is a photovoltaic device with a zinc oxide layer in accordance with a disclosed embodiment. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0009]    Embodiments described herein provide a PV device and a method of forming a PV device which includes a zinc oxide layer provided in a way which increases the amount of photons which reach the absorber layer. The included zinc oxide layer is provided to: (1) eliminate or reduce the thickness of a conventional semiconductor window layer, (2) replace a conventional buffer layer, or (3) both. The reduction of material thickness in the path of incident photons results in enhancement of device performance by allowing more photons to reach the absorber layer. For illustrative purposes, embodiments are described below with reference to a thin film PV device, which may include a PV cell, a collection of cells forming a module, or any portion or combination thereof. However, it should be understood that the embodiments may apply to devices other than thin film devices. 
         [0010]    Now referring to the accompanying figures, wherein like reference numbers denote like features,  FIG. 1  illustrates an exemplary PV device  100 . The device  100  includes a substrate  101 . The substrate  101  is used to protect the PV device  100  from environmental hazards. Since the first layer that may be encountered by light incident on the PV device  100  is substrate  101 , it should be made of a transparent material such as silicate glass, soda-lime glass, or borosilicate glass or another suitable transparent material Over the transparent substrate  101  is an optional barrier layer  103  used to inhibit sodium, which is present in substrate  101  materials, from diffusing to the other layers of the PV device  100 . Sodium diffusion into these layers may adversely affect device efficiency. The optional barrier layer  103  can be a bi-layer of an SnO 2  layer over the substrate  101  and an SiO 2  layer over the SnO 2  layer, a single layer of SiO 2  or SnO 2 , or can be formed of, for example, alumina, or silicon aluminum oxide. The barrier layer  103  can have a thickness of between about 1 Å and about 5000 Å, for example between about 50 Å and about 1000 Å. A TCO layer  105  which functions as one of the electrodes of the device  100  is formed over the barrier layer  103 . Since light has to pass through the TCO layer  105  to reach the semiconductor layers where it is converted to electricity, it may be made of a transparent conductive material such as indium tin oxide (ITO), fluorine doped tin oxide (SnO 2 :F), cadmium stannate (Cd 2 SnO 4 ), indium gallium oxide, or indium titanium oxide. The TCO layer  105  may be formed to a thickness of about 0.2 μm to about 0.5 μm. A buffer layer  107 , is formed over the TCO layer  105  for providing a smooth layer for deposition of a zinc oxide layer  108 . The buffer layer  107  may be made of a metal oxide such as SnO 2 , or a combination of ZnO and SnO 2 , and can be about 25 nm to about 200 nm thick. For example, the buffer layer  107  may be between about 50 nm to about 100 nm thick, or about 75 nm thick. 
         [0011]    Over the buffer layer  107 , a zinc oxide layer  108  is formed adjacent to the buffer layer  107  and in electrical association with the TCO layer  105 . As shown, at least a portion of the zinc oxide layer  108  is in contact with a portion of the buffer layer  107 . The zinc oxide layer is believed to have n-type semiconductor characteristics and has a thickness between about 1 nm and about 500 nm, for example between about 25 nm and about 200 nm, or between about 40 nm and about 75 nm. Over the zinc oxide layer  108 , an n-type semiconductor window layer  109  is formed adjacent to zinc oxide layer  108 . As shown, at least a portion of the window layer  109  is in contact with the zinc oxide layer  108 . Semiconductor window layer  109  is preferably formed of cadmium sulfide, however it should be understood that other n-type semiconductors may be used including, but not limited to, cadmium zinc sulfide. Window layer  109  thickness may be between about 50 Å and about 2000 Å, between about 50 Å and about 1000 Å, between about 75 Å and about 500 Å, or greater than 0 Å and less than about 200 Å. 
         [0012]    Over the window layer  109  a semiconductor absorber layer  111  is formed adjacent to semiconductor window layer  109 . As shown, at least a portion of the absorber layer  111  is in contact with the window layer  109 . Absorber layer  111  is a p-type semiconductor that may be made of, for example, cadmium telluride, copper indium gallium (di)selenide (CIGS), copper indium selenide, copper gallium selenide, or CdS x Te 1-x , which is an alloy of cadmium (Cd), sulfur (S), and tellurium (Te) (where x is greater than zero and less than one and represents the atomic ratio of sulfur to tellurium in the alloy material), as an example, x can be greater than 0 and less than or equal to about 0.3). However it should be understood that other p-type semiconductors may be used. As one alternative, shown in  FIG. 1A , the absorber layer  111  may include a bi-layer of CdS x Te 1-x    111   a  and cadmium telluride  111   b . In an absorber layer  111  including CdS x Te 1-x    111   a  and cadmium telluride  111   b , the CdS x Te 1-x  material  111   a  can be closer than the cadmium telluride material  111   b  to the zinc oxide layer  108 . Absorber layer  111  thickness may be between about 0.5 μm and about 10 μm, between about 1 μm and about 5 μm, or between about 2 μm and about 4 μm. If a CdS x Te 1-x  layer is included in the absorber layer  111 , the CdS x Te 1-x  material  111   a  portion of the absorber layer  111  thickness can be between about 20 nm to about 500 nm. 
         [0013]    After the absorber layer  111  is deposited, the PV device  100  may be treated with a compound comprising chlorine, such as CdCl 2 , and heated to reduce the resistivity of the semiconductor materials through re-crystallization and incorporation of chlorine within the semiconductor materials, particularly the absorber layer. The PV device  100  may be heated to greater than about 400° C., for example, the PV device  100  may be heated to about 440° C., or heated to a first temperature in a first heating at about 440° C., and then heated to a second temperature in a second heating at about 430° C. It should be understood that the chlorine application and heat treatment will vary with the type and thickness of the absorber layer  111  as well as the combination of other PV device layers. 
         [0014]    A back contact layer  113  can be formed adjacent and in electrical association with the absorber layer  111  to form an electrode for conveying electricity out of the PV device  100 . The back contact layer  113  can be formed from a metal, for example, molybdenum, aluminum, copper, gold, alloys thereof, or mixtures of any of the foregoing. A back cover  115  can be formed to provide environmental protection or support for the structure and may be made, for example, as the same materials used to make the substrate  101  or other materials. Optionally, additional materials or layers may be included in the PV device  100 . For example, as shown in  FIG. 1B , a zinc telluride layer  112  may be formed between the absorber layer  111  and the back contact metal layer  113 , which has been experimentally shown to improve device efficiency by reducing electron/hole re-combination losses at the absorber layer  111 /back contact layer  113  interface. The zinc telluride layer  112 , if employed, can have a thickness of about 10 nm to about 500 nm. The zinc telluride layer  112  can also be employed in the  FIG. 1A  structure between the absorber layer  111  and back contact layer  113 . 
         [0015]    The layers of PV device  100 , may be formed using any suitable technique or combination of techniques. For example, the layers can be formed by chemical vapor deposition (CVD), physical vapor deposition (PVD), chemical bath deposition (CBD), low pressure chemical vapor deposition, atmospheric pressure chemical vapor deposition, plasma-enhanced chemical vapor deposition, thermal chemical vapor deposition, DC or AC sputtering, spin-on deposition, spray-pyrolysis, vapor transport deposition (VTD), close space sublimation (CSS) etc. or a combination thereof. These processes are well known in the industry and thus will not herein be explained. 
         [0016]    PV device  100 , illustrated in  FIGS. 1 ,  1 A and  1 B, has improved open circuit voltage (V oc ) (a measure of the maximum voltage available from a solar cell) and short circuit current density as compared to a PV device having the same layer combination as  FIG. 1  but without a zinc oxide layer  108 . It is believed that the zinc oxide layer  108 , having n-type characteristics, also functions as a window layer, and contributes, along with window layer  109 , to maintaining a sufficient p-n junction with the absorber layer  111 . Thus, by adding a zinc oxide layer  108 , the cadmium sulfide window layer  109  itself can be made thinner than in a PV device without zinc oxide layer  108 . For example, in a PV device without zinc oxide layer  108 , window layer  109  thickness may need to be greater than 400 Å to provide a sufficient p-n junction between the window layer  109  and the absorber layer  111 . Zinc oxide also has less optical absorption than cadmium sulfide, thus there is in an overall increase in light passing through the combination of a zinc oxide layer  108  and a thinner (≦400 Å) window layer  109  for photo-conversion within absorber layer  111 . Therefore, photo-conversion efficiency is increased as compared to a PV device with a thicker (&gt;400 Å) cadmium sulfide window layer. 
         [0017]      FIG. 2  illustrates an exemplary embodiment of a PV device  200  similar to PV device  100  including a zinc oxide layer  208  and a window layer  209 , among other layers of PV device  100  described with reference to  FIG. 1 . The layers may be formed using similar techniques, formed to similar thicknesses, and formed of similar materials as those described above with reference to PV device  100 . However, PV device  200  omits buffer layer  107  ( FIG. 1 ). As shown, at least a portion of zinc oxide layer  208  is in contact with TCO layer  105 . PV device  200  has improved open circuit voltage and short circuit current density as compared to a PV device having the same layer combination as  FIG. 1  but without the zinc oxide layer  108 . It is believed that the zinc oxide layer  208  serves as a buffer layer by providing a sufficiently smooth surface for the deposition of the window layer  209 . Furthermore, the zinc precursors used to form zinc oxide costs less than those materials commonly used for buffer layer  107 , such as tin oxide. Therefore, by including zinc oxide layer  208  and omitting a buffer layer  107 , the cost of materials can decrease. 
         [0018]      FIG. 3  illustrates an exemplary embodiment of a PV device  300  similar to PV device  100  including a buffer layer  307  and a zinc oxide layer  308 , among other layers of PV device  100  described with reference to  FIG. 1 . The layers may be formed using similar techniques, formed to similar thicknesses, and formed of similar materials as those described above with reference to PV device  100 . However, PV device  300  omits window layer  109  ( FIG. 1 ). As shown, at least a portion of absorber layer  111  is in contact with zinc oxide layer  308 . PV device  300  has improved open circuit voltage and short circuit current density as compared to a PV device having the same layer combination as  FIG. 1  but without the zinc oxide layer  108 . It is believed that the n-type characteristics of zinc oxide layer  308  serves as a window layer by providing a p-n junction with absorber layer  111 . As noted above, zinc oxide layer  308  has increased transparency over other materials commonly used for window layers, such as cadmium sulfide. As such, the use of a zinc oxide layer  308  as the window layer can allow more photons to reach the semiconductor absorber layer  111  and thus increase photo-conversion efficiency as compared to a more conventional PV device with a cadmium sulfide window layer. Furthermore, zinc precursors used to form zinc oxide costs less than cadmium sulfide. Therefore, by including zinc oxide layer  308  and omitting a cadmium sulfide window layer  109  ( FIG. 1 ), the cost of materials can decrease. 
         [0019]      FIG. 4  illustrates an exemplary embodiment of a PV device  400  similar to PV device  100  including a zinc oxide layer  408  among other layers of PV device  100  described with reference to  FIG. 1 . The layers may be formed using similar techniques, formed to similar thicknesses, and formed of similar materials as those described above with reference to PV device  100 . However, PV device  400  omits buffer layer  107  ( FIG. 1 ) and window layer  109  ( FIG. 1 ). As shown, at least a portion of absorber layer  111  is in contact with zinc oxide layer  408  and at least a portion of zinc oxide layer  408  is in contact with TCO layer  105 . PV device  400  has improved open circuit voltage and short circuit current density as compared to a PV device having the same layer combination as  FIG. 1  but without the zinc oxide layer  108 . Here, it is believed that the n-type characteristics of zinc oxide layer  308  allows it to serve as a window layer by providing the p-n junction with absorber layer  111  while it also serves as a buffer layer by providing a sufficiently smooth surface for the deposition of the semiconductor absorber layer  111 . As noted above, zinc oxide has increased transparency over other materials commonly used for window layers, such as cadmium sulfide, such that the use of a zinc oxide layer  408  as a window layer can allow more photons to reach the semiconductor absorber layer  111  and thus increase photo-conversion efficiency (due to increased current density) as compared to a PV device with a cadmium sulfide window layer. Furthermore, the zinc precursors used to form zinc oxide costs less than cadmium sulfide and materials commonly used as buffer materials, such as tin oxide. Therefore, by including zinc oxide layer  308  and omitting a cadmium sulfide window layer  109  ( FIG. 1 ) and buffer layer  107  ( FIG. 1 ), the cost of materials can decrease. 
         [0020]    Each of the embodiments in  FIGS. 2-4  can also include the absorber bi-layer  111   a ,  111   b  described above with reference to  FIG. 1A  and/or the ZnTe layer  112  discussed above with reference to  FIG. 1B   
         [0021]    The embodiments described above are offered by way of illustration and example. Each layer in PV devices  100 ,  200 ,  300 ,  400  may, in turn, include more than one layer or film. Additionally, each layer can cover all or a portion of the PV device  100 ,  200 ,  300 ,  400  and/or all or a portion of the layer or substrate underlying the layer. For example, a “layer” can include any amount of any material that contacts all or a portion of a surface. It should be understood that the examples provided above may be altered in certain respects and still remain within the scope of the claims. It should be appreciated that, while the invention has been described with reference to the above preferred embodiments, other embodiments are within the scope of the claims. The invention should also not be considered as limited to those embodiments, but is only limited by the scope of the claims appended hereto.