Abstract:
A structure for use in a photovoltaic device is disclosed, the structure includes a substrate, a buffer material, a barrier material in contact with the substrate; and a transparent conductive oxide between the buffer material and the barrier material. The buffer material comprises at least one of CdZnO and SnZnO. The structure can be included in a photovoltaic device. Methods for forming the structure are also disclosed.

Description:
CLAIM FOR PRIORITY 
       [0001]    This application claims priority under 35 U.S.C. §119(e) to Provisional U.S. Patent Application Ser. No. 61/385,398, filed on Sep. 22, 2010, which is hereby incorporated by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention pertains to photovoltaic structures, devices, and methods of forming the same. 
       BACKGROUND OF THE INVENTION 
       [0003]    Photovoltaic devices, such as solar cells, can include a semiconductor, which absorbs light and converts it into electron-hole pairs. A semiconductor junction (e.g., a p-n junction), separates the photo-generated carriers (electrons and holes). A contact allows the current to flow to the external circuit. More recently, photovoltaic devices have used conductive transparent thin films to generate charge from incident light. There is a continuing need to improve performance for such thin film photovoltaic devices. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0004]      FIG. 1  depicts a substrate structure according to an embodiment. 
           [0005]      FIG. 2  depicts a device according to an embodiment. 
           [0006]      FIGS. 3 and 3B  depict the formation of the substrate structure of  FIG. 1 . 
           [0007]      FIG. 4A  Depicts a solar module including the device of  FIG. 2 . 
           [0008]      FIG. 4B  Depicts a solar array including the module of  FIG. 4A . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0009]    In the following detailed description, reference is made to the accompanying drawings which form a part hereof, and in which are shown by way of illustration specific embodiments that may be practiced. It should be understood that like reference numbers represent like elements throughout the drawings. These example embodiments are described in sufficient detail to enable those skilled in the art to practice them. It is to be understood that other embodiments may be utilized, and that structural, material, and electrical changes may be made, only some of which are discussed in detail below. 
         [0010]    A configuration for a substrate structure used for thin-film photovoltaic devices consists of multiple layers deposited over a glass material. An exemplary substrate structure  100  is shown in  FIG. 1 , which includes a substrate  10 , one or more barrier materials  20 , one or more transparent conductive oxides (TCO)  30 , and one or more buffer materials  40 . The TCO material  30  (alone or in combination with other materials, layers or films) can serve as a first contact. Each of these materials ( 10 ,  20 ,  30 ,  40 ) can include one or more layers or films, one or more different types of materials and/or or same material types with differing compositions. 
         [0011]    The substrate  10  can be, for example, glass, such as soda lime glass, low Fe glass, solar float glass or other suitable glass. The barrier material  20  can be silicon oxide, silicon aluminum oxide, tin oxide, or other suitable material or a combination thereof The TCO material  30  can be fluorine doped tin oxide, cadmium tin oxide, cadmium indium oxide, aluminum doped zinc oxide or other transparent conductive oxide or combination thereof The buffer material  40  is described in more detail below. 
         [0012]    The substrate structure  100  can be included in a device  200 , e.g., a photovoltaic device such as a solar cell, as shown in  FIG. 2 . In addition, the device  200  includes a window material  50 , a semiconductor material  60  and a second contact  70 . Each if these materials ( 50 ,  60 ,  70 ) can include one or more layers or films, one or more different types of materials and/or or same material types with differing compositions. 
         [0013]    The window material  50  may be a semiconductor material, such as CdS, ZnS, CdZnS, ZnMgO, Zn (O,S) or other suitable photovoltaic semiconductor material. The semiconductor material  60  can be CdTe, CIGS, amorphous silicon, or any other suitable photovoltaic semiconductor material. The second contact  70  can be a metal or other highly conductive material, such as molybdenum, aluminum or copper. 
         [0014]    Although the materials  10 ,  20 ,  30 ,  40 ,  50 ,  60 ,  70  are shown stacked with the substrate  10  on the bottom, the materials  10 ,  20 ,  30 ,  40 ,  50 ,  60 ,  70  can be reversed such that the second contact  70  is on the bottom or arranged in a horizontal orientation. Optionally, additional materials, layers and/or films may be included in the substrate structure  100  or device  200 , such as AR coatings, color suppression layers, among others. 
         [0015]    The buffer material  40 , which directly contacts the semiconductor materials  60 , is important for the performance and stability of the device  200 . For example, in a device  200  that uses CdTe (or similar material) as the semiconductor material  60 , the buffer material  40  is a relatively resistive material as compared to the TCO material  30 , and provides an interface for the window material  50  and TCO material  30 . Among the solar cell performance parameters, open circuit voltage (Voc) and short-circuit conductance (Gsc) are closely related to the buffer material  40  design. 
         [0016]    According to one embodiment, the buffer material  40  comprises a single layer of GZnO, where G is Cd or Sn. In another embodiment, the buffer material  40  comprises a layer of GZnO and a layer of any other transparent conductive material. In another embodiment the buffer material  40  includes a layer of GZnO and a layer of SnO x . The buffer material  40  may have a thickness from about 0.1 nm to about 1000 nm, or from about 0.1 nm to about 300 nm. 
         [0017]    In one embodiment, a device  200  includes a glass  10 , a barrier material  20  of SiAlO x  (about 2000 Å), a TCO material  30  of CdSt (about 2000 Å), a buffer material  40  of GZnO (about 750 Å), a window material  50  of CdS (about 750 Å), a semiconductor material  60  of CdTe (about 3 μm), and a second contact of a highly conductive material (e.g., molybdenum, aluminum, or copper). 
         [0018]    In another embodiment, a device  200  includes a glass  10 , barrier material  20  comprising a layer of SnO x  and a layer of SiAlO x  (totaling about 500 Å), a TCO material  30  of SnO 2 :F (about 4000 Å), a buffer material  40  of GZnO (about 750 Å), a window material  50  of CdS (about 750 Å), an semiconductor material  60  of CdTe (about 3 μm), and a second contact of a highly conductive material (e.g., molybdenum, aluminum, copper). 
         [0019]    In each embodiment described above, the ratio of G to Zn can be from about 1:100 to about 100:1. 
         [0020]    GZnO material or the entire buffer material  40  may be doped. Dopants can be used to achieve a desired conductivity of the buffer material  40  as compared to the TCO material  30 . In one embodiment, the buffer material  40  is less conductive than the TCO material  30 . Dopants can be n-type or p-type elements. For example, group I elements (e.g., Li, Na, and K) and group V elements (e.g., N, P, As, Sb, and Bi) are p type candidates, and group III elements (e.g., B, Al, Ga and In) and group VII elements (e.g., F, Cl, Br, I, and At) are n-type candidates. In one embodiment, the effective concentration of dopant in the buffer material  40  (or in the GZnO material) is between about 1×10 14  atoms/cm 3  to about 1×10 20  atoms/cm 3 . 
         [0021]    The buffer material  40  provides an interface between the TCO material  30  (highly conductive) and the window material  50  (relatively resistive). To optimize the interface, there should be a good energy band alignment between TCO material  30  and the window material  50 . This can be achieved by adjusting the buffer material  40  doping. For example, if a CdS window material  50  is thin it can become non-conformal and some buffer material  40  will directly contact the semiconductor material  60  (e.g., CdTe), which will change the band alignment. Therefore, depending on the thickness or doping level of the CdS window material  50 , the buffer material  40  doping is selected to provide a good energy band alignment between TCO material  30  and the window material  50 . 
         [0022]    Alternatively, a desired conductivity for the buffer material  40  can be achieved by controlling oxygen deficiencies of sub-oxides. For example, the amount of oxygen deficiency can be altered by changing oxygen/argon ratios during a reactive sputtering process as described in more detail below. 
         [0023]      FIGS. 3A and 3B  depict the formation of the  FIG. 1  substrate structure  100 . As shown in  FIG. 3A , a substrate  10  is provided. The barrier material  20  and TCO material  30  are formed over the substrate  10 . Each of these materials  20 ,  30  can be formed by known processes. For example, the barrier material  20  and the TCO material  30  can be formed by physical vapor deposition processes, chemical vapor deposition processes or other suitable processes. 
         [0024]    As shown in  FIG. 3B , the buffer material  40  is formed over the TCO material  30 . The buffer material  40  can be deposited by physical, chemical deposition, or any other deposition methods (e.g., atmospheric pressure chemical vapor deposition, evaporation deposition, sputtering and MOCVD, DC Pulsed sputtering, RF sputtering or AC sputtering). If a sputtering process is used, the target can be a ceramic target or a metallic target. Further, the sputtering may be conducted using a pre-alloyed target or by co-sputtering from G and Zn targets. 
         [0025]    Arrows  33  depict the optional step of doping the buffer material  40 , which can be accomplished in any suitable manner. 
         [0026]    In one embodiment, the dopant is introduced into the sputtering target(s) at desired concentrations. A sputtering target can be prepared by casting, sintering or various thermal spray methods. In one embodiment, the buffer material  40  is formed from a pre-alloy target comprising the dopant by a reactive sputtering process. In one embodiment, the dopant concentration of the sputter target is about 1×10 17  atoms/cm 3  to about 1×10 18  atoms/cm 3 . In one embodiment, the buffer material  40  is formed by a sputtering process using a target of Cd—Zn or Sn—Zn and a target comprising the dopant, and such targets may be placed adjacent one another during the sputtering process. 
         [0027]    In addition, conductivity of the buffer material  40  can be changed by controlling thermal processing of the buffer material  40 . The buffer material  40  is an amorphous material upon deposition. By thermal processing, e.g., thermal annealing, the buffer material  40  can be converted (in whole or in part) to a crystalline state, which is more conductive relative to the amorphous state. In addition, the active dopant level (and thereby the conductivity) can be varied by thermal processing, e.g., thermal annealing. In this case, both thermal load (i.e., the time of exposure to a temperature and the temperature) and ambient conditions can be manipulated to affect doping levels in the buffer material  40 . For example, a slightly reducing or oxygen-depleting environment during an annealing process can lead to higher doping levels and thus enhanced conductivity accordingly. Furthermore, a thermal treating process can be a separate annealing process after deposition of the buffer material  40  (and before the formation of any other materials on the buffer material  40 ) or the processing used in the depositions of the window material  50  and/or the semiconductor material  60 . The thermal processing can be done at temperatures from about 300° C. to about 800° C. 
         [0028]    Alternatively, a desired conductivity for the buffer material  40  can be achieved by controlling oxygen deficiencies of sub-oxides. For example, the amount of oxygen deficiencies can be altered during the formation of the buffer material  40  by introducing gases and changing the ratio of oxygen to other gasses, e.g., oxygen/argon ratio, during a reactive sputtering process. Generally, for a metal oxide, if it is oxygen deficient, extra electrons of the metal can participate in the conductance, increasing the conductivity of the material. Thus, conductivity of the buffer material  40  can be increased by controlling the deposition chamber gas to be oxygen deficient (i.e., by forming the buffer material  40  in an oxygen deficient environment). For example, supplying forming gas will reduce the available oxygen gas. 
         [0029]      FIG. 4A  depicts a solar module  400 , including devices  200 , which can be solar cells. Each of the solar cells  200  is electrically connected via leads  401  to buses  402 ,  403 . The buses  402 ,  403  can be electrically connected to leads  404 ,  405 , which can be used to electrically connect a plurality of modules  400  to form an array  440 , as shown in  FIG. 4B . 
         [0030]    While disclosed embodiments have been described in detail, it should be readily understood that the invention is not limited to the disclosed embodiments. Rather the disclosed embodiments can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described.