Abstract:
Methods are used to develop and evaluate new materials and deposition processes for use as TCO materials in HJCS solar cells. The TCO layers allow improved control over the uniformity of the TCO conductivity and interface properties, and reduce the sensitivity to the texture of the wafer. In Some embodiments, the TCO materials include indium, zinc, tin, and aluminum.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. Provisional Application Ser. No. 61/778,876 filed on Mar. 13, 2013, which is herein incorporated by reference for all purposes. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates to photovoltaic (PV) devices, and more particularly, to transparent conductive oxide (TCO) layers for a photovoltaic device and methods of forming the same. 
       BACKGROUND OF THE INVENTION 
       [0003]    Solar cells are photovoltaic (PV) devices that convert light into electrical energy. Solar cells have been developed as clean, renewable energy sources to meet growing demand. Solar cells have been implemented in a wide number of commercial markets including residential rooftops, commercial rooftops, utility-scale PV projects, building integrated PV (BIPV), building applied PV (BAPV), PV in electronic devices, PV in clothing, etc. Currently, crystalline silicon solar cells (both mono-crystalline and multi-crystalline) based on high temperature doping and grid firing steps on a cell level are the dominant technologies in the market. These high temperature cell processes make the wafers susceptible to undesirable bowing and yield loss, and limits the necessary thickness reduction of the wafer to reduce the overall cell cost. Furthermore, the annual power output of the systems built with these solar cells suffer from an undesirable high temperature coefficient (&gt;0.4%/C), overall undesirable low cell efficiency at standard test conditions (&lt;20%), and undesirable light-induced degradation. Heterojunction crystalline silicon based solar cells provide a path towards higher efficiency cells without bowing, reduced light-induced degradation (n-type), a lower temperature coefficient (0.3%/C), and a path to thinner and less costly wafers. Heterojunction with an intrinsic thin layer (HIT, trademark of Panasonic) is one of these technological paths. 
         [0004]    The development of heterojunction crystalline silicon (HJCS) based devices represents a daunting challenge in terms of the time-to-commercialization. That same development also suggests an enticing opportunity for breakthrough discoveries. A multilayer system such as HJCS requires management of multiple deposition processes, phase equilibrium considerations, defect chemistries, and interfacial control. The vast phase-space to be managed includes process parameters, source material choices, compositions, and overall integration schemes. The complexity of the HJCS structure, and its interfaces to up-, and down-stream processing, makes it a highly empirical material system. The performance of any device containing thin-film, (opto-) electronically-active layers is extremely sensitive to its interfaces. Interface engineering for electronically-active devices is highly empirical. Traditional R&amp;D methods are ill-equipped to address such complexity, and the traditionally slow pace of R&amp;D could limit any new material from reaching industrial relevance when having to compete with the incrementally improving performance of already established PV fabrication lines, and continuously decreasing panel prices for more traditional cSi PV technologies. 
         [0005]    In one example, the charge collection layers in the heterojunction based crystalline silicon solar cell design listed previously must be formed from TCO materials since the light must pass through them to generate carriers within the silicon. Losses within the HJCS cell design can be decreased by reducing the absorption of light in the TCO layers, increasing the conductivity of the TCO layers, improving the interface quality of the thin film stack, improving the cleaning and texturing processes for the substrate, improving the hydrogen passivation of the substrate, and improving the quality of the intrinsic and doped contact layers to the crystalline silicon, (e.g. amorphous silicon (a-Si:H)) layers. Therefore, there is a need for efficient research and development (R&amp;D) methods for developing and evaluating new materials and processes for use in HJCS solar cells. 
       SUMMARY OF THE DISCLOSURE 
       [0006]    The following summary of the invention is included in order to provide a basic understanding of some aspects and features of the invention. This summary is not an extensive overview of the invention and as such it is not intended to particularly identify key or critical elements of the invention or to delineate the scope of the invention. Its sole purpose is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented below. 
         [0007]    In some embodiments, methods are used to develop and evaluate new materials and deposition processes for use as TCO materials in HJCS solar cells. The TCO layers allow improved control over the uniformity of the TCO conductivity and interface properties, and reduce the sensitivity to the texture of the wafer. In Some embodiments, the TCO materials include indium, zinc, tin, and aluminum. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. The drawings are not to scale and the relative dimensions of various elements in the drawings are depicted schematically and not necessarily to scale. 
           [0009]    The techniques of the present invention can readily be understood by considering the following detailed description in conjunction with the accompanying drawings, in which: 
           [0010]      FIG. 1  illustrates a schematic diagram of a HJCS stack according to some embodiments described herein. 
           [0011]      FIG. 2  illustrates a schematic diagram of a HJCS stack according to some embodiments described herein. 
           [0012]      FIG. 3  illustrates a schematic diagram of a HJCS stack according to some embodiments described herein. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    A detailed description of one or more embodiments is provided below along with accompanying figures. The detailed description is provided in connection with such embodiments, but is not limited to any particular example. The scope is limited only by the claims and numerous alternatives, modifications, and equivalents are encompassed. Numerous specific details are set forth in the following description in order to provide a thorough understanding. These details are provided for the purpose of example and the described techniques may be practiced according to the claims without some or all of these specific details. For the purpose of clarity, technical material that is known in the technical fields related to the embodiments has not been described in detail to avoid unnecessarily obscuring the description. 
         [0014]    As used herein, the notations “Al:ZnO” and “ZnO:Al” will be understood to be equivalent and will describe a material wherein the base material is the metal oxide and the element separated by the colon, “:”, is considered a dopant. In this example, Al is a dopant in a base material of zinc oxide. The notation is extendable to other materials and other elemental combinations. 
         [0015]    In various FIGS. below, a HJCS material stack is illustrated using a cross-sectional view of a simple planar structure. Those skilled in the art will appreciate that the description and teachings to follow can be readily applied to any simple or complex PV solar cell structure, (e.g. a stack with (non-) conformal non-planar layers for optimized photon management). The drawings are for illustrative purposes only and do not limit the application of the present invention. 
         [0016]    The efficiency of HJCS solar devices depends on many properties of the absorber layer(s) and the TCO layers such as crystallinity, grain size, composition and phase uniformity, density, defect concentration, doping level, surface roughness, transparency, conductivity, purity, thickness, etc. 
         [0017]    The manufacture of HJCS solar devices entails the integration and sequencing of many unit processing steps. As an example, HJCS solar device manufacturing typically includes a series of processing steps after the crystalline silicon wafer or kerfless crystalline silicon thin film manufacturing, such as cleaning, texturing, surface preparation, deposition, patterning, etching, thermal annealing, and other related unit processing steps. The precise sequencing and integration of the unit processing steps enables the formation of functional devices meeting desired performance metrics such as efficiency at standard test conditions, temperature coefficient, low light performance, power production, and reliability. 
         [0018]    A discussion of novel TCO materials for use with copper indium gallium selenide (CIGS) solar cells may be found in co-pending and co-owned U.S. patent application Ser. No. 13/310,724, filed on Dec. 3, 2011, which is herein incorporated by reference for all purposes. A discussion of the use of high productivity combinatorial methods for the development of novel materials for use with HJCS solar cells may be found in co-pending and co-owned U.S. patent application Ser. No. 13/719,105, filed on Dec. 18, 2012, which is herein incorporated by reference for all purposes. 
         [0019]      FIG. 1  illustrates a schematic diagram of a HJCS stack according to some embodiments described herein. The HJCS solar cell includes an n-doped crystalline silicon substrate,  102 . Those skilled in the art will understand that there are also HJCS solar cell designs based on p-doped crystalline silicon substrates or even either p-doped or n-doped polycrystalline silicon substrates. The n-doping of the substrate for the HJCS solar cell provides benefits such as reduced photo-induced degradation and a lower temperature coefficient as compared to p-doped substrates. These benefits result in higher annual power output. The surfaces of the n-doped substrate are typically cleaned and textured to improve the light trapping of the solar cell. The texturing is not shown in the figures for simplicity. Intrinsic amorphous silicon layers (i-a-Si:H),  104   a  and  104   b , are deposited on both the front and back surfaces of the solar cell. As used herein, the “front” surface of the solar cell will be understood to that surface that receives the incident light as indicated in  FIG. 1 . The i-a-Si:H layers provide passivation of the textured substrate surface and decrease the charge recombination at these surfaces, thereby increasing the efficiency of the solar cell. The i-a-Si:H layers are typically deposited using techniques such as plasma enhanced chemical vapor deposition (PECVD) or PVD (e.g. sputtering). A p-doped amorphous silicon layer (p-a-Si:H),  106 , is deposited on the i-a-Si:H layer,  104   a , located at the front surface of the solar cell. The p-a-Si:H layers are typically deposited using techniques such as PECVD or PVD (e.g. sputtering). The p-a-Si:H layer,  106 , the i-a-Si:H layer,  104   a , and the n-doped substrate,  102 , collectively form a p-i-n junction that generates and separates charge carriers in response to the incident sunlight. A n-doped amorphous silicon layer (n-a-Si:H),  108 , is deposited on the i-a-Si:H layer,  104   b , located at the back surface (e.g. a second surface) of the solar cell. The n-a-Si:H layers are typically deposited using techniques such as PECVD or PVD (e.g. sputtering). The n-a-Si:H layer,  108 , is typically heavily doped and forms a back surface field (BSF) that reduces charge recombination at the back of the solar cell. A TCO layer,  110   a , is deposited on the p-a-Si:H layer,  106 . The TCO layers are typically deposited using techniques such as LPCVD or PVD (e.g. sputtering). The deposition should be done at temperatures below about 200 C to protect the underlying a-Si layers. Typical examples of TCO materials include fluorine-doped tin oxide, fluorine-doped zinc oxide, tin-doped indium oxide, boron-doped zinc oxide, titanium-doped indium oxide, molybdenum-doped indium oxide, zinc indium oxide, zinc tin oxide, zinc indium tin oxide, cadmium tin oxide, and the like. The front TCO layer,  110   a , serves to collect charge carriers across the front of the solar cell and deliver them to metal conductors,  112   a , used to connect the solar cell to external components of the system. This layer must be transparent to the incident light to maintain high efficiency and must be conductive so that the generated power is not lost during its transmission. A back conductor layer,  110   b , is deposited on the n-a-Si:H layer,  108 . The back conductor layers are typically deposited using techniques such as LPCVD or PVD (e.g. sputtering). The back conductor layer,  110   b , serves to collect charge carriers across the back of the solar cell and deliver them to metal conductors,  112   b , used to connect the solar cell to external components of the system. Typically, the back conductor layer,  110   b , is formed from a TCO material or a highly conductive metal (e.g. Al). The deposition and list of materials for the TCO layer,  110   b , are the same as for TCO layer,  110   a.    
         [0020]    The HJCS solar cell illustrated in  FIG. 1  is based on an n-type substrate. An alternate HJCS solar cell could be based on a p-type substrate. In this configuration, the doping in the p-a-Si:H layer,  106 , would be changed from p-type to n-type (e.g. to form a p-n junction), and the n-a-Si:H layer,  108 , would be changed from n-type to p-type. The processing of these layers would remain as discussed previously. The materials used for the remaining layers would remain as discussed previously. 
         [0021]    The TCO materials listed previously are generally n-type. Those skilled in the art will understand that at the interface between TCO layer,  110   a , and p-a-Si:H layer,  106 , two types of doping are present. However, p-type TCO materials are generally poor quality. Two examples of p-type TCO materials include cobalt zinc oxide, and cobalt nickel oxide. In some embodiments, a bilayer TCO layer may be formed wherein a thin layer of a p-type TCO material is deposited on the p-a-Si:H layer to improve the interface properties and the majority of the TCO layer is formed using a high conductivity n-type TCO material as listed previously. 
         [0022]    The HJCS device performance requires that the TCO material exhibit good conductivity and low absorption in the near infrared region of the spectrum. A common TCO material includes indium-tin-oxide (ITO). However, ITO has its plasma wavelength near 1000 nm (e.g. in the near infrared region) and absorbs sunlight with wavelengths around 1000 nm and longer. A TCO material that is transparent in this region would be beneficial. 
         [0023]    In some embodiments, the TCO material includes indium-tin-aluminum-zinc-oxide (ITAZO). The ITAZO material exhibits good conductivity in the as-deposited state and is stable after heat treatments to 300 C. The ITAZO material can be deposited by co-sputtering from an indium-tin-oxide (ITO) compound target and an aluminum-zinc-oxide (AZO) compound target or from a single target including idium-tin-aluminum-zinc oxide. In some embodiments, oxygen is added to the sputtering atmosphere to ensure that the film is fully oxidized and highly transparent. Typical process conditions for the ITO target include a power density of between 0.5 and 10 W/cm 2 , pressure of between 2 and 10 mtorr, argon flow of between 10 and 40 sccm, oxygen percentage (when used) between 0 and 10%. Typical process conditions for the AZO target include a power density of between 0.5 and 10 W/cm 2 , pressure, argon and oxygen flow and pressure are the same as described above. 
         [0024]    In some embodiments, the ITAZO includes indium at a concentration range between 50 and 90 weight %. In some embodiments, the ITAZO includes tin at a concentration range between 2 and 8 weight %. In some embodiments, the ITAZO includes aluminum at a concentration range between 0 and 2 weight %. In some embodiments, the ITAZO includes zinc at a concentration range between 10 and 45 weight %. 
         [0025]    In some embodiments, the material that forms TCO layers,  110   a  (and, in some embodiments,  110   b ), is augmented with another metal oxide material (e.g. zinc-oxide, antimony-zinc-oxide, indium-zinc-oxide, gallium-zinc-oxide, indium-gallium-oxide, indium-zinc-gallium-oxide, zinc-magnesium-oxide, indium-aluminum-oxide, etc.) formed between the TCO and the a-Si:H layers as an interface layer,  202   a  and  202   b , as illustrated in  FIG. 2 . The metal oxide materials are selected to improve interface contact resistance and to reduce the series resistance of the overall stack. As an example, zinc-oxide forms good ohmic contact to a-Si layers. The reduced series resistance improves the solar cell performance by mainly increasing the fill factor of the solar cell. The thickness of the metal oxide interface layer may be between about 10 nm and about 100 nm. This metal oxide interface layer may be deposited using either LPCVD or PVD (i.e. sputtering). Examples of process parameters for an LPCVD process that may be varied include gas composition, gas concentration, temperature, plasma power, pressure, gas flow rate, substrate bias, etc. Examples of process parameters for a PVD process that may be varied include target composition, gas composition, gas concentration, temperature, plasma power, pressure, gas flow rate, substrate bias, etc. 
         [0026]    In some embodiments, the crystalline indium-tin-oxide material that forms TCO layers,  110   a  and  110   b , is replaced with an amorphous TCO material. As discussed previously, the surface of the substrate is textured before the deposition of the a-Si:H layers and the TCO layers. The textured nature of the substrate surface makes the control of the crystallinity of the TCO layers challenging. The use of amorphous TCO layers allows improved control over the lateral uniformity of the conductivity. Examples of suitable amorphous TCO materials include indium-zinc-oxide, indium-zinc-gallium-oxide, indium-gallium-oxide, indium-aluminum-zinc-oxide, and gallium-tin-zinc-oxide, indium-zinc-oxide doped with metals other than gallium, like . . . These amorphous TCO layers may be deposited using either LPCVD or PVD (i.e. sputtering). Examples of process parameters for a LPCVD process that may be varied include gas composition, gas concentration, temperature, plasma power, pressure, gas flow rate, substrate bias, etc. Examples of process parameters for a PVD process that may be varied include target composition, gas composition, gas concentration, temperature, plasma power, pressure, gas flow rate, substrate bias, etc. 
         [0027]    In some embodiments, the indium-tin-oxide material that forms TCO layers,  110   a  and  110   b , is augmented with an anti-reflection coating,  302 , formed above the TCO layer as illustrated in  FIG. 3 . The anti-reflection coating materials are selected to decrease light reflection from the solar cell and maintain the high conductivity of the TCO materials. As an example, titanium-oxide combined with silicon oxide multi-layers exhibits good anti-reflection properties. The good anti-reflection properties improve the solar cell performance by increasing the light trapping and charge collection of the solar cell. This anti-reflection layer stack may be deposited using either a sol-gel technique or PVD (i.e. sputtering). Examples of process parameters for a sol-gel process that may be varied include sol composition, particle size, particle shape, solvent composition, curing time and temperature, etc. Examples of process parameters for a PVD process that may be varied include layer thickness, number of layers, target composition, gas composition, gas concentration, temperature, plasma power, pressure, gas flow rate, substrate bias, etc. 
         [0028]    In some embodiments, two or more of the metal oxide interface layer, amorphous TCO material, or anti-reflection coating concepts can be combined to improve the performance of the heterojunction solar cell. The benefits of the three concepts are largely complimentary and will each provide performance improvements that are additive. 
         [0029]    Although the foregoing examples have been described in some detail for purposes of clarity of understanding, the invention is not limited to the details provided. There are many alternative ways of implementing the invention. The disclosed examples are illustrative and not restrictive.