Abstract:
A method of forming a photovoltaic device includes forming a thermal stress relieving layer on top of a substrate and forming a sacrificial back electrode metal layer on the thermal stress relieving layer. A semiconductor photon absorber layer is formed on the sacrificial back electrode metal layer, and the absorber layer is reacted with substantially an entire thickness of the sacrificial back electrode metal layer, thereby forming a back ohmic contact comprising a metallic compound of the sacrificial back electrode metal layer and the absorber layer, in combination with the thermal stress relieving layer.

Description:
BACKGROUND 
       [0001]    The present invention relates generally to semiconductor device manufacturing and, more particularly, to a niobium thin-film stress relieving layer for thin-film solar cells. 
         [0002]    Solar cells are photovoltaic devices that convert sunlight directly into electrical power. Generally, p-n junction based photovoltaic cells include a layer of an n-type semiconductor in direct contact with a layer of a p-type semiconductor. When a p-type semiconductor is positioned in intimate contact with an n-type semiconductor, a diffusion of electrons occurs from the region of high electron concentration (the n-type side of the junction) into the region of low electron concentration (the p-type side of the junction). However, the diffusion of charge carriers (electrons) does not happen indefinitely, as an opposing electric field is created by this charge imbalance. The electric field established across the p-n junction induces a separation of charge carriers that are created as result of photon absorption. 
         [0003]    The most common type of solar cell material is silicon, which is in the form of single or polycrystalline wafers. However, the cost of electricity generated using silicon-based solar cells is still higher than the cost of electricity generated by the more traditional methods. Since the early 1970&#39;s there has been an effort to reduce cost of solar cells for terrestrial use. One way of reducing the cost of solar cells is to develop low-cost, thin-film growth techniques that can deposit solar cell quality absorber materials on large area substrates and to fabricate these devices using high-throughput, low-cost methods. 
         [0004]    The increased interest in thin-film photovoltaics has been due primarily to improvements in conversion efficiency of cells made at the laboratory scale, with the anticipation that manufacturing costs can be significantly reduced compared to the older and more expensive crystalline and polycrystalline silicon technology. The term “thin-film” is thus used to distinguish this type of solar cell from the more common silicon based cell, which uses a relatively thick silicon wafer. While single crystal silicon cells still demonstrate the best conversion efficiency to date at over 20%, thin-film cells have been produced which can perform close to this level. As such, performance of the thin-film cells is no longer the major issue that limits their commercial use. Instead, primary factors now driving the commercialization of thin-film solar cells include cost, manufacturability, reliability and throughput, for example. 
       SUMMARY 
       [0005]    In one aspect, a method of forming a photovoltaic device includes forming a thermal stress relieving layer on top of a substrate; forming a sacrificial back electrode metal layer on the thermal stress relieving layer; forming a semiconductor photon absorber layer on the sacrificial back electrode metal layer; and reacting the absorber layer with substantially an entire thickness of the sacrificial back electrode metal layer, thereby forming a back ohmic contact comprising a metallic compound of the sacrificial back electrode metal layer and the absorber layer, in combination with the thermal stress relieving layer. 
         [0006]    In another aspect, a method of forming a photovoltaic device includes forming a thermal stress relieving layer on top of a substrate; forming a sacrificial back electrode metal layer on the thermal stress relieving layer; forming a solution-based, p-type semiconductor photon absorber layer on the sacrificial back electrode metal layer; annealing the absorber layer so as to react substantially an entire thickness of the sacrificial back electrode metal layer with the absorber layer, thereby recrystallizing the absorber layer and forming a back ohmic contact comprising a metallic compound of the sacrificial back electrode metal layer and the absorber layer, in combination with the thermal stress relieving layer; forming a transparent, n-type semiconductor layer over the absorber layer so as to define a p-n junction therebetween; and forming a transparent top electrode on the n-type semiconductor layer. 
         [0007]    In another aspect a photovoltaic device includes a thermal stress relieving layer on top of a substrate; a back ohmic contact on the thermal stress relieving layer; and a p-type semiconductor photon absorber layer on the back ohmic contact; wherein the back ohmic contact comprises a metallic compound of the sacrificial back electrode metal layer and the absorber layer, in combination with the thermal stress relieving layer, and wherein the thermal stress relieving layer has a substantially similar thermal expansion coefficient with respect to the substrate and the absorber layer and a lower Young&#39;s modulus with respect to the sacrificial back electrode metal layer. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0008]    Referring to the exemplary drawings wherein like elements are numbered alike in the several Figures: 
           [0009]      FIG. 1  is a cross sectional schematic view of a thin-film photovoltaic cell; 
           [0010]      FIG. 2  is a cross sectional view of various photovoltaic cell layers, such as the cell schematically depicted in  FIG. 1 ; 
           [0011]      FIG. 3  is a scanning electron micrograph (SEM) image of a molybdenum back electrical contact layer and a CIGS absorber layer formed thereon; 
           [0012]      FIG. 4  is another scanning electron micrograph (SEM) image of a molybdenum back electrical contact layer and a CIGS absorber layer formed thereon; 
           [0013]      FIG. 5  is a flow diagram illustrating a method of forming a solar cell having a thin-film stress relieving layer, in accordance with an exemplary embodiment; and 
           [0014]      FIG. 6  is a cross sectional schematic view of a thin-film photovoltaic cell, such as formed in accordance with the processing illustrated in  FIG. 5 . 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    With respect to thin-film photovoltaic cells as discussed above,  FIG. 1  is a cross sectional schematic view of one exemplary type of cell  100 . The thin-film cell  100  is fabricated on a substrate  102 , such as a sheet of glass, a sheet of metal, an insulating foil or web, or a conductive foil or web, for example. A back electrical contact layer  104  is deposited on the substrate  102 , and serves as an ohmic contact for the cell  102 . One example of such a back contact material layer is molybdenum (Mo), which is currently perhaps the most commonly used material, having a thickness of about 0.5 microns (μm) to about 2.0 μm. A photon absorber layer  106  is formed on the back contact layer  104 . In such a thin-film structure, the absorber layer  106  may be a copper-indium-gallium-sulfide/selenide (CIGS) or copper-zinc-tin-sulfide/selenide (CZTS), p-type semiconductor absorber layer, having an exemplary thickness of about 0.5 μm to 3 μm thick. 
         [0016]    As further illustrated in  FIG. 1 , an n-type semiconductor layer  108  is formed over the absorber layer  106  so as to define a p-n junction therebetween. Relatively speaking, the n-type layer  108  is much thinner than the absorber layer (e.g., about 60 nanometers (nm)), and is desirably highly transparent to solar radiation. An exemplary material for the n-type layer  108  is cadmium sulfide (CdS). Formed upon the n-type layer  108  is a thin, (e.g., about 0.2 μm to about 0.6 μm) transparent top electrode  110 , which completes a functioning cell. The top electrode  110  is both highly conductive and as transparent as possible to solar radiation. Exemplary top electrode materials in this regard include transparent conductive oxides (TCO) such as, for example, zinc oxide (ZnO), indium tin oxide (ITO), aluminum doped ZnO, etc. An optional antireflection (AR) coating  112  is also depicted on the solar cell structure  100  of  FIG. 1 , which can allow a significant amount of extra light into the cell  100 . Depending on the intended use of the cell  110 , the AR coating  112  may be deposited directly on the top electrode  110  (as illustrated), or on a separate cover glass, or both. 
         [0017]    In operation, some photons  114  in sunlight pass through the AR coating  112 , top electrode  110  and n-type layer  108 , and are absorbed by the p-type semiconductor absorber layer  106 . Other photons, depending on the photon energy, may reflect off the cell surface or pass completely through absorber layer  106 . In the latter case, it is possible that photos may be reflected back from the back electrical contact layer  104  and then absorbed by the absorber layer  106 . In any case, photons that are absorbed by the absorber layer  106  give their energy to the crystal lattice of the absorber layer  106 , in turn knocking electrons loose from their atoms. Due to the composition of the cell  100 , the electrons flow in a single direction, and therefore are capable of providing direct current (DC) electricity to an exemplary load  116 . 
         [0018]    In more structural detail,  FIG. 2  is a cross sectional view of various photovoltaic cell layers, such as the cell  100  schematically depicted in  FIG. 1 . For ease of illustration, similar layers in  FIGS. 1 and 2  are designated with the same reference numeral. The exemplary substrate  102  shown in  FIG. 2  may be formed from a material such as glass, metal foil or plastic. In the case of glass, the substrate  102  has a thermal expansion coefficient (TEC) of about 9×10 −6 /K and a Young&#39;s modulus of about 80 gigapascals (GPa). The molybdenum back electrical contact layer  104  has a TEC of about 4.8×10 −6 /K, a Young&#39;s modulus of about 329 GPa, and an electrical resistivity of about 5×10 −8 /Ω·m (ohm-meters). In addition, the relatively thick CIGS/CZTS absorber layer  106  has a TEC of about 9×10 −6 /K, a Young&#39;s modulus of about 60 GPa. 
         [0019]    Physical vapor deposition (PVD) based processes, and particularly sputter based deposition processes, have conventionally been utilized for high volume manufacturing of such thin film layers with high throughput and yield. More recently, techniques for depositing the CIGS/CZTS absorber layer  106  via solution (instead of by traditional vacuum based co-sputtering and co-evaporation deposition processes) have attracted increasing attention. Advantages associated with such solution-based deposition techniques include lower-initial capital cost, higher throughput, and higher material utilization rate. 
         [0020]    For the traditional vacuum based deposition processes, the deposition takes place at an elevated temperature, e.g., about 400° C. to about 600° C., to form polycrystalline semiconductors with large grain size. The cooling process after deposition could lead to high thermal stress at CIGS/CZTS-Mo interface, which in turn can lead to severe film cracking issues. For low-temperature, solution-based processes, a rapid high temperature annealing step after deposition is necessary to transform initially deposited amorphous or nanocrystalline films into high quality, polycrystalline films for solar cell applications. However, such a rapid heating/cooling process may also lead to very high thermal stress at CIGS/CZTS-Mo interface, which in turn can lead to severe film cracking issues. For example,  FIG. 3  is a scanning electron micrograph (SEM) image of a molybdenum back electrical contact layer  302  and a CIGS absorber layer  304  formed thereon. However, due to the thermal stress from annealing, the CIGS absorber layer  304  suffers from cracking and delamination, as indicated at the arrow. 
         [0021]    Other issues associated with rapid high temperature annealing of solution deposited semiconductor films are illustrated in  FIG. 4 . As shown in the SEM image of  FIG. 4 , a molybdenum back electrical contact layer  402  has a CIGS absorber layer  404  formed thereon.  FIG. 4  also depicts a CdS n-type layer  406  on the CIGS layer  404 , and a ZnO/TCO top electrode  408  on the CdS n-type layer  406 . Although the CIGS layer  404  is not cracked in this particular image, multiple voids/defects  410  between the CIGS layer  404  and the molybdenum back electrical contact layer  402  are evident. These voids and defects could adversely affect both the power conversion efficiency of associated solar cells and the yield of integrated solar modules. 
         [0022]    In terms of surface delamination energy, which is a function of material hardness, thickness, and thermal expansion coefficient, a molybdenum back electrical contact layer is not well matched with, for example, a glass substrate and a semiconductor CIGS/CZTS absorber layer. Relatively speaking, molybdenum has a lower CTE with respect to glass and CIGS/CZTS and a high modulus. On the other hand, molybdenum works well as a back contact metal since the reaction between molybdenum and (for example) selenium in CIGS produces a layer of MoSe 2 , which forms an ohmic contact. 
         [0023]    Accordingly, the exemplary embodiments disclosed herein address the above described problems by introducing a thermal stress relieving layer between the solar cell substrate and a molybdenum-based compound. Initially, several materials for the stress relieving layer were considered based on the following desired characteristics: (1) a TEC of about 9×10 −6 /K to match up with glass and CIGS/CZTS; (2) a relatively low modulus; (3) low resistivity; (4) high melting temperature; (5) low reactivity with selenium/sulfur; and (6) low cost, high abundance. 
         [0024]    Based on these criteria, titanium (Ti), while having desirable TEC, modulus and resistivity values, is hard to form a low-stress, thick film, has high reactivity with Se, and is expensive. Titanium nitride (TiN) also has a matching TEC but has a high modulus and is a difficult material for forming a thick film. Platinum (Pt), while having a suitable TEC, is an expensive material for photovoltaic applications. Vanadium (V) has compatible TEC, modulus and resistivity values, but like titanium, has high reactivity with Se. 
         [0025]    In contrast, niobium (Nb), has a compatible TEC (about 7.4×10 −6 /K), low modulus (about 105 GPa), and resistivity (about 15×10 −8 /Ω·m). In addition, niobium has a melting temperature of 2468K, has low reactivity with selenium, is easy to form a low-stress thick film, and has a cost comparable with that of molybdenum. 
         [0026]    Referring now to both  FIGS. 5 and 6 ,  FIG. 5  is a flow diagram illustrating a method  500  of forming a solar cell having a thin-film stress relieving layer, in accordance with an exemplary embodiment.  FIG. 6  is a cross sectional schematic view of a thin-film photovoltaic cell  600 , such as formed in accordance with the processing illustrated in  FIG. 5 . 
         [0027]    As illustrated in block  502  of  FIG. 5 , a conductive thermal stress relieving layer is deposited on a substrate. In the exemplary embodiment of  FIG. 6 , the substrate  602  a material such as glass, metal foil or plastic. In the case of glass, the substrate  602  has a TEC of about 9×10 −6 /K and a Young&#39;s modulus of about 80 GPa. In contrast to a thick molybdenum layer formed directly on the substrate, the cell  600  has a niobium thermal stress relieving layer  604  formed on the substrate  602 , which is thick enough to provide low resistance for electron transport. In an exemplary embodiment, the Nb thermal stress relieving layer  604  may have a thickness of about 0.5 μm to about 2.0 μm. After deposition of the Nb thermal stress relieving layer, a thin back electrode metal layer is formed directly on the thermal stress relieving layer as indicated in block  504  of  FIG. 5 . As indicated above, molybdenum is still a desirable metal for this purpose. However, in contrast to conventional solar cell designs, the sacrificial back electrode metal layer is composed of a thin layer of molybdenum (e.g., on the order of about 60 nm) for better contact and a conductive stress relieving layer. This decreased thickness of a hard molybdenum layer and the adoption of a thick but soft stress relieving layer with better matched TEC minimize thermal stresses at the interfaces between the back electrode metal layer and a subsequently formed absorber layer, without affecting the resistance of the back electrode. 
         [0028]    In block  506  of  FIG. 5 , a semiconductor absorber layer is formed on the back electrode metal layer. The absorber layer may be, for example, CIGS, CZTS, or combinations thereof. Embodiments described herein are applicable to either vacuum-based semiconductor deposition techniques at elevated temperatures or, alternatively to a low temperature (e.g., room temperature) solution-based spin-on technique, followed by a recrystallizing anneal process. 
         [0029]    In either approach, as depicted in block  508 , the device is at some point subjected to an elevated temperature process (either during or after semiconductor absorber layer deposition) to transform the semiconductor film into a high quality, polycrystalline for solar cell applications. For a low temperature, liquid depositon of semiconductor material, this may be followed by a rapid thermal anneal (RTA) or a laser spike anneal. As a result of the elevated temperature process, the molybdenum back electrode metal layer reacts with the absorber to form a compound of, for example, molybdenum selenide (MoSe 2 ) or molybdenum disulfide (MoS 2 ). The molybdenum back electrode may or may not completely react with the absorber, but in any case forms part of an ohmic contact. With respect to  FIG. 6 , the thin, reacted molybdenum-based compound is indicated by a back interface contact layer  605  atop the niobium thermal stress relieving layer  604 , which together define a back electrode. 
         [0030]    As further illustrated in  FIG. 6 , the CIGS/CZTS absorber layer  606  is formed atop the thin, back interface contact layer  605 . Once the CIGS/CZTS absorber layer  606  is formed, the remaining solar cell layers may be formed as indicated in block  510  of  FIG. 5 . These layers include, as shown in  FIG. 6  for example, a thin n-type layer  608  (e.g., about 60 nanometers (nm)) transparent to solar radiation such as CdS), a thin (e.g., about 0.2 μm to about 0.6 μm) top transparent electrode  610 , such as ZnO, ITO, aluminum doped ZnO, etc., and an optional AR coating  612 . 
         [0031]    While the invention has been described with reference to a preferred embodiment or embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.