Abstract:
Methods and apparatus for converting electromagnetic radiation, such as solar energy, into electric energy with increased efficiency when compared to conventional solar cells are provided. A photovoltaic (PV) device generally includes a window layer; an absorber layer disposed below the window layer such that electrons are generated when photons travel through the window layer and are absorbed by the absorber layer; and a plurality of contacts for external connection coupled to the absorber layer, such that all of the contacts for external connection are disposed below the absorber layer and do not block any of the photons from reaching the absorber layer through the window layer. Locating all the contacts on the back side of the PV device avoids solar shadows caused by front side contacts, typically found in conventional solar cells. Therefore, PV devices described herein with back side contacts may allow for increased efficiency when compared to conventional solar cells.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims benefit of U.S. Provisional Patent Application Ser. No. 61/107,966 filed Oct. 23, 2008, which is herein incorporated by reference in its entirety. 
     
    
     BACKGROUND 
       [0002]    1. Technical Field 
         [0003]    Embodiments of the present invention generally relate to photovoltaic (PV) devices, such as solar cells, with increased efficiency and greater flexibility and methods for fabricating the same. 
         [0004]    2. Description of the Related Art 
         [0005]    As fossil fuels are being depleted at ever-increasing rates, the need for alternative energy sources is becoming more and more apparent. Energy derived from wind, from the sun, and from flowing water offer renewable, environment-friendly alternatives to fossil fuels, such as coal, oil, and natural gas. Being readily available almost anywhere on Earth, solar energy may someday be a viable alternative. 
         [0006]    To harness energy from the sun, the junction of a solar cell absorbs photons to produce electron-hole pairs, which are separated by the internal electric field of the junction to generate a voltage, thereby converting light energy to electric energy. The generated voltage can be increased by connecting solar cells in series, and the current may be increased by connecting solar cells in parallel. Solar cells may be grouped together on solar panels. An inverter may be coupled to several solar panels to convert DC power to AC power. 
         [0007]    Nevertheless, the currently high cost of producing solar cells relative to the low efficiency levels of contemporary devices is preventing solar cells from becoming a mainstream energy source and limiting the applications to which solar cells may be suited. Accordingly, there is a need for more efficient photovoltaic devices suitable for a myriad of applications. 
       SUMMARY OF THE INVENTION 
       [0008]    Embodiments of the present invention generally relate to methods and apparatus for converting electromagnetic radiation, such as solar energy, into electric energy with increased efficiency when compared to conventional solar cells. 
         [0009]    One embodiment of the present invention provides a photovoltaic (PV) device. The PV device generally includes a window layer, an absorber layer disposed below the window layer such that electrons are generated when photons travel through the window layer and are absorbed by the absorber layer, and a plurality of contacts for external connection coupled to the absorber layer, such that the contacts for external connection are disposed below the absorber layer and do not block the photons from reaching the absorber layer through the window layer. 
         [0010]    Another embodiment of the present invention is a method of fabricating a PV device. The method generally includes forming a window layer above a substrate, forming an absorber layer above the window layer such that electrons are generated when photons travel through the window layer and are absorbed by the absorber layer, and forming a plurality of contacts for external connection coupled to the absorber layer, such that the contacts for external connection are disposed above the absorber layer and do not block the photons from reaching the absorber layer through the window layer. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    So that the manner in which the above-recited features of the invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
           [0012]      FIG. 1  illustrates multiple epitaxial layers for a photovoltaic (PV) unit in cross-section, in accordance with an embodiment of the present invention. 
           [0013]      FIG. 2  illustrates contacts to the semiconductor layers being on the back side of the PV unit, in accordance with an embodiment of the present invention. 
           [0014]      FIG. 3  illustrates passivation on the edges of the recesses in the emitter layer, in accordance with an embodiment of the present invention. 
           [0015]      FIG. 4A  illustrates the back side of the PV unit, in accordance with an embodiment of the present invention. 
           [0016]      FIG. 4B  illustrates an equivalent electrical circuit of the PV unit of  FIG. 4A , in accordance with an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    Embodiments of the present invention provide techniques and apparatus for converting electromagnetic radiation, such as solar energy, into electric energy with increased efficiency when compared to conventional solar cells. 
       An Exemplary Photovoltaic Unit 
       [0018]      FIG. 1  illustrates various epitaxial layers of a photovoltaic (PV) unit  100  in cross-section. The various layers may be formed using any suitable method for semiconductor growth, such as molecular beam epitaxy (MBE) or metalorganic chemical vapor deposition (MOCVD), on a substrate (not shown). 
         [0019]    The PV unit  100  may comprise a window layer  106  formed above the substrate and any underlying buffer layer(s). The window layer  106  may comprise aluminum gallium arsenide (AlGaAs), such as Al 0.3 Ga 0.7 As. The window layer  106  may be undoped. The window layer  106  may be transparent to allow photons to pass through the window layer on the front side of the PV unit to other underlying layers. 
         [0020]    A base layer  108  may be formed above the window layer  106 . The base layer  108  may comprise any suitable group III-V compound semiconductor, such as GaAs. The base layer  108  may be monocrystalline and may be n-doped. 
         [0021]    As illustrated in  FIG. 1 , an emitter layer  110  may be formed above the base layer  108 . The emitter layer  110  may comprise any suitable group III-V compound semiconductor for forming a heterojunction with the base layer  108 . For example, if the base layer  108  comprises GaAs, the emitter layer  110  may comprise a different semiconductor material, such as AlGaAs (e.g., Al 0.3 Ga 0.7 As). If the emitter layer  110  and the window layer  106  both comprise AlGaAs, the Al x Ga 1-x As composition of the emitter layer  110  may be the same as or different than the Al y Ga 1-y As composition of the window layer  106 . The emitter layer  110  may be monocrystalline and may be heavily p-doped p + -doped). The combination of the base layer  108  and the emitter layer  110  may form an absorber layer for absorbing photons. 
         [0022]    The contact of an n-doped base layer to a p + -doped emitter layer creates a p-n layer  112 . When light is absorbed near the p-n layer  112  to produce electron-hole pairs, the built-in electric field may force the holes to the p + -doped side and the electrons to the n-doped side. This displacement of free charges results in a voltage difference between the two layers  108 ,  110  such that electron current may flow when a load is connected across terminals coupled to these layers. 
         [0023]    Rather than an n-doped base layer  108  and a p + -doped emitter layer  110  as described above, conventional photovoltaic semiconductor devices typically have a p-doped base layer and an n + -doped emitter layer. The base layer is typically p-doped in conventional devices due to the diffusion length of the carriers. 
         [0024]    Once the emitter layer  110  has been formed, cavities or recesses  114  may be formed in the emitter layer deep enough to reach the underlying base layer  108 . Such recesses  114  may be formed by applying a mask to the emitter layer  110  using photolithography, for example, and removing the semiconductor material in the emitter layer  110  not covered by the mask using any suitable technique, such as wet or dry etching. In this manner, the base layer  108  may be accessed via the back side of the PV unit  100 . 
         [0025]    For some embodiments, an interface layer  116  may be formed above the emitter layer  110 . The interface layer  116  may comprise any suitable group III-V compound semiconductor, such as GaAs. The interface layer  116  may be p + -doped. 
         [0026]    Once the epitaxial layers have been formed, the functional layers of the PV unit  100  (e.g., the window layer  106 , the base layer  108 , and the emitter layer  110 ) may be separated from the buffer layer(s)  102  and substrate during an epitaxial lift-off (ELO) process. 
       Exemplary Electrical Contacts 
       [0027]    Electrical contacts may be used to couple the semiconductor layers of the PV unit  100  to wires for connection to other PV units and for external connection to a load. A conventional solar cell typically has contacts on both the front and back sides of the cell. Front side contacts, especially thicker ones, create shadows where light cannot reach the underlying absorber layer to be converted into electric energy. Therefore, the efficiency potential of the solar cell cannot be obtained. Accordingly, techniques and apparatus for contacting the semiconductor layers of the PV unit without introducing shadows are needed. 
         [0028]      FIG. 2  illustrates all electrical contacts to the semiconductor layers being on the back side of the PV unit  100 , according to an embodiment of the present invention. For example, n-contacts  602  may be formed in the recesses  114  to provide an interface to the n-doped base layer  108 , and p-contacts  604  may be formed above the interface layer  116  to couple to the p + -doped emitter layer  110 . The heavy doping in the p + -doped interface layer  116  may facilitate making an ohmic contact. In this manner, efficiency need not be sacrificed by having electrical contacts on the front side of the PV unit to block light and create solar shadows. 
         [0029]    The pattern of the recesses  114  in the emitter layer  110  and the remaining portion of the interface layer  116  for the contacts  602 ,  604  may be based on the desired sheet resistance. The dimensions (e.g., area) of the contacts  602 ,  604  may be very small compared to the dimensions (e.g., area) of a single PV unit  100 . What is more, the pattern of the contacts  602 ,  604  may provide a built-in tolerance against local defects and shadowing. 
         [0030]    The contacts  602 ,  604  may comprise any suitable electrically conductive material, such as a metal or a metal alloy. Preferably, the material for the contacts should not punch through the semiconductor layers during fabrication. Traditional contacts comprising gold (Au) often had this spiking problem. Furthermore, the material for the back side contacts may preferably be capable of being applied at relatively low metallization process temperatures, such as between 150 and 200° C. For example, the contact  602 ,  604  may comprise palladium/germanium (Pd/Ge) to meet these design goals. Palladium does not react with GaAs. 
         [0031]    Whatever material is selected, the contacts  602 ,  604  may be fabricated on the PV unit  100  by any suitable method, such as vacuum-evaporation through a photoresist, photolithography, screen printing, or merely depositing on the exposed portion of the PV units that have been partially covered with wax or another protective material. These methods all involve a system in which the part of the PV unit on which a contact is not desired is protected, while the rest of the PV unit is exposed to the metal. Of these, screen printing may be the most cost effective, helping to decrease the cost of the resulting PV devices. 
         [0032]    Despite all the contacts  602 ,  604  being on the back side of the PV unit  100  to reduce solar shadows, dark current and its stability with time and temperature may still be concerns when designing an efficient PV unit. An exposed p-n layer  112  may be a source of dark current, and larger recesses  114  may be responsible for an increase in dark current. Thus, smaller recesses  114  may be desired. However, there is a tradeoff between reducing the size of the recesses  14  to reduce dark current and the probability of fabricating the n-contacts  602  in the recesses  114  without touching the sidewalls. 
         [0033]    Therefore, for some embodiments, the sidewalls of the recesses  114  may be passivated as another way to reduce the dark current in the PV unit.  FIG. 3  illustrates passivation  702  on the sidewalls (i.e., lateral surfaces) of the recesses  114  in the emitter layer  110 , in accordance with an embodiment of the present invention. The sidewalls may be passivated most likely before—but possibly after—the n-contacts  602  are formed, using any suitable passivation method, such as chemical vapor deposition (CVD) or plasma-enhanced CVD (PECVD). The passivation  702  may comprise any suitable electrically non-conductive material, such as silicon nitride (SiN), SiO x , TiO x , TaO x , zinc sulfide (ZnS), or any combination thereof. 
         [0034]      FIG. 4A  illustrates the back side of the PV unit  100 , wherein all the contacts  602 ,  604  are disposed on the back side. As described above, the n-contacts  602  may be located within the recesses  114  in the emitter layer  110 . The PV unit  100  may have a width w of about 2 to 3 cm and a length l of about 10 cm. 
         [0035]      FIG. 4B  illustrates an equivalent electrical circuit  1500  of the PV unit  100  of  FIG. 4A . One may consider the PV unit  100  as having an efficient miniature solar cell  1502  between each n-contact  602  and p-contact  604 . Within a PV unit  100 , all of the n-contacts  602  are coupled to the same base layer  108  and all of the p-contacts  604  are coupled to the same emitter layer  110 . Therefore, the open circuit voltage (V oc ) of the equivalent circuit  1500  may be modeled as the sum of the open circuit voltages across the miniature solar cells  1502  in series, and the short circuit current (I sc ) may be modeled as the sum of the short circuit currents across the miniature solar cells  1502  in parallel. In essence, the equivalent electrical circuit  1500  of the PV unit  100  may be thought of as a single solar cell with a greater V oc  and a larger I sc  than those of the miniature solar cells  1502  which compose it. 
         [0036]    While the foregoing is directed to embodiments of the invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.