Abstract:
An improved method of doping a substrate is disclosed. The method is particularly beneficial to the creation of interdigitated back contact (IBC) solar cells. A paste having a dopant of a first conductivity is applied to the surface of the substrate. This paste serves as a mask for a subsequent ion implantation step, allowing ions of a dopant having an opposite conductivity to be introduced to the portions of the substrate which are exposed. After the ions are implanted, the mask can be removed and the dopants may be activated. Methods of using an aluminum-based and phosphorus-based paste are disclosed.

Description:
[0001]    This application claims priority of U.S. Provisional Patent Application Ser. No. 61/305,764, filed Feb. 18, 2010, the disclosure of which is incorporated herein by reference in its entirety. 
     
    
     FIELD 
       [0002]    This invention relates to solar cells and, more particularly, to solar cells formed using ion implantation. 
       BACKGROUND 
       [0003]    Ion implantation is a standard technique for introducing conductivity-altering impurities into a workpiece. A desired impurity material is ionized in an ion source, the ions are accelerated to form an ion beam of prescribed energy, and the ion beam is directed at the surface of the workpiece. The energetic ions in the beam penetrate into the bulk of the workpiece material and are embedded into the crystalline lattice of the workpiece material to form a region of desired conductivity. 
         [0004]    Solar cells are one example of a device that uses silicon workpieces. Any reduced cost to the manufacture or production of high-performance solar cells or any efficiency improvement to high-performance solar cells would have a positive impact on the implementation of solar cells worldwide. This will enable the wider availability of this clean energy technology. 
         [0005]    Solar cells typically consist of a p-n semiconducting junction.  FIG. 1  is a cross-sectional view of an interdigitated back contact (IBC) solar cell. In the IBC solar cell, the p-n junction is on the back or non-illuminated surface of the solar cell. Photons  10  enter the solar cell  100  through the top (or illuminated) surface, as signified by the arrows. These photons pass through an anti-reflective coating  104 , designed to maximize the number of photons that penetrate the substrate  100  and minimize those that are reflected away from the substrate. The ARC may be comprised of an SiN x  layer. Beneath the ARC  104  may be a SiO 2  layer  103 , also known as a passivation layer. Of course, other dielectrics may be used. On the back side of the solar cell  100  is an emitter region  204 . 
         [0006]    Internally, the solar cell  100  is formed so as to have a p-n junction. This junction is shown as being substantially parallel to the top surface of the solar cell  100 , although there are other implementations where the junction may not be parallel to the surface. In some embodiments, the solar cell  100  is fabricated using an n-type substrate  101 . The photons  10  enter the solar cell  100  through the n+ doped region, also known as the front surface field (FSF)  102 . The photons with sufficient energy (above the bandgap of the semiconductor) are able to promote an electron within the semiconductor material&#39;s valence band to the conduction band. Associated with this free electron is a corresponding positively charged hole in the valence band. In order to generate a photocurrent that can drive an external load, these electron hole (e-h) pairs need to be separated. This is done through the built-in electric field at the p-n junction. Thus, any e-h pairs that are generated in the depletion region of the p-n junction get separated, as are any other minority carriers that diffuse to the depletion region of the device. Since a majority of the incident photons are absorbed in near surface regions of the device, the minority carriers generated in the emitter need to diffuse to the depletion region and get swept across to the other side. 
         [0007]    As a result of the charge separation caused by the presence of this p-n junction, the extra carriers (electrons and holes) generated by the photons can then be used to drive an external load to complete the circuit. 
         [0008]    The doping pattern is alternating p-type and n-type dopant regions in this particular embodiment. The n+ back surface field  204  may be between approximately 0.1-0.7 mm in width and doped with phosphorus or other n-type dopants. The p+ emitter  203  may be between approximately 0.5-3 mm in width and doped with boron or other p-type dopants. This doping may enable the p-n junction in the IBC solar cell to function or have increased efficiency.  FIG. 6  shows a commonly used pattern for the back side of the IBC solar cell. The metallic contacts or fingers  220  are all located on the bottom surface of the solar cell  100 . Certain portions of the bottom surface may be implanted with p-type dopants to create emitters  203 . Other portions are implanted with n-type dopants to create more negatively biased back surface field  204 . The back surface is coated with a passivating layer  210  to enhance the reflectivity of the back surface. Metal fingers  220   b  are attached to the emitter  203  and fingers  220   a  attaches to the BSF  204 . 
         [0009]    Thus, to form the IBC solar cell, two patterned doping steps may be required. These patterned doping steps need to be aligned to prevent the p+ emitter  203  and the n+ back surface field  204  from overlapping. Poor alignment or overlapping may be prevented by leaving a gap between the p+ emitter  203  and the n+ back surface field  204 , but this may degrade performance of the IBC solar cell. Even when properly aligned, such patterned doping may have large manufacturing costs. For example, photolithography or hard masks (such as an oxide) may be used, but both are expensive and require extra process steps. 
         [0010]      FIG. 2  is a block diagram of a first method to form an IBC solar cell according to the prior art. This process requires two patterned diffusion steps (shown as “Screen Print Patterned Resist”) which must be well aligned to produce the pattern of  FIG. 6 .  FIG. 3  is a block diagram of a second method to form an IBC solar cell. This embodiment performs a first blanket diffusion. The emitter is then etched to expose underlying silicon. The etch mask and the diffusion mask can be the same, although different chemistries are used to etch the oxide mask and to dope the underlying silicon. 
         [0011]    The embodiments of  FIGS. 2-3  both require a large number of expensive process steps to form an IBC solar cell. While the embodiment of  FIG. 3  uses fewer process steps than the embodiment of  FIG. 2 , it is still complicated and still uses multiple wet steps. For these embodiments of  FIGS. 2-3 , the addition of an oxide layer is yet another extra process step. 
         [0012]    Therefore, there is a need in the art for an improved method of doping for solar cells and, more particularly, an improved method of doping for IBC solar cells using ion implantation. 
       SUMMARY 
       [0013]    An improved method of doping a substrate is disclosed. The method is particularly beneficial to the creation of interdigitated back contact (IBC) solar cells. A paste having a dopant of a first conductivity is applied to the surface of the substrate. This paste serves as a mask for a subsequent ion implantation step, allowing ions of a dopant having an opposite conductivity to be introduced to the portions of the substrate which are exposed. After the ions are implanted, the mask can be removed and the dopants may be activated. Methods of using an aluminum-containing and phosphorus-containing paste are disclosed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    For a better understanding of the present disclosure, reference is made to the accompanying drawings, which are incorporated herein by reference and in which: 
           [0015]      FIG. 1  is a cross-sectional view of an IBC solar cell; 
           [0016]      FIG. 2  is a block diagram of a first method to form an IBC solar cell according to the prior art; 
           [0017]      FIG. 3  is a block diagram of a second method to form an IBC solar cell according to the prior art; 
           [0018]      FIGS. 4A-D  are cross-sectional views of a first method to form an IBC solar cell; 
           [0019]      FIGS. 5A-D  are cross-sectional views of a second method to form an IBC solar cell; and 
           [0020]      FIG. 6  is a bottom view of the IBC solar cell of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    The embodiments of the solar cell are described herein in connection with an ion implanter. Beamline ion implanters, plasma doping ion implanters, focused plasma systems, systems that modulate a plasma sheath, or flood ion implanters may be used. However, the gaseous diffusion, furnace diffusion, laser doping, other plasma processing tools, or other methods known to those skilled in the art also may be used. While specific n-type and p-type dopants are listed, other n-type or p-type dopants may be used instead and the embodiments herein are not limited solely to the dopants listed. Furthermore, while one particular embodiment of a solar cell is specifically listed, embodiments of this process may be applied to other solar cell designs or other workpieces such as semiconductor wafers or flat panels. Thus, the invention is not limited to the specific embodiments described below. 
         [0022]      FIGS. 4A-D  are cross-sectional views of a first method to form an IBC solar cell. In  FIG. 4A , an aluminum paste  301  is printed on the solar cell  300  in the pattern of the emitter. Screen printing may be used in one instance, though other printing methods known to those skilled in the art likewise may be used. 
         [0023]    In  FIG. 4B , the solar cell  300  is fired in a furnace. The temperature is sufficiently high so as to allow the aluminum to melt into the solar cell  300 . The furnace may be an in-line furnace that, in one embodiment, operates between approximately 550° C. and 850° C. The aluminum paste  301  melts and forms a eutectic with the silicon of the solar cell  300 . The aluminum paste  301  also forms a doped layer  303  approximately 2-10 μm into the silicon of the solar cell  300 . This doped layer forms the p+ emitter  303  of the solar cell  300 . 
         [0024]    The solar cell  300  is then implanted with a species  310  in  FIG. 4C . The species  310  may be phosphorus and may form the n+ back surface field  304 . In other embodiments, other Group V elements may be used as the n-type dopant. The aluminum paste  301  may serve as a mask that substantially prevents the species  310  from being implanted into the solar cell  300  beneath the aluminum paste  301 . In other words, a majority of the species  310  do not penetrate through the aluminum paste  301 . In some embodiments, at least 90% of the species  310  do not penetrate through the aluminum page  301 . In some embodiments, 100% of the ions may be blocked. 
         [0025]    In one particular embodiment, the side of the solar cell  300  opposite the n+ back surface field  304  may be implanted with phosphorus or another n-type dopant at least partially simultaneously or during a subsequent step. This creates a front surface field. 
         [0026]    In  FIG. 4D , the aluminum paste  301  and the aluminum-silicon eutectic are removed using, for example, a wet step. In some embodiments, this is an acid etch. The doped silicon that was amorphized during the implantation of  FIG. 4C  may resist this wet step. The eutectic is removed to enable proper passivation of the p+ emitter  303 . The implanted dopants are then activated in a thermal activation step, such as an anneal step. An oxide may be grown at least partially during this anneal step in one instance to passivate both the front and back surfaces of the solar cell  300 . 
         [0027]      FIGS. 5A-D  are cross-sectional views of a second method to form an IBC solar cell. A phosphorus doping paste  401  is applied to the solar cell  400  in  FIG. 5A . In other embodiments, n-type dopants such as arsenic or antimony may be used. P-type dopants such as boron, aluminum, gallium or indium may also be used. 
         [0028]    Then, a species  405  is implanted into the solar cell  400  in  FIG. 5B . The species  405  may be boron or another p-type dopant and may form the p+ emitter  403 . The phosphorus doping paste  401  may serve as a mask that substantially prevents the species  405  from being implanted into the solar cell  400  beneath the phosphorus doping paste  401 . In other words, a majority of the species  405  do not penetrate through the doping paste  401 . In some embodiments, at least 90% of the species  405  do not penetrate through the doping page  401 . In some embodiments, 100% of the ions may be blocked. 
         [0029]    In other embodiments, the species  405  may be any of the species listed above, including aluminum, arsenic, antimony, phosphorus, gallium and indium. In other words, a paste containing a p-type dopant may be applied, followed by an implant of ions from an n-type dopant. In other embodiments, such as described above, a paste containing an n-type dopant is applied, followed by an implant of ions from a p-type dopant. 
         [0030]    The phosphorus doping paste  401  may take a longer time to penetrate the substrate than the aluminum paste  301 . Thus, the diffusion step may be combined with an anneal or the activation of the implanted p+ emitter  403  illustrated in  FIG. 5B  to reduce processing time. Thus, at the same time, the implanted boron is activated, and the phosphorus diffuses from the paste into the silicon. In another embodiment, the diffusion of the phosphorus may be done at a different time that the activation of the p-type dopant. 
         [0031]    Thus, in  FIG. 5C , the solar cell  400  is annealed. This activates the p+ emitter  403  and causes phosphorus to diffuse into the solar cell  400  and form the n+ back surface field  404 . Of course, separate thermal steps for phosphorus diffusion and boron activation may be performed. In one instance, an oxide layer is grown on the side of the cell opposite of the n+ back surface field  404  during a thermal or anneal step. 
         [0032]    In  FIG. 5D , the phosphorus doping paste  401  is removed. The phosphorus doping paste  401  may form a phosphorus-silicate glass during the anneal process. An etch step or a buffered HF process may be used to remove the phosphorus doping paste  401  in one instance. 
         [0033]    The embodiments of  FIGS. 4-5  allow doped n-type and p-type regions directly next to or adjacent one another. Thus, the n-type and p-type regions are aligned and the p-n junction may be narrow due to the high dopant concentrations on either side of the junction. Carriers may cross the junction through quantum mechanical tunneling. Thermal steps in the embodiments of  FIGS. 4-5  may be lengthened to enable dopants to diffuse across the junction. As dopants diffuse across this junction, the net carrier concentration at the boundary may be reduced and the depletion depth may be increased. Deeper depletion regions inhibit tunneling. 
         [0034]    In other embodiments, the conductivity of the two dopants used may be the same. For example, it may be beneficial to diffuse a dopant into a portion of a solar cell, while implanting ions of similar conductivity into adjacent portions of the solar cell. 
         [0035]    The present disclosure is not to be limited in scope by the specific embodiments described herein. Indeed, other various embodiments of and modifications to the present disclosure, in addition to those described herein, will be apparent to those of ordinary skill in the art from the foregoing description and accompanying drawings. Thus, such other embodiments and modifications are intended to fall within the scope of the present disclosure. Furthermore, although the present disclosure has been described herein in the context of a particular implementation in a particular environment for a particular purpose, those of ordinary skill in the art will recognize that its usefulness is not limited thereto and that the present disclosure may be beneficially implemented in any number of environments for any number of purposes. Accordingly, the claims set forth below should be construed in view of the full breadth and spirit of the present disclosure as described herein.