Abstract:
A method for manufacturing a solar cell from a p-doped or n-doped silicon substrate having a first main surface used as an incident-light side and a second main surface used as a back side includes: depositing a thin layer onto the second main surface; depositing a dielectric, glass-forming paste onto the second main surface and drying it, in order to cover the thin layer; heating and/or sintering the paste on the second main surface at temperatures greater than app. 577° C., to produce an aluminum dopant layer in the second main surface; and removing the glass layer formed during the heating and/or sintering, as well as an aluminum-silicon eutectic layer formed during the heating and/or sintering, from the second main surface.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a method for manufacturing a solar cell, as well as to a solar cell manufactured according to this method. 
         [0003]    2. Description of the Related Art 
         [0004]    Silicon solar cells are often provided with a metallic coating over an entire surface for reflection and for charge collection on the back side. This backside metallic coating is made of, as a rule, aluminum-based, thick-film paste, which is printed between silver-based soldering surfaces, over a large surface. When sintered above 800° C., the aluminum partially alloys with the upper semiconductor surface by forming the low-melting point (577° C.) AlSi eutectic and recrystallizing, and in the process, it over-compensates for the existing n + -doping from the phosphorus diffusion that had previously occurred all-around, to form highly p-doped (p + -)doping (see F. Huster, 20th European Photovoltaic Solar Energy Conference, Jun. 6-10, 2005, Barcelona, Spain). When the base doping is p, the aluminum-doped, recrystallized surface layer forms a p + -BSF (back surface field) having a p + p-transition (high-low transition). 
         [0005]    For several years, the same cell structure has also been produced on n-doped silicon, using a virtually identical process. Then, the above-mentioned, aluminum-doped surface of the back side becomes the p + -emitter, and the phosphorus-doped layer of the front side becomes the front surface field (FSF). 
         [0006]    A disadvantage of the methods known from the related art is that the printed aluminum paste layer must be app. 40 μm thick (after the sintering), in order to obtain sufficiently deep alloy formation or aluminum doping depth. Due to the bimetallic effect between it and the silicon wafer, a reduction in the wafer thickness below the 180 μm typical up to now results in wafer deformation (so-called bow) that is no longer tolerable. High costs for the solar cell result from the necessary thickness of the silicon wafer and the amount of silicon consequently needed. 
         [0007]    The screen-printed metallic coating of the back side has an imperfect reflection factor of only 65% for the long-wave portions of the sunlight, which penetrate to the back side. An effective reflectivity of &gt;90% would increase the optical path length of the incident light and, therefore, the generation of electron-hole pairs (that is, the current) in the interior of the cell. Consequently, a marked gain in efficiency would be obtained. 
         [0008]    In spite of the field passivation by heavy doping, a metallic surface, both that of an emitter and that of a back surface field (BSF), has a large charge-carrier recombination rate. In order to allow more effective passivation of the aluminum-doped surface of the back side, the thick, screen-printed aluminum layer needed as a dopant source and the AlSi eutectic layer formed between it and the semiconductor surface must be etched off. In the previously known methods, a large amount of hydrochloric acid is necessary for that purpose, due to the thickness of the layers to be etched off. This constitutes a large waste disposal problem. 
       BRIEF SUMMARY OF THE INVENTION 
       [0009]    The subject matter of the present invention is a method for manufacturing a solar cell from a p-doped or n-doped silicon substrate, which has a first main surface used as an incident-light side in a state of operation, and a second main surface used as a back side; the method including the following steps: depositing a thin layer, which mainly includes aluminum, onto the second main surface; depositing a dielectric, glass-forming paste onto the second main surface and drying it, in order to cover the thin layer; heating and/or sintering the paste on the second main surface, in particular, at temperatures greater than app. 577° C., in order to produce an aluminum dopant layer in the second main surface; and removing the glass layer formed during the heating and/or sintering, as well as an aluminum-silicon eutectic layer formed during the heating and/or sintering, from the second main surface, through which the aluminum dopant layer is exposed. 
         [0010]    An advantage of this method is that a solar cell, which has an aluminum dopant layer on the back side and allows passivation of the aluminum-doped back side, is manufactured in a technically simple and inexpensive manner. A further advantage of this is that the aluminum layer is deposited so as to be in direct contact with the entire main surface. In this manner, in the case of melting at the eutectic point, the entire amount of aluminum in the layer may be used directly, that is, without delay and/or without hindrance, for forming a melt. The aluminum-silicon melt flows uniformly on the entire second main surface. In addition, the aluminum layer is completely covered by the glass layer, so that when the aluminum-silicon eutectic melts, the melted layer is not exposed at any place. This means that coalescing to form drops, spattering and/or oxidizing are substantially prevented. Since the glass layer is intended as a temporary cover for the thin aluminum layer and is removed again after the doping, the thickness may be selected to be as low as possible. Thus, the glass layer may be removed again more easily and more rapidly. It is also advantageous that the glass layer has a low expansion coefficient that is similar to the silicon of the silicon substrate, which means that bowing of the silicon substrate is substantially prevented. Therefore, the silicon substrate may have a markedly lower thickness. Together, these result in the efficiency of the solar cell being higher and the manufacturing costs of the solar cell being markedly reduced. 
         [0011]    The heating and/or sintering may proceed at a temperature of at least 800° C. By this means, it is ensured that the aluminum layer forms, together with the silicon, a liquid aluminum-silicon eutectic layer. 
         [0012]    In the method, the paste may be applied by printing, in particular, by screen printing. By this means, the costs of the method may be reduced further. 
         [0013]    In one specific embodiment of the method, the glass layer and the aluminum-silicon eutectic layer are etched away from the second main surface, using, in particular, hydrofluoric acid and hydrochloric acid, respectively. An advantage of this is that the layers are uniformly removed from the second main surface, using a proven method. In addition, it is advantageous that less hydrochloric acid is needed due to the low thickness of the aluminum-silicon eutectic layer, which lowers the costs and the degree of complexity, since the disposal of hydrochloric acid is very expensive and complicated. 
         [0014]    In the method, metallic contact tracks for contacting the silicon substrate, and optionally, bus bars for electrically connecting the metallic contact tracks, may be deposited onto the first main surface; during the heating and/or sintering of the paste on the second main surface, the metallic contact tracks and the optional bus bars being simultaneously heated and/or sintered. The charges near the first main surface are collected by the metallic contact tracks, and the charges of a plurality of metallic contact tracks are collected by optional bus bars. Furthermore, it is advantageous that by jointly heating or sintering the metallic contact tracks and the optional bus bars and the paste on the second main surface, an additional step, which would be necessary for heating and/or sintering the metal contact tracks and the optional bus bars, is eliminated. 
         [0015]    The metallic contact tracks and optional bus bars may be deposited by printing a silver paste and/or spraying on an aerosol ink containing silver and/or extruding a silver paste from thin tubes. A uniform thickness of the metallic contact tracks and optional bus bars is ensured by these cost-effective methods for depositing the metallic contact tracks and optional bus bars. 
         [0016]    In a further specific embodiment of the method, prior to depositing the paste onto the second main surface, a thin dielectric layer, in particular, including an oxide and/or a nitride, is also deposited onto the thin layer to prevent oxidation of the aluminum of the thin layer by air. By this means, it is ensured that the thin aluminum layer is not oxidized, which would have a negative effect on the production of an aluminum eutectic or the aluminum dopant layer. 
         [0017]    In a further specific embodiment of the method, after the aluminum dopant layer is exposed, a passivation layer is also deposited onto the second main surface, the passivation layer is removed from regions of the second main surface to form openings, and, using, in particular, a PVD method, preferably, sputtering and/or vapor deposition, a further aluminum layer is deposited onto the second main surface, in order to contact the aluminum dopant layer in the openings. The efficiency of the solar cell is increased by the passivation layer. In addition, it is advantageous that the second main surface of the silicon substrate is contacted by the further aluminum layer in a technically simple manner. Also, the no-load voltage may be increased. Furthermore, compared to a porous aluminum layer, the passivation layer and the further aluminum layer form, together, an improved infrared light mirror on the second main surface, through which a higher current yield per cell is obtained. 
         [0018]    The passivation layer may be removed from regions of the second main surface, using laser ablation, etching paste and/or ion etching. By this means, the costs and the degree of complexity of the method are further reduced, since proven methods for removing the passivation layer are used. 
         [0019]    In the method, a base layer containing nickel may also be deposited onto the further aluminum layer of the second main surface, in order to allow it to be built up chemically and/or galvanically. By this means, the deposition of further layers onto the second main surface is facilitated considerably. 
         [0020]    In the method, after the deposition of the aluminum layer and the optional nickel-containing layer onto the second main surface, an insulating layer may also be deposited onto the second main surface, and using, in particular, laser ablation, etching paste and/or ion etching, the insulating layer may be removed from regions of the second main surface to re-expose the soldering surfaces, in order to allow subsequent chemical and/or galvanic building-up of the soldering surface openings. By this means, it is ensured that deposited layers for building-up are limited to the soldering surface openings, that is, that deposition of further layers onto only the soldering surface openings is made considerably easier. 
         [0021]    In the method, a solderable layer or sequence of layers including, in particular, nickel, silver, copper and/or tin, may also be galvanically and/or chemically deposited onto the metallic contact tracks of the first main surface and the soldering surface openings of the second main surface; the building-up layer of the first main surface also being able to be different from the layer of the second main surface. By this means, soldering surfaces, which may easily be contacted via soldering, are produced in order to electrically contact the second main surface of the silicon substrate. 
         [0022]    The subject matter of the present invention also includes a solar cell made from a p-doped or n-doped silicon substrate, which has a first main surface used as an incident-light side in a state of operation, and a second main surface used as a back side; the second main surface having an aluminum dopant layer, and the silicon substrate having a thickness of less than app. 200 μm, in particular, less than app. 180 μm. An advantage of this solar cell is that due to the lower thickness, less silicon is needed for the silicon substrate, which reduces costs. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]      FIG. 1-FIG .  14  show cross-sectional views of a silicon substrate after different steps of a method according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0024]    In the following description, like reference numerals are used for parts that are identical or function in the same manner. 
         [0025]      FIG. 1  shows a cross-sectional view of a silicon substrate  1 , which is used as a starting silicon substrate for the method of the present invention. Silicon substrate  1  has a first main surface  2 , which is used as an incident-light side in a state of operation, and a second main surface  3 , which is used as a back side in the state of operation. Silicon  4  of silicon substrate  1  is n-doped or p-doped. Silicon  4  of silicon substrate  1  may be monocrystalline or polycrystalline. First main surface  2  and second main surface  3  have been etched to remove saw damage. In addition, silicon substrate  1  has been textured on both sides. A phosphorus dopant layer  5  is situated in first  2  and second main surface  3 . Phosphorus dopant layer  5  was produced, for example, by diffusion and a subsequent driving-in step. After that, the phosphorus silicate glass produced by the diffusion and subsequent driving-in step was removed from the two main surfaces  2 ,  3 . 
         [0026]      FIG. 2  shows silicon substrate  1  after a first step of the method according to the present invention. In this connection, an antireflection layer or antireflection layer sequence  6  was deposited onto first main surface  2  of silicon substrate  1 , for example, by oxidation and/or PECVD methods or other known methods. Antireflection layer or antireflection layer sequence  6  includes a silicon nitride layer and/or a silicon oxide/silicon nitride layer sequence. This is additionally used as a passivation layer of the first main surface. 
         [0027]      FIG. 3  shows a cross-sectional view of silicon substrate  1  after a further, optional method step. In the method step, phosphorus dopant layer  5  of second main surface  3  is etched away. By this means, the texturing of second main surface  3  is also smoothed out. Potassium hydroxide solution or HF/HNO 3  is preferably used for this. If phosphorus dopant layer  5  of second main surface  3  is not removed, the subsequent doping with aluminum must over-compensate for the phosphorus doping of second main surface  3 . In addition, without this optional method step, the texture of second main surface  3  is retained. 
         [0028]    In the next method step, a thin aluminum layer  7  of a few micrometers, which is as pure as possible, is deposited onto second main surface  3 . An aluminum layer  7 , which is as pure as possible, is to be understood as a layer that essentially contains only aluminum. Thin aluminum layer  7  is deposited over the entire second main surface  3 , for example, by vapor deposition or sputtering, up to a distance from the edge of silicon substrate  1  that is as short as possible. The distance from the edge may also be zero.  FIG. 4  shows silicon substrate  1  after this method step. The thickness of thin aluminum layer  7  is selected with regard to the desired depth of the aluminum dopant layer  8  produced in a subsequent method step. The depth of aluminum dopant layer  8  may be between 1 μm and 10 μm. Aluminum dopant layer  8  acts as an emitter or as a back-surface field as a function of the doping of silicon substrate  1 . 
         [0029]    A cross-sectional view of silicon substrate  1  after a further, optional method step is shown in  FIG. 5 . In the sputtering or vapor deposition system used for depositing thin aluminum layer  7 , for example, thin aluminum layer  7  may be covered with a thin dielectric layer  9 , which preferably includes an oxide and/or a nitride. In this manner, it is ensured that substantially no oxidation of the aluminum of thin aluminum layer  7  by air takes place. 
         [0030]    In the next method step, metallic contact tracks  10 , so-called metal fingers, and optional collecting bars, so-called bus bars, are deposited onto first main surface  2 . The width of metallic contact tracks  10  or metal fingers is as small as possible.  FIG. 6  shows a cross-sectional view of silicon substrate  1  after this deposition. The deposition may be accomplished, using one of the methods known from the related art. Metallic contact tracks  10  and the bus bars are preferably printed with a silver paste, sprayed using aerosol ink containing silver, or extruded from thin tubes. 
         [0031]    First main surface  2  may be provided with, or have, selective doping, i.e., doping at a higher dopant concentration and/or a dopant layer extending deeper into silicon substrate  1 , in particular, underneath the metallic contact tracks and/or bus bars. 
         [0032]      FIG. 7  shows a cross-sectional view of a silicon substrate  1  after the next method step. In this method step, a dielectric, glass-forming paste  11  is deposited onto the entire second main surface  3  and dried. Paste  11  covers thin aluminum layer  7  completely. This cover of thin aluminum layer  7  is intended to be a temporary cover during the method and is removed again after second main surface  3  is doped with aluminum. Therefore, 
         [0000]    the thickness of deposited paste  11  is selected to be as low as possible. The thickness is typically 10 μm-12 μm. 
         [0033]    In the next method step, paste  10  and glass-forming paste  11  are simultaneously sintered or heated on first main surface  2  and second main surface  3 , respectively, at temperatures preferably above 800° C.; the paste being sintered or heated to form metallic contact tracks and optional bus bars. When heated above 577° C., thin aluminum layer  7  and silicon  4  form a liquid eutectic AlSi phase at second main surface  3 .  FIG. 8  shows a cross-sectional view of silicon substrate  1  after this method step. At each time, the thickness of the liquid eutectic layer under glass paste  11  or glass cover  12  is a function of the temperature presently prevailing and the thickness of thin aluminum layer  7 . After briefly heating it above 800° C., in response to cooling off to below the eutectic temperature (app. 577° C.), the aluminum-doped crystalline layer recrystallizes from the inside to the outside to form a p +  layer, which constitutes the emitter in the case of an n-doped silicon substrate  1 , and the back-surface field in the case of a p-doped silicon substrate  1 .  FIG. 8  shows a cross-sectional view of silicon substrate  1  after this method step, in which an aluminum dopant layer  8  is formed on second main surface  3 . Glass paste  11  or glass cover  12  prevents oxidation of the liquid AlSi eutectic layer. 
         [0034]    The remaining eutectic melt in the form of boundary phases of the phase diagram subsequently solidifies to form the AlSi layer having a granular structure. After the doping of second main surface  3  with aluminum, all of the layers on second main surface  3  above aluminum dopant layer  8  are removed. To this end, dielectric layer  12  and optional dielectric layer  9  are initially etched off using hydrofluoric acid, and the remaining layers containing AlSi and aluminum are subsequently etched off using a suitable acid. The silicon substrate  1  after this method step may be seen in  FIG. 9 . The layers on first main surface  2  are essentially not attacked by this etching operation. 
         [0035]    Since the glass layer is only needed temporarily, its thickness may be selected to be as small as possible, preferably, between 10 μm and 12 μm. Because of the lower thickness, less acid is required for re-dissolving or removing this layer. In comparison with the removal of a 40 μm thick aluminum layer or screen-printed aluminum layer according to the related art, an amount of acid, in particular, hydrofluoric acid, is needed that is lower by at least a factor of 4. 
         [0036]    The newly exposed silicon surface of second main surface  3 , which is doped with aluminum, is now coated with a passivation layer  13  suitable for p + -doping (see  FIG. 10 ). Subsequently, passivation layer  13  is locally opened using a known method, for example, using laser ablation, etching paste and/or ion etching. In this context, the regions (openings  15  for local contacts and soldering surface regions  17 ), at which the solderable metallic surfaces (collecting bars or bus bars or soldering contact surfaces) are supposed to be deposited on second main surface  3  in later method steps, are also exposed. A cross-sectional view of the silicon substrate  1  having locally opened passivation layer  13  may be seen in  FIG. 11 . 
         [0037]    The entire second main surface  3  is subsequently covered with a further aluminum layer  14 , using a PVD method known from the related art, e.g., by sputtering or vapor deposition. Further aluminum layer  14  is sufficiently thick and directly contacts aluminum dopant layer  8  of second main surface  3  of silicon substrate  1  in the open regions or openings  15 ,  17 , and in all of the other regions of second main surface  3 , it is situated on passivation layer  13 .  FIG. 12  shows a cross-sectional view of silicon substrate  1  after this method step. In the same PVD system, a thin layer containing nickel is advantageously deposited onto further aluminum layer  14 , in order to be able to more easily deposit, later in the method, a solderable layer in the bus-bar regions or soldering contact surface regions, in a chemical or galvanic process. 
         [0038]    In a further method step, in the same system, a thin dielectric or insulating layer  16  is deposited onto the entire second main surface  3 , in order to limit the building-up to the soldering surface regions. Using one of the known methods (laser ablation, etching paste, ion etching), this dielectric insulating layer  16  is subsequently opened in the soldering surface regions to form soldering surface openings  17 . The silicon substrate after this method step may be seen in  FIG. 13 . By this means, subsequent chemical and/or galvanic building-up  18  exclusive of soldering surface openings  17  is rendered possible or made easier. 
         [0039]    As a final method step, metallic contact tracks  10  of first main surface  2  and soldering contact surfaces  17  of second main surface  3  are built up with a solderable layer sequence  18 ,  19 . This solderable layer sequence  19  is made up of a suitable combination of the metals nickel, silver, copper and/or tin. The deposition of solderable layer sequence  19  may be accomplished galvanically or chemically.  FIG. 14  shows a cross-sectional view of silicon substrate  1  after this last method step. 
         [0040]    At this point, it is emphasized that all of the above-described steps of the method, alone and in any combination, in particular, the details illustrated in the drawing, are claimed as essential to the present invention. Modifications to them are familiar to one skilled in the art. 
         [0041]    In all other respects, the implementation of the present invention is not limited to the above-described examples and emphasized aspects, but only by the scope of protection of the appended claims.