Abstract:
A method for manufacturing high efficiency solar cells is disclosed. The method comprises providing a thin dielectric layer and a doped polysilicon layer on the back side of a silicon substrate. Subsequently, a high quality oxide layer and a wide band gap doped semiconductor layer can both be formed on the back and front sides of the silicon substrate. A metallization process to plate metal fingers onto the doped polysilicon layer through contact openings can then be performed. The plated metal fingers can form a first metal gridline. A second metal gridline can be formed by directly plating metal to an emitter region on the back side of the silicon substrate, eliminating the need for contact openings for the second metal gridline. Among the advantages, the method for manufacture provides decreased thermal processes, decreased etching steps, increased efficiency and a simplified procedure for the manufacture of high efficiency solar cells.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 14/083,141, filed on Nov. 18, 2013, which is a continuation of U.S. patent application Ser. No. 13/333,904, filed on Dec. 21, 2011, now U.S. Pat. No. 8,597,970, issued Dec. 3, 2013, the entire contents of which are hereby incorporated by reference herein. 
     
    
     TECHNICAL FIELD 
       [0002]    Embodiments of the subject matter described herein relate generally to solar cell manufacture. More particularly, embodiments of the subject matter relate to thin silicon solar cells and techniques for manufacture. 
       BACKGROUND 
       [0003]    Solar cells are well known devices for converting solar radiation to electrical energy. They can be fabricated on a semiconductor wafer using semiconductor processing technology. A solar cell includes P-type and N-type diffusion regions. Solar radiation impinging on the solar cell creates electrons and holes that migrate to the diffusion regions, thereby creating voltage differentials between the diffusion regions. In a backside contact solar cell, both the diffusion regions and the metal contact fingers coupled to them are on the backside of the solar cell. The contact fingers allow an external electrical circuit to be coupled to and be powered by the solar cell. 
         [0004]    Efficiency is an important characteristic of a solar cell as it is directly related to the solar cell&#39;s capability to generate power. Accordingly, techniques for improving the fabrication process, reducing the cost of manufacturing and increasing the efficiency of solar cells are generally desirable. Such techniques include forming polysilicon and heterojunction layers on silicon substrates through thermal processes wherein the present invention allows for increased solar cell efficiency. These or other similar embodiments form the background of the current invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]    A more complete understanding of the subject matter can be derived by referring to the detailed description and claims when considered in conjunction with the following figures, wherein like reference numbers refer to similar elements throughout the figures. 
           [0006]      FIG. 1-12  are cross-sectional representations of a solar cell being fabricated in accordance with an embodiment of the invention 
           [0007]      FIG. 13-18  are cross-sectional representations of a solar cell being fabricated in accordance with an another embodiment of the invention 
       
    
    
     DETAILED DESCRIPTION 
       [0008]    The following detailed description is merely illustrative in nature and is not intended to limit the embodiments of the subject matter or the application and uses of such embodiments. As used herein, the word “exemplary” means “serving as an example, instance, or illustration.” Any implementation described herein as exemplary is not necessarily to be construed as preferred or advantageous over other implementations. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background, brief summary or the following detailed description. 
         [0009]    A method of manufacturing solar cells is disclosed. The method comprises providing a silicon substrate having a thin dielectric layer on the back side, and a deposited silicon layer over the thin dielectric layer, forming a layer of doping material over the a deposited silicon layer, forming an oxide layer over the layer of doping material, partially removing the oxide layer, the layer of doping material and the deposited silicon layer in an interdigitated pattern, growing an oxide layer while simultaneously raising the temperature to drive the dopants from the layer of doping material into the deposited silicon layer, doping the deposited silicon layer with dopants from the layer of doping material to form a crystallized doped polysilicon layer, depositing a wide band gap doped semiconductor and an anti-reflective coating on the back side of the solar cell, and depositing a wide band gap doped semiconductor and anti-reflective coating on the front side of the solar cell. 
         [0010]    Another method of manufacturing solar cells is disclosed. The method comprises providing a silicon substrate having a thin dielectric layer on the back side, and a deposited silicon layer over the thin dielectric layer, forming a layer of doping material over the deposited silicon layer, forming an oxide layer over the layer of doping material, partially removing the oxide layer, the layer of doping material and the deposited silicon layer in an interdigitated pattern, etching the exposed silicon substrate to form a texturized silicon region, growing an oxide layer while simultaneously raising the temperature to drive the dopants from the layer of doping material into the deposited silicon layer, doping the deposited silicon layer with dopants from the layer of doping material to form a doped polysilicon layer, covering a first thick layer of wide band gap doped amorphous silicon and anti-reflective coating on the back side of the solar cell, covering an second thin layer of wide band gap doped amorphous silicon and anti-reflective coating on the front side of the solar cell and wherein the thin layer is less than 10% to 30% of the thickness of the thick layer. 
         [0011]    Still another method of manufacturing solar cells is disclosed. The method comprises providing a silicon substrate having a thin dielectric layer on the back side, and a doped silicon layer over the thin dielectric layer, forming an oxide layer over the doped silicon layer, partially removing the oxide layer and doped silicon layer in an interdigitated pattern, growing a silicon oxide layer over the back side of the solar cell by heating the silicon substrate in an oxygenated environment, wherein the silicon layer is crystallized to form a doped polysilicon layer, depositing a wide band gap doped semiconductor on the back side of the solar cell, and depositing a wide band gap doped semiconductor and anti-reflective coating on the front side of the solar cell. 
         [0012]    Still another method of manufacturing solar cells is disclosed. The method comprises providing a silicon substrate having a thin dielectric layer on the back side, and a doped silicon layer over the thin dielectric layer, forming an oxide layer over the doped silicon layer, partially removing the oxide layer and doped silicon layer in an interdigitated pattern, etching the exposed silicon substrate to form a texturized silicon region, growing a silicon oxide layer over the back side of the solar cell by heating the silicon substrate in an oxygenated environment, wherein the silicon layer is crystallized to form a doped polysilicon layer, depositing a wide band gap doped amorphous silicon and an anti-reflective coating on the back side of the solar cell, and depositing a wide band gap doped amorphous silicon and anti-reflective coating on the front side of the solar cell. 
         [0013]    Yet another embodiment for a method of manufacturing solar cells is disclosed. The method comprises providing a silicon substrate having a thin dielectric layer on the back side, and a doped silicon layer over the thin dielectric layer, forming an oxide layer over the doped silicon layer, partially removing the oxide layer and doped silicon layer in an interdigitated pattern, etching the exposed silicon substrate to form a texturized silicon region, growing a silicon oxide layer over the back side of the solar cell by heating the silicon substrate in an oxygenated environment, wherein the silicon layer is crystallized to form a doped polysilicon layer, simultaneously depositing a wide band gap doped amorphous silicon and an anti-reflective coating over the front side and back side of the solar cell, partially removing the wide band gap doped semiconductor and oxide layer to form a series of contact openings, and simultaneously forming a first metal grid being electrically coupled to the doped polysilicon layer and a second metal grid being electrically coupled to an emitter region on the back side of the solar cell. 
         [0014]    An improved technique for manufacturing solar cells is to provide a thin dielectric layer and a deposited silicon layer on the back side of a silicon substrate. Regions of doped polysilicon can be formed by dopant driving into deposited silicon layers, or by in-situ formation of doped polysilicon regions. An oxide layer and a layer of a wide band gap doped semiconductor can then be formed on the front and back sides of the solar cell. One variant involves texturizing the front and back surfaces prior to formation of the oxide and wide band gap doped semiconductor formation. Contact holes can then be formed through the upper layers to expose the doped polysilicon regions. A metallization process then can be performed to form contacts onto the doped polysilicon layer. A second group of contacts can also be formed by directly connecting metal to emitter regions on the silicon substrate formed by the wide band gap semiconductor layer positioned between regions of the doped polysilicon on the back side of the solar cell. 
         [0015]    The various tasks performed in connection with manufacturing processes are shown in  FIGS. 1-18 . Also, several of the various tasks need not be performed in the illustrated order, and it can be incorporated into a more comprehensive procedure, process or fabrication having additional functionality not described in detail herein. 
         [0016]      FIGS. 1-3  illustrate an embodiment for fabricating a solar cell  100  comprising a silicon substrate  102 , a thin dielectric layer  106 , and a deposited silicon layer  104 . In some embodiments, the silicon substrate  102  can be cleaned, polished, planarized, and/or thinned or otherwise processed prior to the formation of the thin dielectric layer  106 . The thin dielectric layer  106  and deposited silicon layer  104  can be grown through a thermal process. A layer of doping material  108  followed by a first oxide layer  110  can be deposited over the deposited silicon layer  104  through conventional deposition process. The layer of doping material  108  can comprise a doping material, or dopant,  109 , but is not limited to, a layer of positive-type doping material such as boron or a layer of negative-type doping material such as phosphorous. Although the thin dielectric layer  106  and deposited silicon layer  104  are described as being grown by a thermal process or deposited through conventional deposition process, respectively, as with any other formation, deposition, or growth process step described or recited here, each layer or substance can be formed using any appropriate process. For example, a chemical vapor deposition (CVD) process, low-pressure CVD (LPCVD), atmospheric pressure CVD (APCVD), plasma-enhanced CVD (PECVD), thermal growth, sputtering, as well as any other desired technique can be used where formation is described. Thus, and similarly, the doping material  108  can be formed on the substrate by a deposition technique, sputter, or print process, such as inkjet printing or screen printing. 
         [0017]      FIG. 4  illustrates the same solar cell  100  from  FIG. 1-3  after performing a material removal process to form an exposed polysilicon region  124 . Some examples of a material removal process include a mask and etch process, a laser ablation process, and other similar techniques. The exposed polysilicon region  124  and layer of doping material  108  can be formed into any desired shape, including an interdigitated pattern. Where a masking process is used, it can be performed using a screen printer or an inkjet printer to apply a mask ink in predefined interdigitated pattern. Thus, conventional chemical wet etching techniques can be used to remove the mask ink resulting in the interdigitated pattern of exposed polysilicon regions  124  and layer of doping material  108 . In at least one embodiment, portions or the entirety of the first oxide layer  110  can be removed. This can be accomplished in the same etching or ablation process in which regions of the deposited silicon layer  104 , and dielectric layer  106  are removed, as shown in  FIGS. 4 and 5 . 
         [0018]    With reference to  FIG. 5 , the solar cell  100  can undergo a second etching process resulting in etching the exposed polysilicon regions  124  to form a first texturized silicon region  130  on the back side of the solar cell and a second texturized silicon region  132  on the front side of the solar cell for increased solar radiation collection. A texturized surface can be one which has a regular or an irregular shaped surface for scattering incoming light, decreasing the amount of light reflected back off the surface of the solar cell. 
         [0019]    With reference to  FIG. 6 , the solar cell  100  can be heated  140  to drive the doping material  109  from the layer of doping material  108  into the deposited silicon layer  104 . The same heating  140  can also form a silicon oxide or a second oxide layer  112  over the layer of doping material  108  and first texturized silicon region  130 . During this process a third oxide layer can be grown  114  over the second texturized silicon region  132 . Both the oxide layers  112 ,  114  can comprise high quality oxide. A high-quality oxide is a low interface state density oxide typically grown by thermal oxidation at temperatures greater than 900 degrees Celsius which can provide for improved passivation. 
         [0020]    With reference to  FIG. 7 , the deposited silicon layer  104  can therefore be doped with the doping material  109  from the layer of dopant material  108  to form a doped polysilicon layer  150 . In one embodiment, forming a doped polysilicon layer can be accomplished by growing an oxide layer while simultaneously raising the temperature to drive the dopants  109  from the layer of doping material  108  into the deposited silicon layer  104 , wherein doping the deposited silicon layer  104  with dopants  109  from the layer of doping material  108  form a crystallized doped polysilicon layer or a doped polysilicon layer  150 . In one of several embodiments, the doped polysilicon layer  150  can comprise a layer of positively doped polysilicon given a positive-type doping material is used. In the illustrated embodiment, the silicon substrate  102  comprises bulk N-type silicon substrate. In some embodiments, the doped polysilicon layer  150  can comprise a layer of negatively doped polysilicon if a negative-type doping material is used. In one embodiment, the silicon substrate  102  should comprise bulk P-type silicon substrate. 
         [0021]    With reference to  FIG. 8 , a first wide band gap doped semiconductor layer  160  can be deposited on the back side of the solar cell  100 . In one embodiment, the first wide band gap doped semiconductor layer  160  is partially conductive with a resistivity of at least 10 ohm-cm. In the same embodiment it can have a band gap greater than 1.05 electron-Volts (eV) acting as a heterojunction in areas of the back side of the solar cell now covered by the first texturized silicon region  130  and by the second oxide layer  112 . Examples of a wide band gap doped semiconductor include Silicon carbide and Aluminum Galium Nitride. Any other wide band gap doped semiconductor material which exhibits the properties and characteristics described above can also be used. The first wide band gap doped semiconductor layer  160  can be composed of a first thick wide band gap doped amorphous silicon layer. 
         [0022]    With reference to  FIG. 9 , a second wide band gap doped semiconductor  162  can be deposited over the second texturized silicon region  132  on the front side of the solar cell  100 . In one embodiment, both the wide band gap doped semiconductor layers  160 ,  162  on the back side and front side of the solar cell  100  can comprise a wide band gap negative-type doped semiconductor. In another embodiment, the second wide band gap doped semiconductor  162  can be relatively thin as compared to the first thick wide band gap doped semiconductor layer. Thus, in some embodiments, the second thin wide band gap doped semiconductor layer can comprise of 10 to 30% of the thickness of the first thick wide band gap doped semiconductor layer. In yet another embodiment both wide band gap doped semiconductor layers  160 ,  162  on the back side and front side of the solar cell respectively can comprise a wide band gap negative-type doped semiconductor or a wide band gap positive-type doped semiconductor. Subsequently, an anti-reflective coating (ARC)  170  can be deposited over the second wide band gap doped semiconductor  162  in the same process. In another embodiment, an anti-reflective coating  170  can be deposited over the first wide band gap doped semiconductor  160  in the same process. In some embodiments, the ARC  170  can be comprised of silicon nitride. 
         [0023]      FIG. 10  illustrates the partial removal of the first wide band gap doped semiconductor  160 , second oxide layer  112  and the layer of doping material  108  on the back side of the solar cell  100  to form a series of contact openings  180 . In one embodiment, the removal technique can be accomplished using an ablation process. One such ablation process is a laser ablation process. In another embodiment, the removal technique can be any conventional etching processes such as screen printing or ink jet printing of a mask followed by an etching process. 
         [0024]    With reference to  FIG. 11 , a first metal grid or gridline  190  can be formed on the back side of the solar cell  100 . The first metal gridline  190  can be electrically coupled to the doped polysilicon  150  within the contact openings  180 . In one embodiment, the first metal gridline  190  can be formed through the contact openings  180  to the first wide band gap doped semiconductor  160 , second oxide layer  112 , and the layer of doping material  108  to connect a positive electrical terminal of an external electrical circuit to be powered by the solar cell. 
         [0025]    With reference to  FIG. 12 , a second metal grid or gridline  192  can be formed on the back side of the solar cell  100 , the second metal gridline  192  being electrically coupled to the second texturized silicon region  132 . In one embodiment, the second metal gridline  192  can be coupled to the first wide band gap doped semiconductor  160 , second oxide layer  112 , and the first texturized silicon region  130  acting as a heterojunction in areas of the back side of the solar cell to connect to a negative electrical terminal of an external electrical circuit to be powered by the solar cell. In some embodiments the forming of metal grid lines referenced in  FIGS. 11 and 12  can be performed through an electroplating process, screen printing process, ink jet process, plating onto a metal formed from aluminum metal nanoparticles or any other metallization or metal formation process step. 
         [0026]      FIGS. 13-18  illustrate another embodiment of fabricating a solar cell  200 . Unless otherwise specified below, the numerical indicators used to refer to components in  FIGS. 13-18  are similar to those used to refer to components or features in  FIGS. 1-12  above, except that the index has been incremented by  100 . 
         [0027]    With reference to  FIG. 13-14 , another embodiment for fabricating the solar cell  200  can comprise forming a first oxide layer  210 , a thin dielectric layer  206 , a doped polysilicon layer  250  over the silicon substrate  202 . The silicon substrate  202  can be cleaned, polished, planarized, and/or thinned or otherwise processed prior to the formation of the thin dielectric layer  206  as discussed similarly above. The first oxide layer  210 , dielectric layer  206  and doped polysilicon layer  250  can be grown through a thermal process. In one embodiment, growing the silicon oxide layer or oxide layer  210  over the back side of the solar cell by heating the silicon substrate  202  in an oxygenated environment, wherein a doped silicon layer is crystallized to form the doped polysilicon layer  250 . In another embodiment, growing the doped polysilicon layer  250  over the dielectric layer  206  comprises growing a positively doped polysilicon, wherein the positively doped polysilicon can be comprised of a doping material  209  such as a boron dopant. In another embodiment, negatively-doped polysilicon can be used. Although the thin dielectric layer  206  and doped polysilicon layer  250  are described as being grown by a thermal process or deposited through conventional deposition process, respectively, as with any other formation, deposition, or growth process step described or recited here, each layer or substance can be formed using any appropriate process as discussed earlier. 
         [0028]    The solar cell  200  can be further processed by partially removing first oxide layer  210 , the doped polysilicon layer  250  and dielectric layer  206  to reveal an exposed region of silicon substrate  220  in an interdigitated pattern using conventional masking and etching processes. In the case of using conventional masking and etching processes, an ablation process can be used. If an ablation process is used, the first oxide layer  210  can be left partially intact over the doped polysilicon layer  250  as illustrated in  FIG. 14 . In another embodiment, a screen print or ink jet printing technique coupled with an etching process can be used. In such an embodiment, the first oxide layer  210  can be etched away from the doped polysilicon layer  250 . 
         [0029]    With reference to  FIG. 15 , the exposed silicon substrate  220  and an exposed region on the front side of the solar cell  200  can be simultaneously etched to form a first texturized silicon surface  230  and second texturized silicon surface  232  for increased solar radiation collection. 
         [0030]    With reference to  FIG. 16 , the solar cell  200  can be heated  240  to a temperature greater than  900  degrees Celsius while forming a second oxide layer  212  on back side and a third oxide layer  214  on the front side of the solar cell  200 . In another embodiment, both the oxide layers  212 ,  214  can comprise of high quality oxide as discussed earlier. 
         [0031]    With reference to  FIG. 17 , the first wide band gap doped semiconductor layer  260  can be simultaneously deposited on the back side and front side of the solar cell. The first wide band gap doped semiconductor layer  260  can be partially conductive having a resistivity greater than 10 ohm-cm. The first wide band gap doped semiconductor layer  260  also can have a band gap greater than 1.05 eV. Additionally, the first wide band gap semiconductor layer can act as a heterojunction in areas of the back side of the solar cell cover the first texturized silicon region  230  and the second oxide layer  212 . 
         [0032]    The first wide band gap doped semiconductor layer  260  can be 10% to 30% thicker than the second wide band gap doped semiconductor layer  262 . In other embodiments, the thickness can vary below 10% or greater than 30% without deviating from the techniques described herein. Both the wide band gap doped semiconductor layers  260 ,  262  can be positively-doped semiconductor, although in other embodiments with different substrate and polysilicon doped polarities, negatively-doped wide band gap semiconductor layers can also be used. Subsequently an anti-reflective coating (ARC)  270  can be deposited over the second wide band gap doped semiconductor  262 . In one embodiment, the anti-reflective coating  270  can be comprised of silicon nitride. In some embodiments, the ARC can be deposited over the first wide band gap doped semiconductor layer  260  as well. 
         [0033]    With reference to  FIG. 18 , the first wide band gap doped semiconductor layer  260  and second oxide layer  212  can be partially removed over the doped polysilicon layer  250  to form a series of contact openings similar to, and with a formative technique similar to, those described above with reference to  FIG. 10-12 . Subsequently, a first metal gridline  290  can be formed on the back side of the solar cell  200  wherein the first metal gridline  290  can be electrically coupled to the doped polysilicon  250  within the contact openings. A second metal gridline  292  can be formed on the back side of the solar cell  200 , the second metal gridline  292  being electrically coupled to the first texturized silicon region or N-type emitter region  230 . In one embodiment, both the first and second metal gridlines can be formed simultaneously. Additional contact can then be made to the first and second metal gridlines  290 ,  292  by other components of an energy system incorporating solar cell  200 . 
         [0034]    While at least one exemplary embodiment has been presented in the foregoing detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or embodiments described herein are not intended to limit the scope, applicability, or configuration of the claimed subject matter in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing the described embodiment or embodiments. It should be understood that various changes can be made in the function and arrangement of elements without departing from the scope defined by the claims, which includes known equivalents and foreseeable equivalents at the time of filing this patent application.