Patent Publication Number: US-2010130014-A1

Title: Texturing multicrystalline silicon

Description:
BACKGROUND 
     The subject disclosure is generally directed to texturing a surface of a multicrystalline silicon using drop jetting technology such as ink jet printing technology. 
     Surface texturing for more efficient light trapping can increase conversion efficiency of multicrystalline silicon solar cells. However, known techniques for surface texturing multicrystalline silicon can be difficult and/or complex. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic block diagram of a drop jetting system that can be used in the disclosed surface texturing techniques. 
         FIGS. 2 ,  3  and  4  are schematic transverse view illustrating an implementation of a technique for texturing a surface of a muliticrystalline silicon substrate. 
         FIGS. 5 ,  6  and  7  are schematic transverse view illustrating an implementation of another technique for texturing a surface of a muliticrystalline silicon substrate. 
         FIGS. 8 ,  9 ,  10 ,  11 ,  12  and  13  are schematic transverse view illustrating an implementation of a further technique for texturing a surface of a muliticrystalline silicon substrate. 
         FIG. 14  is a schematic transverse view illustrating a solar cell that can be produced pursuant to further processing of a muliticrystalline silicon substrate that is selectively surface textured using the disclosed techniques. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a schematic block diagram of an embodiment of a drop on demand liquid drop emitting or jetting system, such as a liquid jet printing or depositing system, that includes a controller  10  and a printhead  20  that can include a plurality of addressable drop emitting drop generators for emitting or depositing drops of liquid  33  onto a receiver substrate  15 . A transport mechanism  40  can be employed to move the substrate  15  relative to the printhead  20 . The printhead  20  receives liquid from at least one liquid containing reservoir  61  that can be attached to the printhead  20  or separate from the printhead and fluidically connected thereto by an appropriate fluidic connection such as flexible tubing. 
     The printhead  20  can comprise a piezoelectric jetting device or a thermal or bubble jetting device. For convenience, using a drop on demand liquid drop jetting system such as that schematically depicted in  FIG. 1  can sometimes be referred to as using ink jet, or ink jet printing, to apply or deposit material on the substrate  15 . 
     A drop on demand liquid drop jetting system such as that schematically depicted in  FIG. 1  can be employed to surface texture multicrystalline silicon (mc-Si for convenience), for example for use as solar cells. By way of illustrative example, drop on demand ink jet printing technology can be suitably modified to deposit or “print” masking materials or etching materials as discussed herein. 
       FIGS. 2 ,  3  and  4  are schematic transverse cross-sectional views illustrating various stages in texturing a predetermined surface  113  of a mc-Si substrate  111 . 
     In  FIG. 2 , a patterned mask  115  is formed on the predetermined surface  113  of the mc-Si substrate  111  using drop on demand liquid drop jetting, whereby for example a patterned mask is deposited or “printed” on the predetermined mc-Si surface using a drop on demand liquid drop jetting system that jets drops of a suitable masking material. An example of suitable masking materials would be a wax that is configured to be in a liquid state during printing or jetting and freezes to a solid state after being printed on the predetermined mc-Si surface. 
     In  FIG. 3 , the masked surface of the mc-Si substrate  111  is etched, for example wet etched using an appropriate wet etching material such as a suitable mixture of nitric acid (HNO 3 ) and hydrofluoric acid (HF). By way of illustrative example, wet etching can be accomplished by spraying liquid etching solution on the masked surface  113 . 
     Alternatively, dry etching techniques, such as plasma etching and reactive ion etching (RIE) can be employed. 
     In  FIG. 4 , the patterned mask  115  is removed to expose a selectively textured surface  113 A. As described further herein, a multicrystalline silicon substrate having a selectively textured surface can be further processed to produce a solar cell as schematically depicted in  FIG. 14 . 
     Alternatively, the patterned mask  115  can comprise a masking material that can be etched away generally simultaneously with the surface  113  of the mc-Si substrate  111 , in which case the structure of  FIG. 4  would be produced after etching. Suitable masking materials that can be wet etched simultaneously with the silicon include wax type masking materials that contain alkanes, esters, and/or other suitable chemicals. 
       FIGS. 5 ,  6  and  7  are schematic transverse cross-sectional views illustrating various stages in texturing a predetermined surface  213  of a mc-Si substrate  211 . 
     In  FIG. 5 , a patterned etching layer  215  is formed on the predetermined surface  213  of the mc-Si substrate  211  using drop on demand liquid drop jetting, whereby for example a patterned etching layer is deposited or “printed” on the predetermined mc-Si surface using a drop on demand liquid drop jetting system that jets a suitable etching material. Examples of suitable etching materials include silicon etching paste available from Merck and/or acid based pastes. 
     In  FIG. 6 , the etching layer is allowed to etch the surface of the mc-Si substrate  211  for an appropriate amount of time to achieve the desired amount of etching. Optionally, the etching layer  215  and the surface  213  can be heated while etching, for example by radiant heating or by placing the structure comprising the substrate  211  and the etching layer  215  in an oven or a belt furnace. 
     In  FIG. 7 , the patterned etching layer  215  is removed to expose a selectively textured surface  213 A. As described further herein, a multicrystalline silicon substrate having a selectively textured surface can be further processed to produce a solar cell as schematically depicted in  FIG. 14 . 
       FIGS. 8 ,  9 ,  10 ,  11 ,  12  and  13  are schematic transverse cross-sectional views illustrating various stages in texturing a predetermined surface  313  of a mc-Si substrate  311 . 
     In  FIG. 8 , a first patterned mask  315  having a first mask pattern is formed on the predetermined surface  313  of the mc-Si substrate  311  using drop on demand liquid drop jetting, whereby for example a patterned mask is “printed” on the predetermined mc-Si surface using a drop on demand liquid drop jetting system that jets a suitable masking material. Examples of suitable masking materials include a wax that is configured to be in a liquid state during printing or jetting and freezes to a solid state after being printed on the predetermined mc-Si surface. 
     In  FIG. 9 , the masked surface of the mc-Si substrate  311  is etched, for example wet etched or dry etched. 
     In  FIG. 10 , the first patterned mask  315  is removed to expose a partially textured surface  313 A. Alternatively, the patterned mask  315  can comprise a masking material that can be etched away generally simultaneously with the exposed portions of the surface  313  of the mc-Si substrate  311 , in which case the structure of  FIG. 10  would be produced after etching. 
     In  FIG. 11 , a second patterned mask  325  having a second mask pattern is formed on the predetermined surface  313  of the mc-Si substrate  311  using drop on demand liquid drop jetting, whereby for example a patterned mask is “printed” on the predetermined mc-Si surface using a drop on demand liquid drop jetting system that jets a suitable masking material. For example, the second mask pattern covers at least some of the regions that were etched using the first mask  315 , so as to leave unmasked at least some portions of the regions that had been protected by the first mask. 
     In  FIG. 12 , the masked partially textured surface of the mc-Si substrate  311  is etched, for example wet etched or dry etched. 
     In  FIG. 13 , the second patterned mask  325  is removed to expose a selectively textured surface  313 B. Alternatively, the patterned mask  325  can comprise a masking material that can be etched away generally simultaneously with the exposed portions of the surface  313 A of the mc-Si substrate  311 , in which case the structure of  FIG. 13  would be produced after etching. 
       FIG. 14  is a schematic sectional view of a solar cell that can be made pursuant to further processing of a p-type multicrystalline silicon substrate that has been selectively surface textured pursuant to the foregoing techniques. The solar cell includes an n+ emitter layer in the portion of the substrate that includes the selectively textured surface and an antireflection layer such as silicon nitride (SiNx) disposed on the selectively textured surface. By way of illustrative examples, the n+ emitter layer can be formed by diffusing donor doping material, while the SiNx antireflection layer can be formed by known techniques such as chemical vapor deposition or physical vapor deposition. Metal electrodes such as silver gridline or busbar can be deposited on the non-textured portions of the selectively textured surface after formation of the emitter layer and the antireflection layer. For completeness, the solar cell of  FIG. 14  is shown as including an AL-BSF (aluminum Back Surface Field) layer and an Al backside electrode layer on the back surface of the substrate. The Al backside electrode layer can be deposited for example by screen printing, and the AL-BSF layer will be formed during the electrode firing process when the screen printed Al reacts with the Si substrate to form the AL-BSF layer. 
     Depending on implementation, the disclosed techniques can be embedded/integrated into existing in-line wet processing systems. 
     The claims, as originally presented and as they may be amended, encompass variations, alternatives, modifications, improvements, equivalents, and substantial equivalents of the embodiments and teachings disclosed herein, including those that are presently unforeseen or unappreciated, and that, for example, may arise from applicants/patentees and others. Unless specifically recited in a claim, steps or components of claims should not be implied or imported from the specification or any other claims as to any particular order, number, position, size, shape, angle, color, or material.