Classifications

• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10F—INORGANIC SEMICONDUCTOR DEVICES SENSITIVE TO INFRARED RADIATION, LIGHT, ELECTROMAGNETIC RADIATION OF SHORTER WAVELENGTH OR CORPUSCULAR RADIATION
  • H10F77/00—Constructional details of devices covered by this subclass
  • H10F77/70—Surface textures, e.g. pyramid structures
  • H10F77/703—Surface textures, e.g. pyramid structures of the semiconductor bodies, e.g. textured active layers

Definitions

• the invention generally relates to sheet wafers and, more particularly, the invention relates to texturing sheet wafers.
• One of the primary goals of the solar module industry is to achieve "grid-parity," which generally means that the cost per produced watt of electricity from a solar module is comparable to the cost per produced watt of electricity produced from conventional means (i.e., from the "grid").
• grid parity generally means that the cost per produced watt of electricity from a solar module is comparable to the cost per produced watt of electricity produced from conventional means (i.e., from the "grid").
• grid parity the solar industry strives both to reduce the cost of producing solar modules and improve module energy conversion efficiency.
• solar cells formed from sheet wafers typically are fabricated by immersing an entire ribbon crystal wafer within an expensive acid chemistry (e.g., using sulfuric acid, which is expensive both to obtain and discard). Although effective, this process is expensive, thus ultimately increasing the cost of the cell.
• a method of texturing a sheet wafer 1) provides a sheet wafer having a front surface with a preliminary average surface roughness, and a back surface, and then 2) positions the sheet wafer on a transport mechanism. The back surface of the sheet wafer is positioned to face downwardly.
• the method sandblasts the front surface of the sheet wafer as it is supported by the transport mechanism to cause the front surface to have an intermediate average surface roughness.
• the preliminary average surface roughness is less than the intermediate average surface roughness.
• Illustrative embodiments sandblast by directing sand toward the front surface of the sheet wafer. While directing sand toward the front surface, the method may direct a positive pressure toward the front surface of the sheet wafer to remove excess sand.
• the method also may sandblast the front surface of the sheet wafer as the wafer is moving on the transport mechanism.
• the method may sandblast by stopping movement of the transport mechanism, and then directing sand toward the front surface of the sheet wafer after the transport mechanism stops moving.
• Some embodiments may sandblast both when moving, and when stopped.
• the transport mechanism may include a moving belt. Accordingly, the front face of the belt supports the back surface of the sheet wafer.
• the method may direct sand at the front surface of the sheet wafer for between about 40 and 60 seconds.
• the sheet wafer is quite fragile as it may have a maximum thickness area of less than about 200 microns.
• sandblasting may produce a texture on the front surface of the sheet wafer that extends between about 7 and 20 microns into the front surface of the sheet wafer.
• the method may further process the sheet wafer to form a solar cell having a front face.
• the roughened front surface of the sheet wafer forms the front face of the solar cell.
• Some embodiments further process the wafer.
• the method may acid etch the front surface to have a final average surface roughness that is greater than the intermediate average surface roughness.
• Figure 1 A schematically shows the front surface of a sheet wafer textured in accordance with illustrative embodiments of the invention.
• Figure IB schematically shows the back surface of a sheet wafer textured in accordance with illustrative embodiments of the invention.
• Figure 2A schematically shows a perspective view of a sandblasting machine as it textures a sheet wafer.
• Figure 2B schematically shows a cross-sectional view of the sandblasting machine across line B-B.
• Figure 3 shows a process of texturing a sheet wafer in accordance with illustrative embodiments of the invention.
• Figure 4 schematically shows a photovoltaic cell formed from a sheet wafer textured in accordance with illustrative embodiments of the invention.
• Figure 5 shows a process of forming the photovoltaic cell shown in figure 4 in accordance with illustrative embodiments of the invention.
• a method sandblasts the front surface of a sheet wafer to increase its surface roughness. Accordingly, applications requiring such roughness, such as photovoltaic cells, should reflect substantially less light from its surface. Details of illustrative embodiments are discussed below.
• FIGS 1A and IB schematically show opposite sides of a single sheet wafer (“wafer 10") configured in accordance with illustrative embodiments of the invention.
• the sheet wafer 10 may be similar to those known by the trademark STRING RIBBON crystals, distributed by Evergreen Solar, Inc. of Marlborough, MA.
• STRING RIBBON crystals distributed by Evergreen Solar, Inc. of Marlborough, MA.
• Such type of sheet wafers may be formed by using processes described in various patents naming Emanuel M. Sachs as the inventor.
• US Patent No. 4,661,200 shows a type of sheet wafer, a STRING RIBBON crystal, and teaches a method of forming such type of sheet wafer.
• a sheet wafer is formed from a process that grows the (generally planar) wafer directly from molten material.
• This is in contrast to conventional wafers formed from cast wafer techniques, which cut generally planar wafers from a large ingot of solidified wafer material.
• a sheet wafer forms directly from a melt, while a wafer formed using cast wafer techniques requires another step to make it generally planar; namely, a sawing step.
• this sawing step can serve as the basis for a texture on solar cells formed from cast wafers, thus avoiding significant texturing steps for the purposes described herein for photo voltaics.
• Sheet wafers often do not have this benefit— instead, they often are smooth and thus, require this extra texturing step.
• the wafer 10 shown in Figures 1 A and IB is generally planar with a generally rectangular shape and a relatively large surface area on its front and back surfaces 12 and 14.
• Figure 1 A shows the front surface 12
• Figure IB shows the back surface 14.
• the wafer 10 may have a width of about 3 inches, and a length of 6 inches.
• the length can vary significantly depending upon the application, while the width also can vary depending upon the separation of its two strings. Accordingly, discussion of specific lengths and widths are illustrative and not intended to limit various embodiments the invention.
• the thickness of the wafer 10 varies and is very thin relative to its length and width dimensions. Specifically, the wafer 10 may have a thickness ranging from about 130 microns to about 320 microns across its width. In fact, recent innovations have driven sheet wafer thickness to a maximum of less than about 200 microns, or an average thickness of less than about 200 microns. Despite this range, the wafer 10 may be considered to have an average thickness across its length and/or width. As such, the sheet wafer 10 is very fragile when subjected to any force, such as that of a sandblasting machine 16.
• the wafer 10 may be formed from any of a wide variety of materials and crystal types, such as multi-crystalline, single crystalline, polycrystalline, micro crystalline or semi-crystalline materials.
• the wafer 10 may be formed from polysilicon.
• the wafer 10 shown in Figures 1 A and IB may be used for a number of applications, such as solar cells. It nevertheless should be noted that the wafer 10 may be used in other applications and thus, is not limited to use in solar cells. In a similar manner, discussion of a string ribbon wafer is for illustrative purposes only and not intended to limit a number of other embodiments.
• the wafer 10 has a textured front surface 12 and, conversely, a substantially smooth backside surface 14.
• the back surface 14 illustratively is covered with a layer of aluminum, or a passivation portion, such as an oxide, and conductive contacts (e.g., formed from silver).
• FIG. 2 A schematically shows a perspective view of a sandblasting machine 16 that may be used for that purpose in accordance with illustrative embodiments of the invention.
• figure 2B schematically shows a cross-sectional view of the sandblasting machine 16 across line B-B. It should be reiterated that the sandblasting machine 16 shown in the figures is merely schematic and not intended to limit various embodiments of invention.
• the sandblasting machine 16 has a movable conveyor belt 18 that supports and transports one or more of a plurality of sheet wafers 10.
• FIGS 2 A and 2B schematically show the sandblasting machine 16 processing two separate sheet wafers 10 at the same time during the texturing process (discussed below).
• the belt 18 supports the back surface 14 of the sheet wafer 10, thus exposing the front surface 12 to a stream of sand.
• the conveyor belt 18 has a plurality of openings/holes 24 for removing excess sand from its front surface 12.
• the interior of the sandblasting machine 16 has a sandblast port 20 for directing sand onto the front surface 12 of the wafer 10.
• This port 20 may be positioned at any of a plurality of different positions (e.g., directly above the front surface 12 of the wafer 10, or at an angle to the front surface 12), and implemented as plural ports 20.
• a single port 20 is shown merely for simplicity.
• the sand has characteristics commonly found in sand used within similar sandblasting machines. Moreover, the sand can be substantially dry or wet to some extent.
• the sandblasting machine 16 optionally may have one or more air nozzles 22 for blowing excess sand from the front surface 12 of the wafer 10. As noted, this excess sand can be directed off the edge of the belt 18, or through the holes 24 in the belt 18. Although not shown, this excess sand may be recycled and again used to texture of the front surface 12 of the wafer 10.
• Figure 3 shows a process of texturing/roughening the sheet wafer 10 in accordance with illustrative embodiments of the invention. It should be noted that for simplicity, this described process is a significantly simplified version of an actual process used to texture the sheet wafer 10. Accordingly, those skilled in the art would understand that the process may have additional steps not explicitly shown in Figure 3. Moreover, some of the steps may be performed in a different order than that shown, or at substantially the same time. Those skilled in the art should be capable of modifying the process to suit their particular requirements.
• step 300 positions a doped wafer 10 on the conveyor belt 18.
• wafers intended to be integrated into a photovoltaic cell can be doped with a conventional dopant, such as phosphorus or boron.
• the process passes the wafer 10 through the sandblasting machine 16 and through a gentle stream of sand for a predetermined period.
• the sheet wafer 10 passes steadily and slowly through the stream of sand.
• the sheet wafer 10 may be in a stationary position underneath the sandblast port 20 for a set period of time.
• the sheet wafer 10 is moved into the stream of sand and stopped for a
• sheet wafers 10 exposed to the gently directed stream of sand for between about 40 and 60 seconds produce satisfactory results. Such wafers 10 produced a texture that extended to depths of between about 7 and 20 microns into the test wafers 10. Some wafers 10, however, have textures that were more shallow than about 7 microns.
• the sheet wafer 10 is extremely fragile. Accordingly, to maintain a high yield, the sandblasting machine 16 must be specifically calibrated to ensure that the sand does not strike the front surface 12 with too much force. To that end, those skilled in the art should consider any of a variety of factors to prevent wafer breakage. Among other things, those factors may include the thickness of the wafer 10, the material of the wafer 10, the type of sand used, the intended roughness of the wafer 10, the temperature of the wafers 10 during processing, and the physical dimensions of the wafer 10. After preliminary assessment and a number of testing runs, one skilled in the art can determine the appropriate manner of configuring the sandblasting machine 16.
• step 304 in which the air nozzles 22 optionally apply a positive air pressure to the front surface 12 of the wafer 10 to remove some of the excess sand.
• This optional step which can be performed after step 304 or at the same time, must ensure that the air flow does not damage the wafers 10 or interfere with the sand as it falls on the front surface 12.
• the wafer 10 should be sufficiently textured and thus, may be removed from the belt 18 (step 306).
• this process can texture sheet wafers 10 in series.
• the conveyor belt 18 may support multiple wafers 10 in a side-by-side configuration along its width.
• the sandblasting machine 16 may serially texture plural sets of side-by-side wafers 10.
• the back surface 14 should not have any more than a negligible change in its average roughness.
• follow-up processes may apply a simple chemical etch to the front surface of the wafer 10.
• illustrative embodiments may pass the wafer through a texturing chemistry.
• the texturing chemistry includes hydrofluoric acid— but not sulfuric acid— and/or nitric acid. Some embodiments use other acids that have a similar result to that of hydrofluoric acid.
• the wafer 10 may be immersed in a bath of the texturing chemistry to further texture the front side. Note that immersion also exposes the back side of the wafer, thus using more texturing chemistry. Alternatively, the wafer 10 may be floated to immerse only the front side in the texturing chemistry.
• these follow-up processes may borrow portions of the processes described in co-pending patent application number 12/546,942, entitled, “Single-Sided Textured Sheet Wafer,” filed on August 25, 2009, and naming Guenther Grupp and Brian McMullen as inventors, the disclosure of which is incorporated herein, in its entirety, by reference.
• This texturing chemistry does not necessarily have sulfuric acid. Accordingly, sandblasting avoids use of this problematic type of acid.
• FIG. 4 schematically shows a top view of a photovoltaic cell 25 that may incorporate a wafer 10 textured in the manner described above. This is but one of any number of different types of photovoltaic cells that may use the noted wafer 10. Accordingly, discussion of this specific photovoltaic cell 25 is not intended to limit some embodiments of the invention.
• the same reference numbers for the front and back surfaces 12 and 14 of the wafer 10 are used for the front and back surfaces of the cell 25.
• the top surface of the cell 25 has an antireflective coating (not shown) to capture more incident light, and a pattern of deposited/integral conductive material to capture electric current.
• the conductive material includes a plurality of thin fingers 26 traversing generally lengthwise (horizontally from the perspective of the figure) along the wafer 10, and a plurality of discontinuous busbars 28 traversing a generally along the width (vertically from the perspective of the figure) of the front surface 12 of the wafer 10 (also referred to as a "substrate").
• each of the busbars 28 has regularly spaced discontinuities along their lengths.
• the busbars 28 are generally arranged in a pattern that is more or less perpendicular to the fingers 26.
• busbars 28 and fingers 26 may form the busbars 28 and fingers 26 in different orientations.
• the fingers 26, busbars 28, or both could traverse in a random manner across the front surface 12 of the wafer 10, at an angle to the fingers 26 and busbars 28 shown, or in some other pattern as required by the application.
• the photovoltaic cell 25 also has a plurality of tab conductors 30 (referred to generally as “tabs 30") electrically and physically connected to the busbars 28.
• the tabs 30 may be formed from silver, silver plated copper wires, or silver plated copper wires to enhance conductivity.
• the tabs 30 transmit electrons gathered by the fingers 26 to the busbars 28 and then to a metallic strip 32, which is connectible to either an external load or another photovoltaic cell 25 (e.g., as shown in Figure 1).
• Illumination of the front surface 12 of the wafer 10 generates charge carriers; namely, holes and electrons.
• the back face (not shown) of the cell 25 does not receive light and thus, may be completely covered to maximize its efficiency in collecting charge carriers.
• the back face of the cell 25 has a back surface metallic covering (e.g., aluminum) with an exposed bottom contact (not shown) shaped to correspond with the shape of the prior noted metallic strip 32.
• the photovoltaic cell 25 therefore serially connects with similar photovoltaic cells by connecting their metallic strip 32 to its bottom contact, and/or by connecting its metallic strip 32 to their bottom contacts.
• the bottom contacts may be embodied by one or more small pads to which the strip 32 is electrically connected.
• Figure 5 shows a process for forming the photovoltaic cell 25 shown in Figure 4 in accordance with illustrative embodiments of the invention. It should be noted that for simplicity, this described process is a significantly simplified version of an actual process used to form a photovoltaic cell 25. Accordingly, those skilled in the art would understand that the process may have additional steps not explicitly shown in Figure 5. Moreover, some of the steps may be performed in a different order than that shown, or at substantially the same time. Those skilled in the art should be capable of modifying the process to suit their particular requirements.
• the process begins at step 502, which provides the doped, textured substrate wafer 10 produced by the process of Figure 3.
• the process diffuses a junction into the substrate 12 (step 504).
• a P-type string ribbon wafer 10 may form a very thin layer of N- type material to the front surface 12 of the wafer 10.
• this layer may have a thickness of about 0.3 microns.
• the process may apply this layer by spraying a phosphorous compound onto the front face 12 of the wafer 10, and then heating the entire substrate 12 in a furnace.
• the junctions may be formed by other means and thus, the noted techniques are discussed for illustrative purposes only.
• step 506 by depositing the above noted electrically insulating, antireflective coating (not shown) to the front surface 12 of the wafer 10.
• the antireflective coating may be formed from conventional materials, such as silicon nitride.
• step 508 processes the back surface 14 of the wafer 10.
• conventional screen-printing processes first form a bottom contact (not shown) from a silver paste on the wafer 10, and then mask the bottom contact to form the bottom surface metallic covering (e.g., formed from aluminum, not shown).
• the process begins processing the front surface 12 by forming the arrays of fingers 26 and busbars 28 (step 510).
• illustrative embodiments screen-print a highly conductive paste over a mask on the front surface 12 of the wafer 10.
• the mask has the desired pattern for fingers 26 and busbars 28.
• Illustrative embodiments deposit one layer of conductive material only, although some embodiments can deposit multiple layers.
• various embodiments use a silver paste to form the fingers 26 and busbars 28.
• This step may deposit the fingers 26 as a substantially continuous line of the conductive material. Accordingly, fingers 26 formed this way should be free from breaks along their lengths. Despite these efforts, however, during or after processing, any of the fingers 26 may form one or more breaks along their lengths (referred to as "unintentional breaks"). Consequently, the resultant finger(s) 26 in turn often have one or more irregularly spaced breaks. Such breaks also may have irregular shapes.
• Fingers 26 formed by processes to have no breaks thus are considered not to be discontinuous even if they have one or more such breaks.
• fingers engineered with spaces/discontinuities/breaks along their length, whether they are regularly or irregularly spaced are considered to be discontinuous.
• fingers engineered without spaces/discontinuities/breaks along their lengths are considered to be continuous— even if they have the noted unintentional breaks. The same discontinuous and continuous
• such embodiments may use inkjet printing or aerojet printing.
• the process passes the wafer 10 through a furnace at a high temperature for a short amount of time.
• the process may pass the wafer 10 through a furnace at 850 degrees C for approximately 1 second.
• This short but quick heating effectively solidifies the conductive paste, and causes the conductive paste to "fire through" the antireflective coating.
• the conductive paste penetrates through the antireflective coating to make ohmic contact with the wafer 10.
• the fingers 26 and busbars 28 contact the wafer 10 in a manner that causes their respective current- voltage curves to be substantially linear.
• step 512 secures busbar pads 30 (part of fingers 28) to the busbars 28.
• step 514 by affixing the metal strip 32 (see Figure 2A) to the busbars 28. Any conventional means for making this connection should suffice, such as conventional soldering techniques.
• the resulting photovoltaic cell 25 can be combined with other similar cells into panel form to form a photovoltaic panel/module.

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• Photovoltaic Devices (AREA)
• Container, Conveyance, Adherence, Positioning, Of Wafer (AREA)
• Helmets And Other Head Coverings (AREA)
• Mechanical Treatment Of Semiconductor (AREA)

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• 2012
  • 2012-03-01 WO PCT/US2012/027266 patent/WO2012118960A2/en not_active Ceased
  • 2012-03-01 MX MX2013010077A patent/MX2013010077A/es not_active Application Discontinuation
  • 2012-03-01 TW TW101106818A patent/TW201251099A/zh unknown
  • 2012-03-01 CA CA2885922A patent/CA2885922A1/en not_active Withdrawn

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