Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
  • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
  • H01L31/02—Details
  • H01L31/0224—Electrodes
  • H01L31/022408—Electrodes for devices characterised by at least one potential jump barrier or surface barrier
  • H01L31/022425—Electrodes for devices characterised by at least one potential jump barrier or surface barrier for solar cells
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B64—AIRCRAFT; AVIATION; COSMONAUTICS
  • B64G—COSMONAUTICS; VEHICLES OR EQUIPMENT THEREFOR
  • B64G1/00—Cosmonautic vehicles
  • B64G1/22—Parts of, or equipment specially adapted for fitting in or to, cosmonautic vehicles
  • B64G1/42—Arrangements or adaptations of power supply systems
  • B64G1/44—Arrangements or adaptations of power supply systems using radiation, e.g. deployable solar arrays
  • B64G1/443—Photovoltaic cell arrays
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
  • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
  • H01L31/04—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof adapted as photovoltaic [PV] conversion devices
  • H01L31/041—Provisions for preventing damage caused by corpuscular radiation, e.g. for space applications
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
  • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
  • H01L31/04—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof adapted as photovoltaic [PV] conversion devices
  • H01L31/042—PV modules or arrays of single PV cells
  • H01L31/048—Encapsulation of modules
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
  • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
  • H01L31/04—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof adapted as photovoltaic [PV] conversion devices
  • H01L31/042—PV modules or arrays of single PV cells
  • H01L31/05—Electrical interconnection means between PV cells inside the PV module, e.g. series connection of PV cells
  • H01L31/0504—Electrical interconnection means between PV cells inside the PV module, e.g. series connection of PV cells specially adapted for series or parallel connection of solar cells in a module
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
  • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
  • H01L31/04—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof adapted as photovoltaic [PV] conversion devices
  • H01L31/054—Optical elements directly associated or integrated with the PV cell, e.g. light-reflecting means or light-concentrating means
  • H01L31/056—Optical elements directly associated or integrated with the PV cell, e.g. light-reflecting means or light-concentrating means the light-reflecting means being of the back surface reflector [BSR] type
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
  • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
  • H01L31/18—Processes or apparatus specially adapted for the manufacture or treatment of these devices or of parts thereof
  • H01L31/1876—Particular processes or apparatus for batch treatment of the devices
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
  • Y02E10/00—Energy generation through renewable energy sources
  • Y02E10/50—Photovoltaic [PV] energy
  • Y02E10/52—PV systems with concentrators
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
  • Y02P70/00—Climate change mitigation technologies in the production process for final industrial or consumer products
  • Y02P70/50—Manufacturing or production processes characterised by the final manufactured product

Definitions

• the present invention relates to solar ceils.
• the invention relates to methods and apparatuses for efficient packaging of solar cells for space applications. Description Of The Related Art
• Photovoltaic cells are well-known devices which convert solar energy into electrical energy. Solar cells have long been used to generate electrical power in both terrestrial and space applications. Solar cells offer several advantages over more conventional power sources. For example, solar cells offer a clean method for generating electricity. Furthermore, solar cells do not have to be replenished with fossil fuels. Instead, solar cells are powered by the virtually limitless energy of the sun. However, the use of solar cells has been limited because solar cells are a relatively expensive method of generating electricity. Nonetheless, the solar cell is an attractive device for generating energy in space, where low-cost conventional power sources are unavailable.
• solar cells manufactured for space use have been of fairly small dimensions. Commonly, solar cells have had dimensions of 4 cm x 6 cm or less. In space applications, hundreds or thousands of the small solar cells are interconnected together to form large solar arrays. In a typical manufacturing process, a solar cell manufacturer delivers hundreds or thousands of separate, unconnected, solar cells to a solar array assembler. Often, the solar ceil manufacturer mounts a coverglass over each cell to protect the cells against space radiation and other environmental conditions. Alternatively, the solar cell manufacturer may ship bare cells to the array assembler. The array assembler then must provide the coverglass. The array assembler also interconnects the individual cells into large, suitably sized panels.
• One preferred embodiment of the present invention is a modular, glass covered solar cell array suitable for use in space.
• the modular, glass covered solar cell array includes at least physically and electrically interconnected solar celis. At least a portion of both cells are covered by a common, substantially transparent cover. The transparency of the cover will not substantially degrade when exposed to a space radiation environment.
• at least one cell is comprised of single crystal silicon.
• at least one cell is comprised of GaAs.
• at least one cell is comprised of multijunctions.
• At least one cell has an area of at least 100 cm 2 . In another embodiment, at least one cell is at least 150 ⁇ m thick, in still another preferred embodiment, at least one cell is thinner than 150 m
• the solar array cells are interconnected by at least one conductor mounted on the backs of the cells.
• at least one cell has at least one contact for receiving the conductor. The contact wraps from a first side of the cell to a second side of the cell.
• at least one cell has at least one contact for interconnection to at least one other solar cell array.
• the solar cell array includes at least one bypass diode mounted to at least one surface of one of the cells, in another embodiment, the solar cell array includes contacts for connection to at least one externally mounted bypass diode.
• Figure 1 a illustrates one embodiment of a solar ceil front side
• Figure 1b illustrates one embodiment of a solar cell back side
• Figure 1c illustrates one embodiment of a solar cell having wrap around contacts
• Figure 2 illustrates one embodiment of a standard power module
• Figure 3 illustrates components of one embodiment of the standard power module
• Figure 4 illustrates the results of a power density tradeoff analysis
• Figure 5 illustrates the results of a cost versus coverglass quantity tradeoff analysis
• Figure 6 illustrates the results of a cost versus coverglass coating tradeoff analysis
• Figure 7 illustrates the results of a cost versus adhesive mass tradeoff analysis
• Figure 8 illustrates the results of a radiation analysis for one relevant operating environment
• Figure 9 is a graph illustrating mission fluence versus shielding.
• Figure 10A illustrates one embodiment of an array of interconnected standard power modules
• Figure 10B is a detailed diagram of a portion of the interconnected standard power modules of Figure 10A.
• Figure 11 illustrates one embodiment of a solar cell manufacturing process; and
• Figures 12A, B illustrate one embodiment of a standard power module manufacturing flow.
• a baseline modular, glass covered solar array 200 which is interchangeably called a standard power module (SPM), consists of a solar cell circuit matrix that can be readily used to build up a variety of array sizes.
• SPM configuration consists of: • A large area ( " 20 cm x 30 cm) ce ⁇ a doped borosilicate coverglass microsheet 202, nominally 100 ⁇ m thick, which provides radiation resistant shielding for charged and uncharged particles.
• coverglass materials and dimensions can be used as well.
• each solar cell has at least one space qualified N/P wraparound contact 102, although other configurations may be used as well.
• each solar cell has two contacts 102, 104, one for each cell doping polarity.
• Two silver plated Invar ribbon interconnectors 206 are approximately 50 ⁇ m thick, and may be welded to each solar cell 100 to provide space qualified thermally matched series connection.
• One skilled in the art will understand that other interconnect technologies, such as mechanically bonding, crimping and soldering may also be used
• transparent silicone adhesive 300 bonds the solar cell circuit to the coverglass 202 and provides a space qualified non darkening resilient interface.
• the solar cell design is space qualified with a single crystal silicon design.
• the materials and the cell are optimized for performance in an AMO spectrum (the spectrum found at Earth's orbit around the sun, outside of Earth's atmosphere) and the space radiation environment.
• AMO spectrum the spectrum found at Earth's orbit around the sun, outside of Earth's atmosphere
• space radiation environment As illustrated in Figure 1 c, a shallow junction N on P structure 106 which provides satisfactory radiation resistance may be used.
• terrestrial silicon cells have lower radiation resistance because they use deep junctions to prevent punch through of the screen printed contacts.
• the preferred base silicon material is space qualified, Czochralski grown, boron doped, single crystal silicon with a base resistivity of 1 3 ohm cm
• This resistivity is a compromise (radiation resistance vs initial power) for many missions that don't see radiation fluences greater than about 3E15 e/cm 2 (1 MeV equivalent) where 10 ohm-cm material becomes advantageous.
• the silicon wafer of the preferred embodiment has been designed to provide cost reductions compared to conventional designs, while at the same time maintaining those attributes that are important for maintaining radiation resistance, such as the resistivity and minority carrier lifetime.
• GaAs/Ge and multijunction solar cells may be used in the SPM instead of silicon solar cells.
• the GaAs/Ge and multijunction solar cells such as those manufactured by TECSTAR, can provide efficiencies greater than 24%.
• the AITiPdAg contact on the back side of the solar cell has an evaporated aluminum BSR layer under the TiPdAg contact to reflect the long wavelength light which consequently reduces the absorptivity ( ⁇ ) to 0.71.
• a two-sided wraparound (WA) contact configuration 102 as shown in Figures lai c is used due to its many advantages.
• the contact pick-off at two opposite ends, halves the parasitic series resistance losses associated with 10 cm dimensions.
• the contacts 102, 104 are located primarily on the back of the cell 100, the front contact area is reduced, resulting in an increased active area.
• the coverglass can cover the entire front surface, without introducing stress risers, resulting from top-to-bottom interconnects.
• the coverglass also provides full cell protection from atomic oxygen and high energy particle radiation effects.
• the design provides a "give away area" on the rear surface below 5%, adding less than 1 % to the resistive losses.
• the contact 102 is electrically coupled to N- material, while the contact 104 is electrically coupled to P+ material.
• One embodiment of the front of the cell 100 illustrated in Figure 1a has a dimension D1 of approximately 10 cm, a dimension W1 of approximately 10 cm, a bus bar width dimension X1 of approximately 0.1 cm, and a bus bar spacing of approximately 4.57 cm.
• One embodiment of the back of the cell 100 illustrated in Figure 1b has a contact width dimension T1 of approximately 0.2175 cm, and an N channel contact length dimension L1 of approximately 6.55 cm, and a P channel contact length L2 of approximately 2.29 cm.
• the estimated conversion efficiency of one embodiment of a 10 cm x 10 cm, WA contact cell 100 is approximately 12.1 % (1 ,367 W/m 2 , AMO). This estimate is based on data from a prior art 13.7% efficient 4 cm x 6 cm cells with dual a ⁇ tireflection coating derated by the following factors:
• the solar cell should preferably be as thin as practical, and 62 ⁇ m silicon cells have been fabricated and used to make flight qualified arrays, and may be used in the present invention.
• BSF back surface field
• a 200 ⁇ cell thickness is used to minimize breakage losses associated with large area cells and to increase delta efficiency by 6%. This performance versus weight trade-offs is discussed further in the SPM discussion below.
• the solar cell antireflection coating is a single layer TiOx chemical vapor deposition (CVD) coating instead of the more expensive evaporated dual AR coatings.
• CVD chemical vapor deposition
• Both coating types can be used, have been space qualified, and have space flight heritage.
• Isc short circuit current
• the additional Isc (short circuit current) performance advantage has been desired in spite of the additional costs and expensive evaporator equipment encountered.
• the CVD method to form the AR coating in used in one embodiment. This coating will not delaminate or degrade the cell's mechanical integrity or electrical performance when operated over the specification environment and mission life.
• a preferred embodiment baseline SPM design is shown in Figures 2 and 3 and consists of six, 10 cm x 10 cm, 200 ⁇ m BSR silicon solar cells 100 interconnected with 50 ⁇ m thick silver-Invar ribbons 206. However, even larger area coverglass sizes may be used. Each SPM 200 is covered with a 100 ⁇ m thick, cerium oxide coverglass 202 which is bonded to the cells with Nusil CV7-2500 adhesive 300. Component selection was based on system trade studies that evaluated electrical and mechanical performance, mass, reliability, manufacturability, size, cost, schedule and heritage.
• the preferred embodiment of the baseline SPM design advantageously has the following properties:
• the SPM 200 has a length Y2 of approximately 30.17 cm, a width Y3 of approximately 20.135 cm, and a typical cell gap and edge spacing Y1 of approximately 0.025 cm.
• FIG. 4 shows an approximate power to mass ratio of 100 Watts/kg at End of Life (EOL) for the 0.020 cm/0.010 cm cell/coverglass thickness combination.
• EOL End of Life
• the power density (W/kg) studies were performed to select the coverglass/cell thickness that would provide an optimum W/kg.
• the 0.015 cm/0.010 cm and 0.020 cm/.OIO cm cover and cell design options provide significantly higher values than the other combinations. These two combinations were subsequently evaluated based on manufacturability (yields) and the net cost of producing all of the required SPMs.
• the WA cell configuration allows for low cost SPM manufacturing. Manufacturing operations may be performed with the solar cells face side down for glassing, interconnecting, testing and marking. This approach reduces handling and greatly increases throughput.
• Another advantage of the preferred embodiment baselmed cell is the interconnect.
• the interconnect may be a low cost silver plated Invar ribbon formed for stress relief as part of the automation process and then welded to each cell of the SPM.
• the silver-plated Invar interconnect material has a thermal coefficient of expansion which is closely matched with silicon. This reduces weld and thermal elongation stresses resulting in a highly reliable interconnection.
• the stress relief has been designed to absorb the .008 cm movement within the SPM and the .051 cm movement between SPMs.
• the interconnect is protected from the effects of atomic oxygen by the design of the SPM which shelters the interconnects by covering them with the solar cell, cover adhesive and coverglass.
• Techniques other than welding may be used to attach the interconnects.
• soldering or crimping techniques may be used as well.
• the coverglass used in one preferred embodiment was selected by size, thickness and the need for coatings.
• Figure 5 depicts the reduced cost savings of more than 71 % by processing six cells under one piece of glass as compared to fabricating an assembly of one cell with one piece of glass
• Figure 6 provides the results depicting the cost advantage of approximately 35 percent by using uncoated glass versus coated glass expressed in terms of dollars per watt at end of life
• performance enhancements can be gained with the use of antireflective (AR) coatings with possible increased cost and added complexity at processing (having to keep track of the coated side).
• AR antireflective
• coverglass performance experiments have been conducted, and the results show the uncoated coverglass provides a one percent gain as compared to the bare solar cell which is attributable to the improved optical match of the cover overlaid on the cell. It is also possible to use a coverglass that has increased rigidity when treated by a process called “toughened.”
• the toughened glass process is widely used for military aircraft windshield applications and involves cutting the coverglass to it's final size, then chemically treating it to replace the sodium ions with potassium ions Atomic oxygen erosion of the coverglass will be less than 1 micron when exposed to 1.5 x 10 21 A/cm 2 .
• SPM design facilitates SPM integration into large solar arrays.
• silver-plated Invar interconnects protrude from the positive and negative edges of the SPM to allow for welding of the interconnects between each module.
• the advantage of this approach is that all SPM level interconnecting is accomplished by overlapping the interconnect tabs and welding or soldering them together.
• the SPMs are sized and configured to allow for robotic intervention for a convenient pick and place operation.
• Figure 10A illustrates multiple SPMs 200 serially connected into a larger array 1000, thereby providing higher output voltages than are achievable if the SPMs were connected in parallel.
• the array may optionally be protected using external diodes 1002.
• the SPMs may be connected in parallel and the diode may be mounted within the SPMs.
• Figure 10B is a detailed diagram of a portion 1004 of the interconnected standard power modules of Figure
• Interconnects 206 couple the N channel contacts 102 of one cell 100 to the P channel contacts of the adjoining cell.
• silicon solar cells are used because they have the inherent capability to withstand reverse bias voltages experienced when celis in strings are partly or totally shadowed. Many satellites have flown without bypass diode protection, but it is customary to include a bypass diode connected across several silicon ceils to prevent overheating or degradation of shadowed celis.
• bypass diode protection is provided at the array level or on the cells themselves.
• the diode is typically mounted onto a cell surface.
• placing one bypass diode externally for every 20 cells in series (10 SPMs) serves to protect the cells from degradation after the shadow is removed.
• the solar cells are screened to identify and reject cells having reverse bias problems, so that no bypass diodes are required.
• a preferred embodiment of the SPM has excellent on-orbit performance characteristics. Performance predictions indicate that the EOL on-orbit power-to-mass ratio will be 100 W/kg with an areal power density of 87 W/m 2 . These levels of performance translate into significant system-level benefits.
• the array mass savings can be used to increase the payload mass and augment the capacity of the revenue-generating communications systems.
• the areal power density performance enables a smaller area array, which reduces on-orbit drag, also reducing the fuel (and mass) that needs to be carried for station-keeping. Power Analysis
• a power analysis uses typical loss factors, including Af - the array-level loss factor of 0.938.
• the P SPM (EOL, on-orbit) and P s ⁇ c (deliverable configuration) values are highlighted in the analysis, and the Rf, Sf, Af, and Tf ⁇ factors are identified.
• the P s ⁇ c value is based on SPM operation at the maximum power point. This value may be for acceptance testing and operation at voltages other than the maximum power point. For example, P s ⁇ c would be 9.064 W (5.80 % reduction) if SPM testing is conducted at:
• the cell dimensions are effective dimensions for a rectangular cell having the equivalent area to a preferred embodiment of the cell having cropped corners.
• a coverglass insertion factor of 1.01 is used. This value results from improved optical coupling to the single-layer AR-coated cell.
• the temperature coefficients used are end of-life values, and are applied in the analysis after all the life factors have been taken. These values were determined from Figures 3.15 • 3.18 of the Jet Propulsion Laboratory (JPL) Solar Cell Radiation Handbook (JPL Publication 82 69). The radiation factors are discussed further below. The UV darkening factor is based on similarity. The operating temperature was determined iteratively. Radiation Analysis
• Typical energy spectra for charged particle environments were enhanced by interpolating energies for which radiation damage coefficients are defined by JPL. Energy spectra values are shown in Figure 8 for relevant environments, trapped electrons, trapped protons, and solar protons. This is done to increase the resolution and accuracy of the subsequent calculations for equivalent radiation fiuence.
• Tfn Isc (uA/C-cm2) 36 0.297 .30
• the effective shielding was determined for both the front side and back side of the SPM.
• the values for the baseline design are given in Tables 4 and 5.
• the 200 ⁇ m physical cell thickness equates to 150 ⁇ m shielding thickness because of reduced red and infrared response in the cell base.
• the calculated reliability, R SPM , of the SPM is equal to the calculated probability of success.
• the probability of success is related to component and weld joint failures.
• the solar cell failure rate, ⁇ is expressed in units of "number of failures per operating hour.” The failure rate of 1 x 10 9 failures per soiar cell operating hour was used based on the JPL solar cell array design handbook.
• a mission life of ten years and the on-orbit temperature of 56°C were used to determine the failure rate.
• the reliability predictions were calculated for three scenarios: 1) two cells in series; 2) six cell in series; and, 3) two ceils in series and three in parallel.
• the solar cell failure rate of 1 x 10 9 failures/hour is valid at 30°C.
• the on-orbit temperature was calculated to be 56°C (see SPM performance for details).
• the failure rate increases by 5% per 10°C increase in temperature.
• R SPM 0.9994 Scenario 3, 2 Cells in Series by 3 in Parallel
• the SPM design is highly reliable.
• wafers used in the SPM may be wire sawed.
• the wafers then undergo surface preparation.
• the wafers are chemically etched to remove surface contaminants and damage from the sawing process.
• the wafers may be acid etched using automated production equipment to prepare the surface and control wafer thickness.
• the front surface is preferably smooth and free of work damage and provides for a shallow (0.15 ⁇ m) N+/P junction with good characteristics.
• the back surface polish provides high reflectance for longer wavelengths, reducing solar absorptance giving lower on-orbit cell temperature.
• this etch method provides a well rounded edge for proper deposition of insulating layers and contact materials. Automated handling from coin-stacked wafers to Teflon boats may be used. The transfer ensures that the two WA edges are etched and returned to a cassette, with all edges correctly aligned.
• the post-etch wafer thickness is controlled using SPC procedures which measure and control thickness by etchant temperature, concentration and immersion time.
• the manufacturing equipment provides automation which allows for cassette-to-cassette wafer processing to achieve high throughput and yields.
• a diffusion mask is used on the back surface of the wafer prior to N+ diffusion to prevent doping of the back side of the wafer during the diffusion process, providing uniform and repeatable SiO, layers.
• An ellipsometer is used to measure the oxide thickness and the process is controlled by using SPC methods.
• the CVD production equipment is available from Watkins-Johnson, and uses a continuous belt process with automatic cassette-to-cassette loaders. The wafers are transported through the furnace on a belt where silane and oxygen are mixed to form S ⁇ 0 2 on the heated back surface of the wafers. The loaders allow for high throughput and high yields.
• Diffusion is the primary technique for forming the shallow N+/P junction.
• the wafers are semi- automatically transferred from polypropylene cassettes into a quartz diffusion boat.
• a transfer device is used to transfer the wafers to minimize wafer chipping and breakage and to maintain the alignment of the designated WA edges.
• the solar cell junction is formed in clean quartz boats and tubes.
• Phosphorus oxichlo ⁇ de (P0CI 3 ) and oxygen are used to form phosphorus pentoxide (P 2 0 5 ) on the surface of the wafers.
• the high temperature « 850°C) drives the phosphorus into the wafer to form the junction.
• the sheet resistance of the diffused layer is controlled using SPC methods. This control ensures proper junction depth and therefore the good cell performance, both before and after orbital irradiation
• the MRL production rate diffusion furnace uses advanced process control capability to control diffusion depths and sheet resistance values. Quartz boats are manually loaded on a quartz paddle to start the process. The computer control system automatically positions the wafers in the hot zone of the furnace, initiates the proper gas flow sequence, and slowly transfers the wafers out of the process tube to cool. The wafers are then transferred back to polypropylene boats.
• Oxide Etch Process 1108 This process removes the oxide formed during the diffusion mask process (CVD) and removes the thin glassy oxides formed during the diffusion process.
• the etching solution uses a mixture of hydrofluoric acid and deiomzed water.
• the oxide etch process is totally automatic and uses the same equipment type as for acid etch. Wafers are staged at the front of the machine in cassettes. The machine picks up the cassettes and automatically processes them through the etch and rinse steps. It also automatically loads and replenishes the chemicals using a chemical dispense system.
• a dielectric oxide is preferably formed on the edges of the wafers, where the WA metal is deposited, to prevent shorting of the P/N junction.
• the dielectric is formed in the WA areas using CVD S ⁇ 0 2 .
• Mechanical masks are used to define the pattern for isolation of the N contact on the front and back sides of the wafers.
• the oxide thickness is controlled using SPC criteria.
• the Watkins-Johnson machine is the same type as used for CVD of S ⁇ 0 2 for the diffusion mask process. It is a reliable, continuous belt process. The operator positions a mask properly on a wafer, and clamps it with a clip. The wafer and mask assemblies are manually placed on the belt of the CVD S ⁇ 0 2 furnace, and S ⁇ 0 2 is deposited in the designated WA areas. (6) Front/Back Metal Evaporation 1112, 1114
• Front (TiPdAg) and back (AITiPdAg) metal contact depositions are performed in high volume vacuum deposition evaporators used extensively for space cell manufacturing.
• CHA Industries Mark 50 vacuum deposition system is specifically designed for flexibility and long term reliability to support high volume production.
• Precision metal masks with -50 ⁇ m wide grid openings, are used to form the front grid and bus bar pattern. The mask maintains ceil performance while eliminating the usual expensive, labor intensive photolithography processing.
• a mask is also used for the back side to ensure the proper electrical isolation needed on the back side between the P and N metal contacts.
• the metal thickness are controlled using SPC methods
• A/R Coating Process 1116 The celis are coated with an atmospheric pressure chemical vapor deposited (CVD) antireflective coating on the front surface.
• the coating operation is a low cost, high volume, continuous belt process using equipment provided by BTU International Vaporized tetraisopropyltitanate (TPT) and water are mixed and introduced to the heated surface of the wafers where T ⁇ 0 2 is formed With the WA cell design, masks are not required to shield the contacting areas from the AR coating for additional cost reduction (8) Sintering Process 1118
• Sintering uses a belt furnace supplied by the Thermco Company. Hydrogen and nitrogen are mixed in the furnace to minimize oxidation of the metals during the process.
• the wafers are placed directly on the moving belt, and process controllers ensure that the correct heating rate, dwell time and cooling rate are maintained for optimum contact sintering.
• the wafers are then removed from the belt and placed in coin stacks.
• Solar cells are tested with automated cell test equipment incorporating an integrated AMO solar simulators.
• the cells are separated into electrical groups based on their individual outputs measured at a constant test voltage.
• Coin stacks of cells are placed in a load stacker magazine on the tester.
• a robotic transfer system automatically places the cells under the solar simulator for testing, and then places the cells in the proper electrical group. The operator only needs to pick up the counted stacks of tested cells and to identify electrical groups.
• the sorted cells are transferred to the assembly area for assembly into SPMs.
• An automated ceil handling system is used to provide the accuracy and repeatability needed for high volume assembly.
• Cells may be roboticaily removed from a cassette and positioned on a six-cell fixture (ceil pallet) that maintains the cell alignment throughout the glassing process. Only minimal mechanical contact with the active surface of the cell occurs. Contact of the cells edges and the active surfaces with metal surfaces is eliminated. Unlike a standard front to-back contact cell, no handling steps involving awkward "tabbed" cells are required.
• the positioning and adhering of the cell onto a cell pallet removes possible inherent cell bow in addition to maintaining the cell's location with respect to the other five celis. Varying cell gaps due to cell dimensional tolerances become less critical when interconnects do not pass through them. Having no interconnects on the cell back enables the cells to be placed flat against the pallet for precise control of the adhesive and SPM assembly thickness.
• Adhesive materials are packaged in precise mix ratios for accuracy. Prior to dispensing, the adhesive mix is "de gassed" in a vacuum chamber to remove entrapped air and minimize voids during glassing.
• the cell/adhesive/glass sandwich is automatically transferred to a conveyor where adhesive "wicking" progresses under ambient conditions under the weight of the cell pallet
• the cell pallet remains aligned with the glass pallet via guide pins which allow the sandwich to come together until the cell pallet bottoms-out on the glass pallet and the adhesive gap is fixed. Even with adhesive viscosity changes over the allowable pot life of the material, full wicking is achieved in the time allotted Any excess adhesive flows into the gaps between the cells. Without any interconnects attached to the cell's active side, the adhesive gap design is the thinnest and produces the lightest overall weight.
• the fixture assembly passes through a tunnel oven profiled to bring each fixture up to temperature gradually, to thus prevent warping of the assembly or uneven cure of the adhesive.
• the conveyor belt width is oversized to allow a large range of adjustment without limiting throughput.
• the end of the conveyor passes through a cooling section designed to bring the assembly back to ambient conditions without stressing the SPM.
• the conveyor system ensures that each SPM is exposed to identical process parameters unlike a batch oven system that is subject to the human variable.
• the cell pallet is removed from the assembly, leaving the glass pallet containing the module to be manually transferred to the interconnecting station.
• Interconnect And Inspect 1210 Automated interconnection technology is used to mechanically form individual interconnects from a reel of ribbon and then roboticaily position them for welding. Redundant welding performed with state-of-the-art "constant power" controlled equipment ensures superior reliability and weld strength consistency by automatically adjusting to variances in cell/interconnect surface conditions via process feedback. All portions of the interconnect, including the formed strain relief and welded areas, are fully exposed and free from adhesive contamination unlike any of the front to back contact options. When contact must be made to the front side of a cell, the interconnects strain relief and weld joints become buried in adhesive during the glassing process, prompting concern with having to endure differential thermal stresses between the front and back-side of the cell.
• welding solely to the cell backside after glassing creates intimate support of every portion of the cell with an adhesive/glass layer producing the lowest mechanical stresses.
• Welding to a cell's front side typically requires a pre-glassing weld station, and special handling of interconnected cells, causing increased attrition and manufacturing difficulties.
• a weld station also attaches the necessary array interconnects to the SPM as detailed above.
• an integrated visual inspection identifies candidates to be separated for rework on another weld station to prevent any interruption in product flow. The SPM is then separated from the glassing pallet and transferred to the cleaning station. (6) Clean. Electrical Test And Mark 1212
• Visual in-process inspection is performed by line operators. Final inspection of the SPM may be independently performed by quality assurance personnel in accordance with an approved Acceptance Test Plan.

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• 1999
  • 1999-04-12 AU AU57692/99A patent/AU5769299A/en not_active Abandoned
  • 1999-04-12 EP EP99944982A patent/EP1078393A4/de not_active Ceased
  • 1999-04-12 WO PCT/US1999/007907 patent/WO1999060606A2/en active Application Filing

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