Thinning Solar Silicon Discs

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Originally published on Solar Love.

Carrying out applied research throughout the entire process chain of wafer-based silicon solar module production, Fraunhofer CSP is currently working on improving the quality of wafer-thin silicon discs and in producing them as cheaply as possible.

Wafer-Thin Silicon Discs on Leading Edge of German R&D

Time is Money

With an 80% share of the global production of solar cells and modules, wafer-based silicon solar cells are highly profitable. However, time is money, all the way down to the production of these desirable little grey wafer-thin silicon discs used as the base plate for solar cells.

Fraunhofer CSP Silicon Wafers Group Manager Dr. Stephan Schönfelder spoke recently about Fraunhofer’s R&D efforts to improve these. “Roughly a third of the costs for a silicon solar module is accrued before production of the wafer even starts,” he said, adding that his research is focused on reducing costs.

Wafer-Thin Silicon Disc R&D

Dr. Schönfelder, a mechanical engineer who wrote his doctoral thesis at the Fraunhofer Institute for Mechanics of Materials (IWM) in Halle, Germany, oversees a research project named “DiaCell – innovative wafering technologies from the substrate to the photovoltaics module.” Schönfelder’s thesis was based on the mechanics of wafer-thin silicon substrates, making him the right man for his current R&D.

Setting up an industry-compatible pilot production line at Fraunhofer CSP, wafers are put through the entire production chain. Testing and optimizing, new developments and production improvements run through the chain repeatedly. “As research cannot just rest at having a fantastic idea that changes the intermediate product in the desired way,” said Schönfelder, “you also have to look at what influence this idea has on the successive elements in the value chain.” An additional requirement of the DiaCell wafer-thin silicon R&D project involves computer simulation of all silicon wafer production processes.

silicon wafers © fraunhofer CSP

Diamond Saw Pros and Cons

Holding a wafer-thin silicon disc between his two index fingers, Schönfelder explained that it is the industry standard, roughly 180 micrometres thin. He said that his research project is about producing even thinner silicon wafers, as well as reducing the breakage rate. Regional German solar industry partners in Fraunhofer’s DiaCell project include SILTECTRA from Dresden, bubbles & beyond from Leipzig, and Innotech Solar from Halle.

DiaCell refers to the name of the diamond wire saw involved in the research. Holding it up to the light, Schönfelder said, “No, the diamond wire does not sparkle.” The wire looks grey compared to standard shiny steel slurry wire saws with reddish-gold brass coating. The diamond coating feels rough, and, on the positive side, is more effective, producing a faster dissipation of silicon blocks in the highly prized wafer-thin discs.

Reducing costs for the entire value chain is the mission of the DiaCell research project. In the course of experimentation, diamond wire-sawn wafers also revealed some negative sides. First, the wafers break easily in the sawing direction. Furthermore, chips resulted from the diamond wire saw, contaminating the wafer-thin silicon discs with very fine powder. Several complex chemical decontamination steps were thus required in the process.

Additionally, current material loss from every wire-sawn wafer-thin silicon disc is nearly the same width as one standard wafer, around 180 micrometers. This sawing gap created by the wire cutting process is incredibly expensive, representing a nearly 50% material loss.

Exploring Wafer Splitting Strategies

With project partner SILTECTRA, another research effort is developing wafer-splitting strategies to produce zero material loss. Instead of sawing the wafer into slices, with wafer splitting, a special polymer is glued to both sides of the wafer. When in a frozen state, the special polymer layer contracts, developing a strong enough force to split the wafer into slices.

Developing “intelligent fluids” with Fraunhofer CSP, bubbles & beyond is researching other money-saving strategies. Hoping to cut out whole sections in the production process, designing “intelligent fluids” is a critical endeavor for every aspect of process development.

The “MechSi” of Wafer-Thin Silicon Discs

Fraunhofer Electrical Engineer Jens Schneider is leading another aspect of the wafer-thin silicon disc research project, dealing with “modelling the mechanical behaviour of thin silicon substrates and solar cells.” Dr. Schneider reported, “‘MechSi’ (his project title) is on the one hand investigating what influence new production processes have on the wafer. Whether the wafer-thin silicon discs are made by standard slurry wire saws, diamond wire saws or by splitting, determines their different properties in the end.”

The MechSi team is investigating the mechanical behaviour of the wafers when processed into solar cells and modules. Schneider anticipated that manufacturing recommendations would result from this research. Here again, reducing costs is dependent on reducing breakage. “When introducing new processes it is ultimately also about reducing the breakage rate,” Schneider said.

bipv solar panels screenshot © fraunhofer CSP

Standing in front of a structure similar to a roof truss, Schneider points to the mounted building-integrated PV (BIPV) solar modules. Exposed to extreme wear, these wafer-based silicon solar cells function as roofing, as well as capture sunlight to produce solar energy.

Intersolar 2015 Innovation Award Nominee

Recently attending the “Intersolar Europe” trade fair in Munich, Fraunhofer CSP participated for the fourth year. This year, the company was even nominated for an Innovation Award. Nominated for research on potential-induced degradation (PID), this is a frequently occurring defect mechanism as a result of operating solar modules at high system voltages in a damp environment.

Fraunhofer CSP R&D efforts identified short circuiting in solar cells that can lead to output losses as a result of crystal defects. PIDcon, a test process plus corresponding device developed with Freiberg Instruments simplifies quality testing of solar cells and modules during production, saving material, energy, and costs.

Professor Jörg Bagdahn, Head of Fraunhofer CSP in Halle, announced, “We are delighted that PIDcon has been nominated for the Intersolar Award 2015.” Bagdahn continued, “Experiences of previous visits to trade fairs have shown that our research expertise is in extremely high demand and that Intersolar gives us access to important contacts with manufacturers, suppliers and other partners in the industry.”

solar cells screenshot © fraunhofer CSP

Picture Credits: All images © Fraunhofer CSP

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7 thoughts on “Thinning Solar Silicon Discs

  • When the saw is the same thickness as the slice it cuts, it looks as if the technology is reaching its limit. Radical future improvements may depend on a shift to vapour deposition – the way perovskite cells are made already. Not that these don’t face other large obstacles on the rocky path to commercialisation.

    • VPD doesn’t produce the big single crystals you get with block pouring or ignot pulling – afaik.

      Anyone ever thought of using laserlight to penetrate the crystal and focus in there to break up the structure (3D laser engraving/cracking)?

    • I thought perovskite cells were made by drying a liquid solution?

  • This is cool stuff. I had no idea material loss was this much:

    “Additionally, current material loss from every wire-sawn wafer-thin silicon disc is nearly the same width as one standard wafer, around 180 micrometers. This sawing gap created by the wire cutting process is incredibly expensive, representing a nearly 50% material loss.”

    Man, that’s a lot of wasted material. Mother nature made the tree and the sawdust can be either compressed into MDF or burned for electricity. But in the case of PVs, man spent a lot of time and money making the stuff that ultimately gets wasted. So I went googling for an analogy in sawmilling. Here’s a paper from US Forest Service:

    “Factors Determining Lumber Recovery in Sawmilling”

    The following factors influence lumber recovery during the sawmilling process and are examined in detail in this report:

    (1) Log diameter, length, taper, and quality.
    (2) Kerf width.
    (3) Sawing variation, rough green-lumber size, and size of dry-dresser lumber.
    (4) Product mix.
    (5) Decisionmaking by sawmill personnel.
    (6) Condition and maintenance of mill equipment.
    (7) Sawing method.

    From this mid 1980s report on 1970s computer simulations, it looks like 50 percent recovery is a dream upper limit unattainable goal. Nonetheless, research always needs analogies to move forward. Or not make the same mistakes.

    • Yes this was something that I learned about years ago when getting specialty wood cut for my furniture business.
      An experienced and qualified sawmill operator is an artisan in their own league that can make or break the profitability of a private mill. It is amazing how they can look at a raw log and tell you the dimensions and board feet that can be produced within a couple of inches.
      At that time even CAD run saws couldn’t compete with the experience of these old masters for utilizing every usable inch of a tree.

  • There just has to be a better way than casting ingots and then slicing them up, that’s so energy- and material-inefficient…

    It looks like some research work is being done with sintered silicon powders for PV’s, there seem to be a few scholarly articles out there.

  • The cost of Solar PV panels has to get below $0.40/W at close to 20% conversion efficiency to compete with FirstSolar CdTe and possibly to compete with CIS/CIGS (Solar Frontier?, Hulk?, Siva?) in the next few years. The race will keep going from there. – October 2014
    “SunEdison Predicts New FBR Polysilicon Process/Facility Will Lead To $0.40/W Solar Modules”

    1366 is in commercial production of it’s (silane?) vapor deposition technology, aiming to significantly reduce the cost of silicon wafers. I think nakedChimp is correct this is multi-crystalline Si, not mono-crystalline. Crystal Solar is also pursuing VPD.

    Cracking off wafers using ion implantation is a possibility, although the machines are expensive and I haven’t seen any articles revealing it coming to market. I find it hard to believe sawing ingots is the final answer.

    SunEdison and others are bringing down the cost of purified silicon production. Purified silicon was someting like $40/kg before 2007. Many seem to be able to produce now for $15/kg or less. SunEdison is claiming production cost below $6/kg will be reached using FBR, reference here: – October 2014
    “SunEdison adds polysilicon plant to China ambitions – Bloomberg”

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