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.

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.

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.

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 a 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.”