Highly coveted in the solar industry: wafer-thin silicon discs
Published: 1 July 2015 - Marianne Evans
Emerging from the crisis stronger – this phenomenon should also apply to the solar industry. Under this premise the Fraunhofer Center for Silicon Photovoltaic CSP in Halle carries out applied research throughout the entire process chain for the production of wafer-based silicon solar modules. CSP is taking part in the “Intersolar Europe” trade fair for the solar industry in Munich for the fourth time. This year it has even been nominated for an Innovation Award.
“No, the diamond wire does not sparkle,” said Stephan Schönfelder, holding it up to the light. The wire looks grey compared to the shiny reddish gold steel wire with brass coating. The diamond coating feels rough and produces a faster dissipation of silicon blocks in the coveted thin wafers. Time is money – with the production of these grey discs that are used as the base plate for solar cells too. Wafer-based silicon solar cells have an 80% share of the global production of solar cells and modules. “Roughly a third of the costs for a silicon solar module is accrued before production of the wafer even starts,” said Stephan Schönfelder and he added that the research is about reducing these costs, even more so due to the crisis on the photovoltaics market. Schönfelder coordinates the research project which the diamond wire saw is involved even in the name: “DiaCell – wafering technologies from the substrate to the photovoltaics module”. The mechanical engineer wrote his doctoral thesis at the Fraunhofer Institute for Mechanics of Materials IWM in Halle. Its theme is based in the field of the mechanics of thin silicon substrates. He is therefore an expert when it comes to the mechanical properties of thin silicon wafers and in producing them in high quality but as cheaply as possible.
Schönfelder holds a disc between his two index fingers. It is roughly 180 micrometres thin, which is the standard, he said, adding that this research project is about producing even thinner silicon wafers, among other things. At the same time they are looking at reducing the breakage rate too. Partners in the project come from regional industry: SILTECTRA from Dresden, bubbles & beyond from Leipzig and Innotech Solar from Halle.
An industry compatible pilot line has been set up at Fraunhofer CSP where the wafers run through the entire production process chain. Existing and new production technologies are tested and optimised here over and over again. As research cannot just rest at having a fantastic idea that changes the intermediate product in the desired way, you also have to look at what influence this idea has on the successive elements in the value chain, says Schönfelder. DiaCell has the job of developing technologies that reduce costs for the entire value chain. As a result, mechanical experiments showed that the diamond wire sawn wafers also have one disadvantage: they break easily in the sawing direction. Furthermore, the diamond wire saw causes chips: very fine powder that contaminates the wafer disc and entails several complex chemical steps in the cleaning process. To save time and money here too, bubbles & beyond is developing so-called “intelligent fluids” together with Fraunhofer CSP. The development of fluids goes hand in hand with the process development and may cut out whole sections in the entire production process in the end.
CSP is researching splitting the wafer without any material loss with the project partner SILTECTRA. After all, every sawing gap causes material loss in the ballpark of one wafer thickness. That is a great deal in relation to the approx. 180 micrometre thin standard disc and may mean up to 50% material loss.