At solar cell manufacturers, Würth Solar in Germany, current production yield is at 90 per cent and rising. Würth Solar GmbH & Co. KG in Schwäbisch Hall (Germany) has been producing solar modules using CIS technology since October 2006. Manufacturing printed circuit boards (PCBs) on glass substrates is a difficult job. These CIS coating pioneers were world leaders in developing solar modules with dimensions of 1200 mm x 600 mm at their Marbach pilot installation back in 2000. Since the start of production in Germany, Würth Solar has collaborated with AIT Göhner GmbH, a Stuttgart-based integration partner of Cognex, to implement data tracking using In-Sight® ID readers in order to boost productivity and optimize product quality.
Determine the origin of defects – before it’s too late!
The modules from Würth Solar are made up of multi-layer copper/indium/selenium solar cells in series, with a coating thickness of four microns. Even in poor light conditions, they guarantee a high output. It is important in production of these cells to collect all the quality data from the substrate plates in each production machine. However, the individual quality results arrive only at the end of the production process, when the performance of the 120 x 60 cm modules is tested in the solar simulator. Therefore, it is only at this point that it becomes clear whether a module really will have the desired output (approx. 80 watts), perhaps due to fluctuations in quality. This means that Würth Solar engineers only discover how good their results are at the end of the complex coating processes. If quality data have not been collected beforehand, it is too late to determine the origin of defects.
Prevent confusion using Data Matrix
Seamless quality assurance requires the glass plates to be assigned to individual inspection stations by means of Data Matrix code identification. Quality data are recorded every third or fourth step of the production process. The Data Matrix code is used to test whether the plate assigned to the data collected to date really has been processed. Glass breakage can be one of the reasons for data sets being created for a plate which no longer exists. Data Matrix coding prevents confusion.
The challenge - find an ID Reader capable of reading code in difficult conditions
The code is applied as an engraving using a laser system. The challenge came in the form of selecting a suitable vision system to read the codes. The real difficulty with lasered Data Matrix codes on glass is the low contrast between the engraving and the surrounding surface. After various products had been tested, it was decided to use Cognex ID readers - the In-Sight 5400/5100 vision sensors. The ID readers detect and decode even difficult codes on problematic surfaces, achieving read distances of over two meters in the process. 19 processing steps follow the lasering, the so-called "birth" of the plate including the application of the base coating plus subsequent laser structuring giving the plates their cellular shape. A quality inspection is important at this point, as the key production step follows once the plate has been washed. The plates have their CIS coating applied at 600 °C. After a chemical treatment to apply a buffer coating, the Data Matrix code is checked again. Zinc oxide is then applied forming a light-permeable front contact for the solar cell, completing the photodiode. In the subsequent final finishing stage, the plates are given additional bores for the guide electronics for the contacts.
600 solar panels a day!
Finally, the coated substrate glass is permanently bonded to the cover glass. Thus combined, the CIS solar panel is ready for a huge variety of applications, ranging from classic sun collectors to electricity generation on roofs and house facades to integrated product and stand-alone solutions.
Cognex ID Readers allow each work station to maximise output
This brief outline of the complex production process shows the many factors affecting the quality of CIS solar modules. The seamless tracking of production data using the Cognex ID reader created the basis for perfecting individual workstations. Würth Solar is pleased with the reliability and simple, self-explanatory handling of the ID reader and the AIT software not to mention the results. As early as 2008, it plans to use further production facilities to almost double the existing output of 600 solar modules a day to 1,100 modules.