The difficulty here is that the laser production technology used does not permit the channel to be simply transferred to the rear of the cell. Alternatives such as the wet-etching method also have their disadvantages in the form of more expensive processes and a strong negative impact of the environment. The researchers at Merck are therefore using a new etching paste from the isishape® range, whose products have already been successfully used to mass produce touch panels and high-efficiency solar cells.

And their efforts have borne fruit. “With the paste, it is possible to etch an insulating edge on the back of the wafer in a way that conserves resources and is environmentally friendly,” says Köhler. The entire process consists of just three steps: printing, etching, and cleaning. In addition, it is carried out at a temperature of about 220 °C. As the temperature used is quite low compared to those used in other processes, this approach saves energy, conserves the cell’s material, and does not require aggressive solvents. “We can therefore use open water circulation systems to clean the wafers,” adds Köhler.

Merck demonstrated the method’s effectiveness during a pilot test at a well-known electronics company, where around 1,000 wafers were successfully isolated using the paste. In addition, Merck is cooperating with the machine manufacturer Schiller Automation, which developed the "SE lab" pilot facility exclusively for the use of the special paste isishape SolarEtch® SiD. Moreover, Schiller-Automation is incorporating the entire process of edge isolation into a standard process. “With this machine, solar cell manufacturers can test the method in a near-production setting and experience the benefits of our edge insulation process themselves,” says Köhler.

Higher performance machines of the same type could apply the paste to as many as 3,600 wafers—with diameters of 156 millimeters—per hour. The production process also encompasses a special quality assurance step, during which the wafer is irradiated with UV light after the paste has been applied to the edge. If the wafer’s edge glows, the light-sensitive paste is present at that point. In contrast, non-reflective areas indicate chipped spots on the wafer, for example, which can result in undesirable short circuits. Such wafers are thus easily recognized and can be immediately separated from the others and reintroduced to the production process after they have been cleaned.

According to Köhler, the use of a non-contact method to apply the paste onto the wafer is also groundbreaking. “This technology opens up completely new possibilities when it comes to structuring semiconductor surfaces,” he says. "And not only in relation to solar cells."