Processing techniques for monolithic interconnection of solar cells at wafer level

Monolithic interconnected modules are large-area high-voltage photovoltaic devices that are realized through solar cell segments that are series-connected via interconnection trenches during wafer processing. This paper investigates different processing techniques, giving detailed information about each processing sequence. In the first approach, a wet chemical etching procedure and photo-defined polyimide as dielectric film is used, leading to minimal trench width of 86 µm. Furthermore, we examine the applicability of advanced processing techniques. An alternative dry-etching process using reactive ion etching in inductively coupled plasma is investigated. With this technique, smooth and near-vertical sidewalls could be achieved, whereas the undercuts in etching profile, which are inevitable when using wet chemical etching, could be avoided. Moreover, an accurate processing technique for plasma-enhanced chemical vapor deposited SiNx as a dielectric film for electrical isolation is presented. This allowed sufficient isolation to be achieved, as well as enabling precise structuring. With these advanced processing techniques, trench widths of 57 µm were realized, resulting in a reduction of area losses due to interconnection from 9% to below 6%.