In order to overcome the inherent limitations of screen printing during solar cell production, such as a locked aspect ratio and finger distortions when narrowing down on screen opening, there are a few advanced printing technologies that can help to solve these issues to a certain extent. Double printing is one such technology that is well known and is also extensively employed in mass production.

Double printing simply follows a ‘print on print’ approach, which means, after applying the first print, a second layer of metallization pattern is added on the earlier printed contacts with a second printer. The second layer not only neutralizes peaks and valleys, and interruptions, if any, it also helps in building the finger height. High alignment accuracy of the second printer is the prerequisite for this approach.

With double printing, the first layer could be any standard front-contacting paste, as the role of the paste remains more or less the same. The paste for the second layer can be tailored to provide higher conductivity without worrying about sintering attributes. This concept has been in mass production for some time now. Some paste suppliers even recommend using the same kind of front-contacting paste for both layers, while others are offering a package consisting of a pair of pastes for double printing.

Double printing helps in reducing series resistance of contacts. Since it has two hits, even if the first layer is not perfect with possible finger disruptions or high number of peaks and valleys, the second layer makes up for it. Double printing is particularly advantageous when a cell maker wants to maintain good printability and productivity, while still aiming to reduce the risk of finger disruptions at the same time. According to R&D lab manager Marco Galiazzo at screen printer supplier Applied Materials, for technologies such as shingling, where a good finger cross-section is becoming important, cell manufacturers are going back to double printing.

However, when using two pastes, one limitation of the approach is that the glass frit in top layers melts and mixes with the paste of the bottom layer, disturbing the chemistry of the whole pastes system. Again, due to several variables associated with the technology, screen opening and laydown also vary. As a reference, a few pastes makers put the average screen opening in mass production with double printing at 20 µm. However, according to Applied’s Galiazzo, the best customers of his company are at 22 µm. Some cell producers cooperating with Heraeus as part of its fineline project are already using 18 µm openings, which is also the lowest level indicated by a few other paste producers. As for the laydown, double printing enables 80 to 100 mg in most of the cases, but some aggressive customers are already at between 70 mg and 80 mg.

Double printing is also one of the subsections in the TaiyangNews Market Survey on Metallization Pastes 2019/2020. As per the products listed in the survey, the most recent models from different companies support finger openings from 18 to 30 µm and laydown of 50 to 100 mg (see graphic).

For details on further important aspects of Metallization Pastes, see TaiyangNews Market Survey Metallization Pastes 2019/20.