Paste development for electrochemical screen printing to structure metal layers of back contact solar cells

An electrochemical etching process is developed to realize the contact pattern of back contact solar cells. It combines ECM technology (Electrochemical Machining) with screen printing to allow local removal of metallic layers for maskless formation of electrically isolated conductive paths. The present work focused on waterbased paste development for this application. A plain sodium nitrate solution is used as electrolyte which allows electrochemical etching. Thickener and rheological additives are added to form a screen-printable paste. Additionally different additives are varied to improve printing paste behavior and printing results. The homogeneity of the printing paste is a major factor in the quality of etched structures. The multi-level homogenizing process developed in the present work reduces the particle size by about 35%. Using the optimized NaNO3-based paste, etched lines are formed in 100 nm aluminum layers by the Electrochemical Screen Printing (ESP) process. The smallest lines show widths of 80 mm and the adjacent aluminum regions are electrically separated up to a resistance of 1.6 MO. The process time is less than 2 seconds for 156x156 mm2 samples. Further process modifications ensure another particle size reduction of 33%. First IBC test structures are etched with line widths of 160 mm by using the novel process.