Influence of firing temperature and silver–aluminum paste intermixing on front contact quality and performance of TOPCon silicon solar cells

We present a microstructural analysis of the front metallization in TOPCon solar cells by using scanning electron microscopy, Raman spectroscopy, Kelvin probe microscopy and atomic force microscopy. Correlative imaging at the cross section of front contacts interface with secondary electrons and with back-scattered electrons is a suitable method to identify contact formation in small localized areas where silicon intermixes with elements from the metallization paste. Three different types of intermixing have been identified; two shallow types are located either in the valley between the pyramids of the surface texture or on their facets near the tips. For contacts formed at higher temperature we detected sporadically a third type whose depth typically exceeds one micrometer. Likely this type of contact pierces through the emitter region, leading to losses in the open circuit voltage by creating shunts across the p-n junction of the cell. By statistically evaluating the dimensions of the intermixed areas we estimate electrical contact area fractions of 0.3% and 1 % for a firing temperatures of 780 °C and 840 °C, respectively. These area fractions are consistent with discrepancies between our measured values for contact resistivity and reported data for ideal full area contacts.