Differently shaped Ag crystallites and four current transport paths at sintered Ag/Si interface of crystalline silicon solar cells

The comprehensive understanding of the Ohmic contact mechanism on the front metalization is highly desirable for further improvement of crystalline silicon solar cells performance. However, there are still controversial views about current transport paths between silver electrode and silicon emitter. To clarify this significant issue, we applied the selective acid etching (AE) process and the method of mechanical stripping process to prepare proper samples for direct observation of the Ag/Si contact interface. It is revealed that four kinds of Ag crystallites are grown with variant shapes and sizes on the pyramid tips, {111} planes, edges and tiny bumps at valleys of the textured Si surface resulting from anisotropic etching of Si emitter by the flowing molten glass frit during firing and the consequent epitaxially growth of Ag on the etching pits during cooling. All these Ag crystallites are in direct contact with Si emitter, serving as the most important contact points on the Si side. Meanwhile, three kinds of spherical Ag particles are formed in the resolidified glass frit during cooling which are crucial tunneling paths, conducting between glass-covered Ag crystallites on silicon and Ag bulk above. As a whole, four current transport paths are suggested mainly via Ag crystallites, Ag particles or both. In addition, the four electron transport paths are further confirmed by the Kelvin probe force microscopy characterization. The clarification and detailed understanding of all current transport paths are key points to design better silver paste and sintering process, and hence to improve the solar cell performance.