Nano-Ag colloids assisted tunneling mechanism for current conduction in front contact of crystalline Si solar cells

It has long been known that performance of industrial silicon solar cells depends critically on the firing conditions (e.g., temperature-time profile) of the screen-printed thick film Ag conductors (for front-side contact). Using dual beam (focused ion and electron) microscopy and transmission electron microscopy (TEM), we have investigated the microstructure of the front-side contact/solar cell emitter interface fired at temperatures from below to above optimal. We found that as we swept through optimal firing condition, the interfacial glass layer separating the emitter and the Ag-bulk evolved from one richly decorated with nanometer-size Ag colloids into one with progressively more Ag crystallites attached directly to the silicon emitter surface. To improve the statistical relevance of our microscopy work, we used contact resistance maps to guide our sampling. This allowed us to further relate the appearance and disappearance of microstructures (e.g. nano-Ag colloids, Ag crystallites, residual SiN_x:H layer... etc.) to contact resistance. Finally, we have used selective acid etching technique to investigate the two-dimension distribution of Ag crystallites in cells fired at different temperatures. These microstructural observations offered new insights supporting the existence of a tunneling mechanism of current flow (¿nano-Ag colloid assisted tunneling¿ model), in the absence of Ag-crystallites, in screen-printed industrial multicrystalline-Si solar cells.