Abstract
Good quality contacts between metal and silicon emitter are crucial for high crystalline solar cell efficiencies. We investigate the impact of defects originating from electrically inactive phosphorus on contact formation within silver thick film metallized silicon solar cells. For this purpose, emitters with varying sheet resistance, depth, and dead layer were metallized with silver pastes from different generations. Macroscopic contact resistivity measurements were compared with the microscopic contact configurations studied by scanning electron microscopy. The density of direct contacts between Ag crystallites grown into Si and the Ag finger bulk is essential for low contact resistivity. The presence of glass-free regions needed for such direct contacts depends on the paste composition and on the surface texture, and does not vary with the Si emitter properties. Indeed, the decrease in contact resistivity correlates with increasing density of Ag crystallites embedded in the Si surface. Furthermore, the density of Si surface-embedded Ag crystallites scales proportional to the electrically inactive P and is independent of the sheet resistance. Using the newest silver paste, the Ag crystallite density is independent of the emitter doping, but the Ag crystallite size increases as a function of the thickness of the dead layer. Transmission electron microscopy characterization of the excess P-doped Si crystal lattice shows that significant strain and Si bond weakening may play a major role for both Ag crystallite nucleation and growth. Finally, we studied Si crystal defects by metallizing nanocracks, dislocations, and grain boundaries and found that Ag crystallite nucleation is defect-property dependent. Copyright © 2013 John Wiley & Sons, Ltd.
