Impact of excess phosphorus doping and Si crystalline defects on Ag crystallite nucleation and growth in silver screen‐printed Si solar cells

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.

Impact of boron doping profiles on the specific contact resistance of screen printed Ag–Al contacts on silicon

“…The wet-chemical etching 1 process is performed such that the bulk Ag-Al contacts, the glass layer, the passivation layer stack, and all crystallites penetrating the boron-doped silicon (emitter)-either in direct contact with the bulk metal or separated from it by an intermediate thin glass layer [21]-are removed.…”

Electrochemical nature of contact firing reactions for screen‐printed silicon solar cells: origin of “gray finger” phenomenon

“…The interfaces of fire‐through Ag contacts with the textured Si emitter exhibit a highly non‐uniform and complex microstructure that contains Ag crystallites grown into the Si emitter, a glass layer and pores separating the sintered Ag gridline from the Si emitter, and Ag particles/colloids dispersed in the glass layer. The current conduction mechanism across the fire‐through contact interface has been extensively studied, in particular the role of the Ag crystallites grown into the Si emitter in current conduction . Li et al .…”

Impact of Extended Contact Cofiring on Multicrystalline Silicon Solar Cell Parameters

“…Also note that mc-Si cells are not textured to reduce potential variations due to the step. The highly doped emitter has been selected for improving contact quality and maximizing Ag-crystallite creation thanks to the presence of a high amount of inactive phosphorous [15]. Although this emitter can limit efficiency values, the main experimental objective is to assure a high quality contact, and to further analyze the evolution of the contact and the rest of cell parameters during the extended contact co-firing.…”