Detailed structural and electrical characterization of plated crystalline silicon solar cells

Abstract

In this paper a detailed study on the electrical characteristics of Ni/Cu/Ag plated p-type Passivated Emitter Rear Contact (PERC) silicon solar cells is reported. By comparing the cell performance of different cell groups, pseudo Fill Factor (pFF) degradation by laser-induced defects, is observed. High-power (hard) laser ablated cells exhibit a lower efficiency and faster degradation with thermal ageing. We also present structural and electrical characterization to further visualize and identify the responsible defects, which turn out to be laser-ablation-induced dislocations, penetrating the silicon emitter and base. Increasing the laser fluence gives rise to a higher dislocation density. These dislocations are further confirmed as active generation-recombination centers by Deep Level Transient Spectroscopy (DLTS) analysis. The laser ablation induced dislocations are unavoidable because even at insufficient laser fluence (0.48 J/cm2) to fully open the dielectric stack, the density is already at the level of 106/cm2. A substitutional nickel peak is also detected by DLTS, suggesting nickel diffusing into the silicon base during the sinter step. Whereas no copper levels are found in the p-type silicon base by DLTS even after thermal aging at 235 °C.