Laser Assisted Patterning of a-Si:H: Detailed Investigation of Laser Damage

Abstract

A detailed investigation of the laser damage to amorphous silicon (a-Si:H) layers patterned by laser ablation (LA) and wet chemical etching is presented. This approach can be applied to pattern the rear side of silicon hetero-junction interdigitated back-contact solar cells. Only the top sacrificial a-Si: H laser-absorbing layer of an a-Si:H/SiOx/a-Si:H/c-Si stack is ablated. Laser damage in the bottom a-Si:H layer and a-Si:H/c-Si interface is analyzed by both scanning electron microscopy and transmission electron microscopy. We show that the a-Si:H/c-Si passivation is degraded by laser damage and that this degradation can be diminished by increasing laser processing speed. This is attributed to a decrease of laser-irradiated area, and particularly smaller overlapping zones of adjacent laser pulses. The re-passivation quality after LA and wet etching is similar to that of as-passivated samples. This indicates that laser damage is not present in the bulk c-Si substrate but only in the a-Si: H passivation layer, which is removed during subsequent wet etching, thus allowing high quality repassivation.