Damage-free ultraviolet nanosecond laser ablation for high efficiency back contact solar cell fabrication

Selective laser ablation of dielectric films for local contact formation is an attractive process simplification for high efficiency silicon solar cell fabrication. In high efficiency applications, the goal of laser ablation is spatially selective removal of surface dielectric layer(s) with minimal modification to the electronic properties of the substrate, equivalent to the performance benchmark set by photolithography processing. In this work, we present detailed characterisation of direct, 248 nm, nanosecond laser ablation of a Si₃N₄/SiO₂ dielectric stack for rear contact openings in a high efficiency interdigitated back contact solar cell. The efficacy of the ablation process is determined by the influence of the laser irradiation on three properties of the ablated region: pre-existing near surface dopant profiles, recombination activity, and the resistivity of the contact formed through the ablated opening. It is found that sufficient optical attenuation occurs in the Si₃N₄ layer at 248 nm to perform direct ablation. Despite visual evidence of near surface silicon melt formation, there is no measurable change in dopant distribution. Furthermore, we show that there is no increase in recombination activity beyond that expected for the underlying diffused and unpassivated surface, and contact resistivity below 0.1 mΩ cm² is achieved for vacuum evaporated aluminium on boron-diffused silicon. Back contact solar cells processed with laser ablated contact openings demonstrate conversion efficiencies equivalent to photolithography processed controls. An independently verified small-area device efficiency of 23.5% with an open-circuit voltage of 696 mV is achieved, a record efficiency for a laser processed crystalline silicon solar cell.