Impact of surface topography and laser pulse duration for laser ablation of solar cell front side passivating SiN_x layers

摘要

Local contact openings in SiN_x layers that passivate the front side of solar cells offer an attractive alternative to the current standard "fire-through" screen printing process for front grid fabrication. Additionally, this technology can be used for enabling a selective emitter. In the present paper, we investigate laser ablation of SiN_x layers on planar and textured silicon surfaces for various laser wavelengths and pulse durations in the nanosecond (ns) to femtosecond (fs) range. We characterize the dark J-V characteristics of diodes with laser contact openings in the SiN_x layer passivating the emitter. Our results show that on alkaline textured surfaces the ablation by a ns laser produces less damage than by an ultrashort pulse laser. The dark currents of alkaline textured diodes treated with picosecond (ps) or fs lasers are one order of magnitude higher than those of ns laser treated diodes. High ideality factors furthermore indicate crystal damage in the ～500 nm deep space charge region of the diodes. Scanning electron microscope and transmission electron microscope images of textured samples, confirm the presence of extensive and deep crystal damage after ps laser ablation, which are not observed in laser treated samples with planar surfaces. Correspondingly, for planar surfaces we find for both, ns and for ps laser ablated regions, emitter saturation current densities J_0e,abl of ～2 pA/cm~2. The recombination in textured samples in contrast differs vastly for ns and ps laser ablation. The ns laser results in an only slightly increased value of 3.7 pA/cm~2 while the ps laser treated sample was not evaluable due to severe crystal damage leading to effective lifetimes of <5 μs.