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

Abstract

Local contact openings in SiNx 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 SiNx 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 SiNx 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 J0e,abl of ∼2 pA/cm2. 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/cm2 while the ps laser treated sample was not evaluable due to severe crystal damage leading to effective lifetimes of <5 μs.

Mechanisms of lifetime degradation in Si/ARC samples patterned by laser lift-off

“…For large facet sizes and shallow melt depths, the delay time introduced by long-route recrystallization could be sufficient for melt regions near the pyramid tips to solidify either spontaneously or with (111)-templated epitaxy, leading to defective morphologies such as reported in Ref. [9]. In contrast, when the laser melting is deep enough to provide a quasi-planar melt/{001} interface from which to template, the predicted recrystallization can clearly be defect-free (as shown in Figs.…”

“…ARC patterning at ps and fs (vs. ns) pulse length irradiation has shown some promise in reducing Si damage [1, 8,9] due to the shallower heating associated with shorter pulses. However, the trend towards shallower heating runs counter to the desire for the deeper melt depths preferred for emitter diffusions.…”

Damage-free laser patterning of silicon nitride on textured crystalline silicon using an amorphous silicon etch mask for Ni/Cu plated silicon solar cells

“…These variations in laser pulse intensity are detrimental towards the ablation process given that Hernandez et al demonstrated a decrease in generation current (J G ) and open circuit voltage (V OC ) with an increase in laser pulse intensity [1]. Further, the non-uniform heating that can occur causes threading dislocations at the peaks of the pyramids, as deep as 1 μm [8].…”

“…However, a major challenge for a laser ablation process is the c-Si damage typically induced by the laser. While 'low damage' laser patterning is trivial on smooth surfaces, performing such patterning on surfaces textured by the standard mono-crystalline Si alkaline etch process proves more difficult [7][8][9].…”

Light-Trapping Properties of a Diffractive Honeycomb Structure in Silicon

“…for local contact openings) on the planar side of the wafer [20] and reducing surface recombination.…”