Superb Improvement of the Picosecond Laser Ablation of Dielectrics Via a Top-Hat Beam for High-Efficiency Solar Cells

In this study, a picosecond (ps) laser (wavelength 532 nm) with a top-hat beam was used to ablate a dielectric stack on a polished surface to improve the quality of the ablated surface. Homogeneous removal was achieved at an appropriate fluence for the ablation, since the uniform absorption of laser energy in the stack occurred by using a top-hat beam and a polished surface process. When the fluence was in the range of 0.51–0.74 J/cm² for a top-hat beam, the direct ablation played a leading role, since more energy was absorbed in the stack rather than in the silicon, resulting in minimum damage. However, the surplus peak fluence of a ps laser with a Gaussian beam penetrated the stack and consequently caused heat shock to the silicon, and partial lift-off ablation generated thermal damage with a depth of more than 2 μm. Average efficiency of 23.40% for the passivated emitter and rear contact (PERC) cells applied with the superior ablation exceeded that for the reference cells applied with the nanosecond laser ablation with a Gaussian beam by 0.11% absolute. The significant improvement of open-circuit voltage (V_oc), short-circuit current (I_sc) and fill factor (FF) was mainly due to the reduced recombination at the rear contacts and decreased leakage current. The mechanisms of the laser ablation with a top-hat beam resulting in reduced damage are also applicable to other lasers with different wavelengths and can be used for improving the efficiency of other cells where a low level of damage in the ablation process is needed to decrease the surface recombination.