Analysis of nanosecond and femtosecond laser ablation of rear dielectrics of silicon wafer solar cells

Laser ablation of rear dielectrics of PERC (Passivated Emitter and Rear Cell) type silicon solar cells is by far the most cost-effective method to open a variety of patterns for local contact formation. In this paper, rear dielectric ablation of p-type PERC solar cells with an implanted phosphorus emitter is investigated using two laser sources with green wavelength and with pulse durations in the nanosecond (ns) and femtosecond (fs) range. The ns laser source emits 38 ns pulses and the ablation behaviour is linear for the entire range of available pulse fluences. The fs laser source emits 480 fs pulses and has two operating regimes, gentle and strong, based on the incoming pulse fluence. To evaluate the impact of laser induced damages on PERC solar cells, four different laser conditions are considered: fs gentle regime, fs strong regime, ns low fluence and ns high fluence. The solar cells' one-sun I-V parameters show that the cells processed in the fs strong regime have a severe reduction in the open-circuit voltage by almost 40 mV, fill factor loss by similar to 7% and efficiency loss by 3-4%. Fill factor loss analysis shows that a loss of almost 7% absolute comes entirely from 42 recombination, revealing a possibility of severe damage to the front side emitter. The solar cells processed with the other three laser conditions have comparable I-V characteristics. SEM analysis of the front surface of samples with laser treated rear surface shows that the fs strong regime produces severe damage to the front-surface texture resulting in lower J(sc), whereas this effect is not observed for the other laser conditions.