Impact of femtosecond laser processing on dielectric layers for solar cell applications

Laser ablation using ultrashort pulses is becoming more relevant in the fabrication of solar cells due to their ability to produce well-defined grooves. In this work, laser grooving of dielectric layers using laser pulses of 480 femtosecond and 515 nm (green) wavelength at various pulse fluences and pulse overlaps is presented. The dielectric is either a single-layer film (SiNX) or a double-layer stack (AlOX/SiNX or SiO2/SiNX). An analytical model that enables one to determine the laser groove depth and width by calculating the equivalent fluence along the midpoint of the groove and by extracting the single-pulse ablation properties (threshold fluence, spot radius, and energy penetration depth) of the dielectrics is presented. The modeled groove dimensions match with the experimentally measured values, thereby allowing precise patterning of the dielectrics. Micromachining considerations such as the ablation rate and ablation efficiency are calculated from the modeled groove depth which enables one to optimize the ablation process. The optimum fluence zone, where the process yields >90% of its maximum ablation rate and efficiency, is identified to be just above the threshold fluence. Moreover, the results indicate that the optimum fluence zone remains practically unaffected irrespective of the chosen laser average power. Hence, the precise calculation of groove dimensions and processing under the optimum fluence zone provides a well-defined and efficient laser grooving process that is essential for high-efficiency solar cell architectures. (C) 2018 Laser Institute of America.