Process Characterisation of Picosecond Laser Ablation of SiO2 and SiNx Layers on Planar and Textured Surfaces

Local contact openings in dielectric passivating layers of solar cells reduce recombination losses and offer an attractive alternative to “fire-through” processes such as screen printing of grid lines on the front side. Laser ablation with ultra short pulses is particularly suitable for contactless industrial processing of high-efficiency silicon solar cells. In the present paper, we investigate picosecond laser ablation of passivating SiO2 and SiNx layers on planar and textured silicon surfaces for various wavelengths of laser radiation. We characterise ps-laser ablated contact openings on 40 100 Ω/sq emitters by lifetime and contact resistance measurements. Our results show that for infrared and for visible wavelengths the local emitter saturation current densities in laser treated areas are 1 to 4 pA/cm2 when ablating SiNx and SiO2 layers from planar surfaces. However, ps-laser ablation on alkaline textured surfaces results in server crystal damage. Specific contact resistance values of < 1 mΩcm2 are achieved. We characterise the total recombination current densities by measurements of the J-V characteristics of laser treated diodes. Assuming 3% metallised area the efficiency loss compared to wet chemically treated solar cells is very low and amounts to 0.2% absolute.