Recombination at laser-processed local base contacts by dynamic infrared lifetime mapping

Abstract

Laser-processed local metal contacts to Si solar cells are a promising approach, to combine high efficiency and low production cost. Understanding carrier transport and recombination in locally contacted solar cells requires numerical simulations with experimentally verified input parameters. One of these input parameters is the reverse saturation current density J0,cont at the local base contact. We determine J0,cont by means of area averaged charge carrier lifetime measurements and an analytical model, which distinguishes between recombination at the metal contacts and at the passivated interface in between the contacts. The calibration-free dynamic infrared lifetime mapping technique is used. We measure local reverse saturation current densities J0,cont=2×103 to 2×107 fA/cm2 at metal contacts to p-type float-zone material with resistivities ρ=0.5 to 200 Ω cm. Laser contact openings (LCOs) formed by laser ablation of an amorphous Si/SiNx passivation stack and subsequent physical vapor deposition of aluminum are used as contact formation technique. As well laser fired contacts (LFCs) are applied to the same passivation stack and metallization. We observe no difference in J0,cont between LCO and LFC. Our results indicate degradation of the passivation stack by the laser treatment in the vicinity of the LCO and LFC.