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Study of dielectric layers for bifacial n-type silicon solar cells with regard to optical properties, surface passivation quality, and contact formation
We have studied surface passivation layers for the application on n -type p+ nn+ bifacial silicon solar cells. Thereby, we have examined their optimal composition and thickness with regards to passivation quality, optical properties, and especially the contact formation during a co-firing step. These parameters were addressed in separate investigations: 1) simulation of the optical properties of a bifacial silicon solar cell, 2) measurement of the passivation quality on lifetime samples, 3) measurement of contact resistance (of aerosol printed fingers) to analyze the contact formation during the co-firing process, and 4) differential scanning calorimetry measurement were conducted to fundamentally understand reactions during contact formation in a fast firing furnace. The passivation layers tested were silicon nitride (SiNx), titanium oxide (TiO 2), and silicon oxide (SiO2) on lowly phosphorus-doped silicon n+-layers, whereas aluminum oxide (Al2O3) stacks, capped with SiNx and TiO2, were studied on lowly boron-doped silicon p+-layers. The results show that a dielectric stack, consisting of 10-nm-thick Al2O3 and 60-nm-thick SiNx layers on the boron-diffused silicon front side and a single 50-nm SiNx layer on the phosphorus-diffused silicon rear side, provides low emitter saturation current density (J0e), high optical absorption current density, and low contact resistance for printed and co-fired contacts.