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 (SiN_x), titanium oxide (TiO 2), and silicon oxide (SiO₂) on lowly phosphorus-doped silicon n⁺-layers, whereas aluminum oxide (Al₂O₃) stacks, capped with SiN_x 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 SiN_x layers on the boron-diffused silicon front side and a single 50-nm SiN_x layer on the phosphorus-diffused silicon rear side, provides low emitter saturation current density (J_0e), high optical absorption current density, and low contact resistance for printed and co-fired contacts.