TiN_x and TiO_x/TiN_x Barrier Layers for Al-Based Metallization of Passivating Contacts in Si Solar Cells

Abstract

A solution is presented for Ag-free contacting of passivating contacts for crystalline Si-based solar cells. In particular, a Ti-based diffusion barrier layer is introduced between the polycrystalline-Si of the passivating contact and standard screen-printed Al-paste, in order to prevent Al/Si alloying during contact sintering. This solution enables contact formation with industrial standard Al-pastes while maintaining high surface passivation quality of the underlying passivating contact. Metallization-induced implied open circuit voltage losses remain below 10 mV, with values > 720 mV before metallization. Contact resistivity values are below 3 Ωcm on average at substrate peak firing temperatures of 700 °C. This approach provides an alternative to Ag-based contact electrodes, while still maintaining high surface passivation quality and enabling low contact resistivity through the use of a conductive barrier layer. Thermal stability of the diffusion barrier and the impact of layer properties on its functionality are investigated. The impact of a TiOx seed layer on diffusion barrier layer orientation is examined. Seed layer dependent properties, such as crystal orientation, are analyzed with X-ray diffraction. TiNx layer orientation changes with introduction of the TiOx seed layer, enabling a higher peak firing window. Diffusion profiles of Si, Al, and Ti are investigated via secondary-ion mass spectrometry.