Multi-characterization study of interface passivation quality of amorphous sub-stoichiometric silicon oxide and silicon oxynitride layers for photovoltaic applications

Abstract

Si solar cells have achieved a world record efficiency of 26.7% as a result of both improvement of Si ingot growth and optimal passivation of surfaces and interfaces. In this framework, a clear understanding of the electronic, optical, structural and passivation properties of innovative Si based layers is mandatory. The present study reports on the characterization of amorphous sub-stoichiometric silicon oxide (a-SiOx) and silicon oxynitride (a-SiOxNy) layers and their surface passivation properties. The layers have been deposited on float zone Si wafers (2 Ω cm, (100)-oriented, 250 µm thick) by plasma enhanced chemical vapour deposition (PECVD) adding increasing fractions of N2O and CO2 to the SiH4 flux during deposition to increase the energy band gap of the layers. Composition, optical properties, light induced electronic transitions and minority carrier lifetimes of Si wafers passivated with these layers have been investigated by Fourier-transform infrared spectroscopy (FTIR), spectral ellipsometry, surface photovoltage (SPV) spectroscopy and photo conductance decay (PCD). The overall characterization of the layers has allowed us to understand the effect of increasing N2O and CO2 flux ratios during deposition on the interface properties. The present study establishes the importance of the approach of using multiple characterization methods in the evaluation of the passivation capability of layers that combine large optical band gap and surface passivation.