Ultrathin atomic layer deposited niobium oxide as a passivation layer in silicon based photovoltaics

Abstract

Atomic layer deposited (ALD) niobium oxide (NbOx) films were investigated for their passivation properties through minority carrier lifetime measurements and compared to a well-known passivating material, aluminum oxide. ALD alumina is known to passivate by a combination of field-effect passivation from fixed charges and chemical passivation from hydrogenation of dangling bonds. It was hypothesized that niobium oxide films passivate by varying degrees of the same mechanisms found in alumina. The effects of ALD oxygen source (water or ozone) and varying anneal temperatures were correlated to passivation quality. Lifetimes of sub-1 nanometer films were specifically investigated. It was found that water is a superior oxidant for passivation relative to O3. Thermally activated NbOx films deposited with water have near equivalent or superior lifetimes to alumina of the same thickness after equivalent annealing at temperatures up to 350 °C. Chemical analyses by x-ray photoelectron spectroscopy (XPS) were used to investigate the suspected mechanisms of passivation. It was suggested that field-effect passivation is the dominating mechanism in NbOx films based on correlations between band movement as probed by XPS and the lifetime data presented in this work. This work provides new insights into the applicability of niobium oxide as a passivating selective contact for silicon photovoltaics with the goal of reaching new record efficiencies in solar cells.