Production of a temporally stable chemically enhanced ultra-thin HfO 2 interlayer with excellent passivation for use in photovoltaic passivating contacts.
Incorporation of carrier-selective passivating contacts is on the critical path for approaching the theoretical power conversion efficiency limit in silicon solar cells. We have used plasma-enhanced atomic layer deposition (ALD) to create ultra-thin films at the single nanometre-scale which can be subsequently chemically enhanced to have properties suitable for high-performance contacts. Negatively charged 1 nm thick HfO 2 films exhibit very promising passivation properties – exceeding those of SiO 2 and Al 2O 3 at an equivalent thickness – providing a surface recombination velocity (SRV) of 19 cm s −1 on n-type silicon. Applying an Al 2O 3 capping layer to form Si/HfO 2/Al 2O 3 stacks gives additional passivation, resulting in an SRV of 3.5 cm s −1. Passivation quality can be further improved via simple immersion in hydrofluoric acid, which results in SRVs < 2 cm s −1 that are stable over time (tested for ∼50 days). Based on corona charging analysis, Kelvin probe measurements and X-ray photoelectron spectroscopy, the chemically induced enhancement is consistent with changes at the dielectric surface and not the Si/dielectric interface, with fluorination of the Al 2O 3 and underlying HfO 2 films occurring after just 5 s HF immersion. Our results show that passivation is enhanced when the oxides are fluorinated. The Al 2O 3 top layer of the stack can be thinned down by etching, offering a new route for fabrication of ultra-thin highly passivating HfO 2-containing nanoscale thin films.