Stable chemical enhancement of passivating nanolayer structures grown by atomic layer deposition on silicon

Abstract

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 HfO₂ films exhibit very promising passivation properties – exceeding those of SiO₂ and Al₂O₃ at an equivalent thickness – providing a surface recombination velocity (SRV) of 19 cm s⁻¹ on n-type silicon. Applying an Al₂O₃ capping layer to form Si/HfO₂/Al₂O₃ 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 Al₂O₃ and underlying HfO₂ films occurring after just 5 s HF immersion. Our results show that passivation is enhanced when the oxides are fluorinated. The Al₂O₃ top layer of the stack can be thinned down by etching, offering a new route for fabrication of ultra-thin highly passivating HfO₂-containing nanoscale thin films.