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

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 similar to 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.

Automated summary

Incorporation of carrier-selective passivating contacts is crucial for achieving the theoretical power conversion efficiency limit in silicon solar cells. Using plasma-enhanced atomic layer deposition (ALD), we created ultra-thin films that can be chemically enhanced for high-performance contacts. 1 nm thick HfO2 films with negative charge show promising passivation properties, surpassing SiO2 and Al2O3 at equivalent thicknesses. Additional passivation is achieved by applying an Al2O3 capping layer, resulting in a surface recombination velocity (SRV) of 3.5 cm s(-1). Immersion in hydrofluoric acid further improves passivation quality, with stable SRVs < 2 cm s(-1) over 50 days. Chemical enhancement occurs at the dielectric surface and fluorination of the Al2O3 and underlying HfO2 films is observed after just 5 s of HF immersion. The Al2O3 top layer can be thinned down by etching, providing a new route for fabricating highly passivating HfO2-containing nanoscale thin films.