Exploring hafnium oxide’s potential for passivating contacts for silicon solar cells

We investigate the potential of ultrathin nanometre-scale HfO2 films deposited by atomic layer deposition for passivating contacts to silicon focusing on variations in film thickness. A peak in passivation quality as assessed by carrier lifetime measurements, is reported for 2.5 nm thick films annealed at 475 °C, for which a surface recombination velocity <1 cm/s is determined. For samples <2.5 nm thick, there is a marked decrease in passivation quality. X-ray diffraction highlights a change from crystallised monoclinic HfO2 to amorphous, as film thickness decreases from 10 nm to 2.5 nm. Kelvin probe results indicate that as-deposited 2.5-10 nm films have similar effective work functions, although the work function of 1 nm films is considerably lower. Upon vacuum annealing, all films experience a reduction in effective work function. An initial investigation into the contact resistivity in a passivating contact structure utilizing HfO2 reveals a strong annealing temperature dependence, with the lowest resistance achieved below 375 °C, followed by a decrease in performance as temperature increases towards the optimal temperature for passivation (475 °C). Limitations in the existing contact structure are discussed.