Hydrogen passivation of poly-Si/SiO_x contacts for Si solar cells using Al₂O₃ studied with deuterium

The interplay between hydrogenation and passivation of poly-Si/SiO_x contacts to n-type Si wafers is studied using atomic layer deposited Al₂O₃ and anneals in forming gas and nitrogen. The poly-Si/SiO_x stacks are prepared by thermal oxidation followed by thermal crystallization of a-Si:H films deposited by plasma-enhanced chemical vapor deposition. Implied open-circuit voltages as high as 710 mV are achieved for p-type poly-Si/SiO_x contacts to n-type Si after hydrogenation. Correlating minority carrier lifetime data and secondary ion mass spectrometry profiles reveals that the main benefit of Al₂O₃ is derived from its role as a hydrogen source for chemically passivating defects at SiO_x; Al₂O₃ layers are found to hydrogenate poly-Si/SiO_x much better than a forming gas anneal. By labelling Al₂O₃ and the subsequent anneal with different hydrogen isotopes, it is found that Al₂O₃ exchanges most of its hydrogen with the ambient upon annealing at 400 °C for 1 h even though there is no significant net change in its total hydrogen content.