Surface passivation of boron diffused emitters for high efficiency solar cells

In order to utilize the full potential of solar cells fabricated on n-type silicon, it is necessary to achieve an excellent passivation on boron-doped emitters. As SiO2, the most effective passivation for highly doped n-type surfaces, does not show a sufficient performance on highly boron doped surfaces some effects that possibly lead to this gap in performance are investigated. Especially the question of boron redistribution during oxidation is the focus of this work. The field effect induced either by corona charge or fixed charge in the surface layer is known to strongly affect the surface passivation quality on silicon solar cells. Typical passivation layers used for high-efficiency solar cells as SiO2 and SiNx feature a built-in positive charge. For highly doped p-type surfaces however, it is shown experimentally that the passivation quality is strongly enhanced by a high density of negative charge. Thus, the negative-charge dielectric Al2O3 is applied as surface passivation layer on high efficiency n-type c-Si solar cells. An independently certified solar cell efficiency of 23.2 % confirms the excellent passivation quality of this negatively charged dielectric on a boron emitter.