Manipulation of stored charge density and polarity in anodic aluminium oxide for silicon solar cell passivation

This paper introduces a method to manipulate stored charge density and polarity in anodic aluminium oxide (AAO) for silicon solar cell passivation. The method involves the use of a pulse current source to anodise aluminium layers instead of the typically used constant current/voltage source, with the test structures experiencing positive and negative cycles periodically. By tuning the positive cycle percentage, it is demonstrated that the effective stored charge can be manipulated in a range from -5.2×10¹¹ to 2.5×10¹² q/cm² when the AAO is formed over a thin silicon dioxide layer. Photoconductance decay measurements on symmetrical n-type test structures with ∼ 150 Ω/□ boron-diffused surfaces were used to estimate the recombination current density associated with the diffused surface (J_op+), which was then correlated to the stored charge. As the effective stored charge changed from positive towards negative values, the J_op+ was reduced from 210 fA/cm² to 100 fA/cm². The stored charge in various test structures was monitored constantly to study the stability. It was found that the positive stored charge deteriorated with time while the negative charge was relatively stable.