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Abstract
Aluminium oxide (AlOx) plays a critical role in increasing silicon solar cell efficiency and is used as a rear side surface passivation layer for the passivated emitter and rear cell (PERC). Although the majority of commercial silicon solar cells are based on the PERC structure, there is still insufficient knowledge available in literature regarding the properties of the AlOx layer. In this thesis, the impact of deposition parameters on surface passivation quality is studied. It is found that plasma power must be sufficient in order to ionise the precursor gases to acquire thermally stable AlOx layer. The ratio between the precursor gases is also found to have a significant impact on the surface passivation quality in both as-deposited and fired states. Interestingly, despite the large difference in precursor gas ratios, which leads to AlOx layers having different properties, all these layers are found to be almost stoichiometric. Further investigation indicates that the different surface passivation results may be explained by the different hydrogen depth profile across the AlOx. Apart from chemical composition, change in the interfacial layer between silicon (Si) and AlOx layer also has a strong impact on passivation quality. Nano-scale investigation of the Si/AlOx interface reveals that a thermal process after the AlOx deposition can change its properties drastically. Thicker silicon oxide layer at the Si/AlOx interface effectively reduces the interface defect densities. The subsequent thermal process also changes the chemical configuration of the AlOx layer and it is shown to be the origin of negative charge (Qtot) within the layer. It is found that the ratio of the two chemical configurations (tetrahedral- to octahedral- aluminium ratio) and the resulting Qtot has a linear relationship. The root cause of surface passivation degradation at elevated temperature in the dark is also investigated. It is hypothesised that hydrogen may have a dominant role in the degradation. Using different sample structures, the modulated effective lifetime is correlated with hydrogen migration between the Si surface and the bulk.