Hierarchy of Grain Boundary Impurity Gettering and Passivation in Photovoltaic Multicrystalline Silicon

This thesis examines grain boundaries, dislocations and metallic impurities in photovoltaic multicrystalline silicon (PV mc-Si). The interaction dynamics between hierarchy of grain boundaries (GBs), nickel, iron, and intragrain defects during thermal and hydrogenation processes have been studied. Localized Deep Level Transient Spectroscopy (DLTS) analysis was applied to quantify the changes induced by rapid thermal annealing (RTA) in trap states and accompanying decrease of carrier recombination in respect to the hierarchy of GBs and the dislocation density. A unique in-situ microscopic masking process for Schottky diode preparation which practically enables the localized probing of specific electrical active defect structure has been developed to investigate the local properties of mc-Si wafers. The reduction of recombination activity at GBs after RTA was found to be dominated by impurity decoration and less influenced by the relative structural mismatch for a specific type of GB. A new type of spectra was observed in PV thin mc-Si and its origins are attributed to the additional barrier height caused by GB hetro-junction.;The gettering capabilities of GBs were found to follow the sigma order in high sigma GB group but no consistent gettering relationship was found between gettering strength and low sigma group (O3 and twins). Electron bean induced current (EBIC) result shows that most of dislocation sites and twins (intragrain defects) present a weak or no impurity gettering tendency if they co-exist with GBs. GBs present as a more favored gettering sites to attract impurities under annealing condition. GB gettering strength presents a higher dependence on the hierarchy of crystal defect and lower dependence on the impurity type. GB plane also plays an influential role in the gettering behavior of GBs. On the other hand, the concentration of electrically "inactive" metals due to the presence of structural defect was quantified by subtracting the difference between the bulk metallic impurity concentration and electrical trap concentration. The electrical inactive level of nickel was found a least one order of magnitude higher than iron.;Silicon nitride hydrogenation was found to be effective on both coincidence site lattice (CSL) GBs and random GBs (R). This passivation also demonstrated significant passivation effects on dislocations, small angle (SA) GBs and twin boundaries (TBs) in mc-Si. Besides GB character, the local conditions like stress field also demonstrate an essential influence on the hydrogenation result. It was found that the main mechanism of SiN passivation at GB is to shift the trap energy level toward a shallower level so that the minority carrier recombination decreases. Relatively smaller grains present higher lifetime and also a better lifetime improvement after hydrogenation. The pure impact of GB mismatch on the density of interfacial states and deep level states were characterized by multi-frequency IV/CV and DLTS by using bonded wafer as GB model. The density of interfacial states at hetero -- GB was found to be 1.5 times higher than homo -- GB. Dislocations from GB misfit and induced by oxidation process were found to present different trap energy levels.