Defect studies in silicon photovoltaics: gettering mechanisms and defect characterisations
Creators: name: Le, Tien
Publisher: name: The Australian National University
Year: 2024

description: This thesis is dedicated to advancing the understanding of gettering mechanisms and defect recombination activities for the metallic impurities in crystalline silicon (Si). Initially, metallic impurities in the Si wafer bulk are major efficiency-limiting factors in Si solar cells. Gettering can significantly lower the metal concentrations. Despite previous studies on gettering by dielectric materials, including silicon nitride and aluminium oxide films, the underlying behaviours and mechanisms remain partially elucidated. This thesis investigates the gettering kinetics and mechanisms of those two common dielectric materials in Si solar cells. By monitoring the kinetics of iron loss from the Si wafer bulk i.e. by measuring the lifetime of wafers for the associated and dissociated states of iron-boron metastable complex, it is confirmed that silicon nitride and aluminium oxide gettering mainly occurs via segregation, in contrast to precipitation gettering, which is driven by the supersaturation effect, even at a low annealing temperature of 400oC. Simulation of gettering kinetics with the diffusion-segregation assumption suggests the presence of an interfacial diffusion barrier slowing the transport of iron impurities from the Si wafer bulk to the silicon nitride gettering regions. In the case of aluminium oxide films, there is a much-reduced diffusion coefficient in the aluminium oxide layers. Additionally, the thesis investigates the impact of depth-wise inhomogeneous iron distributions on iron concentration measurements in Si wafers. It highlights significant measurement errors due to nonuniform Fe distribution, particularly during early gettering stages. A 'threshold' Fe concentration below 1E13 cm-3 is recommended to minimise errors. Factors like surface passivation and wafer thickness also influence measurement accuracy. Subsequently, with an improved understanding of metallic gettering mechanisms, bulk defects in n-type Czochralski (Cz) Si solar cells are compared and identified. This part of the thesis assesses the bulk quality of industrial Cz-grown phosphorus-doped n-type Si wafers along an ingot. The minority charge carrier lifetimes of the Cz-Si wafer bulk before and after phosphorus diffusion gettering were assessed by applying room-temperature superacid surface passivation. A substantial enhancement in the bulk lifetime of all n-type Cz-Si wafers along the ingot was observed, indicating the effectiveness of a gettering step for such wafers and the presence of getterable metallic impurities in these wafers. By experimentally monitoring the lifetime changes upon a gettering anneal and simulating the gettering kinetics based on different metal diffusivities, iron is identified as a limiting defect for the tail part of the ingot. A dissolved iron concentration of ~1E12 cm-3 is estimated from the bulk lifetimes of the tail wafers. This approach also serves as a novel method to identify iron or other getterable metals with moderate diffusivities, such as chromium, in n-type Si wafers. Finally, a comprehensive re-evaluation of the iron-gallium (FeGa) recombination parameters in Si using injection-dependent lifetime spectroscopy (IDLS) is presented. Ga-doped Si wafers (of varying resistivities) with precise concentrations of intentional iron contamination in the Si wafer bulk, through ion implantation and distribution, were used. The presence of interstitial Fei and FeGa, and their lifetime-limiting effects in these Si wafers, were confirmed through measuring the effective minority carrier lifetime changes during the FeGa dissociation-association processes. Error analysis was employed to find the combination of defect parameters that best fit the experimental data and to ascertain the range of uncertainty associated with the IDLS best-fit results. The extracted defect parameters are also verified by experimentally measuring the crossover point of Fei and FeGa lifetime curves.