Mickael Albaric

This works aims at contributing to increase the understanding of crystal defects and their distortions in dislocated Silicon Czochralski ingots. A popped-out ingot (PO), which experienced a premature detachment from the melt during the tail pulling step and an ingot with grown-in dislocations (GID), were studied. In each ingot, two wafers, cut at different heights, were characterized by photoluminescence (PL) mapping and Rocking Curve Imaging (RCI). Distortions and misorientations between crystal domains were quantified on large surface areas (up to one fourth) on the wafers. Dislocation densities were found to be in the order of a few 105 dislocations/cm2 in both ingots. In PO wafers, slip bands were found to be the main distortion structure. Additionally, round-shaped dislocation cells of sizes ranging from 100 to 300 µm were observed. Dislocation densities and misorientations were found to decrease further from the detachment front. Different defects were identified in GID ingots: cellular dislocation pattern (500 to 1400 µm in diameter), boundaries, sub-grains and dislocation sources. Regions with the highest distortions where found exhibit smaller dislocation cells. A good correlation is obtained between the distortion level distributions and thermal excess stress distribution maps calculated in the literature. The misorientation between domains separated by a boundary evolves more quickly along the defect propagation direction than the dislocation cell sizes and associated dislocations densities. While sub-grain boundaries have a known effect on the charge carrier lifetimes, our result reveal the unexpected impact of dislocations cells, which are not visible on PL maps, on carrier lifetimes.

The use of cut-cells in most of the current modules is now the norm to maximise the final product performances. But the integration of such new cell configurations also comes with new challenges, especially if small cell size dimensions such as shingle are considered. Indeed, specific edge passivation processes are often used to recover for the cut-cell losses. Alumina Oxide or Polymer deposition are the most studied approaches, with in general promising benefits proven at cell level. However, only few communications after module integration are available, in particular about the impact of these additional layers on final module reliability. We show for example that it is crucial to avoid direct deposition of AlOX over the cell metal pattern in the future interconnection area, as TC (Thermal Cycling) resilience of such modules is clearly degraded. Proper edge localization of the AlOX layer is needed to recover the initial reliability behaviour. To preserve the interconnection quality, three different approaches are investigated: (1) edge localisation of the layer by wafer stacking during deposition (2) introduction of adapted copper plating solutions allowing metal growth through the passivation layer (3) edge passivation process directly applied on final strings just before lamination.

The hydrogenation step contributing to the high efficiencies (>25%) reached with poly-Si/SiOx passivated contacts solar cells is still poorly understood. In this study, Fourier transform infrared spectroscopy (FTIR) is used to follow the different bonding configurations of H during the fabrication process. The carrier lifetime degradation upon annealing is correlated to an important loss of Si–H bonds, from both the a­‑Si:H film and the SiOx interfaces. The subsequent hydrogenation step results in the formation of a small number of Si–H bonds near the crystalline silicon c-Si/SiOx interface, associated with the low stretching mode (LSM) and correlated to a significant lifetime improvement. These bonds feature a preferential orientation, as shown by polarized measurements.