Surface lifetime investigation for characterization and enhancement of passivating contacts in c-si solar cells
In the context of high-efficiency solar cells based on crystalline silicon (c-Si), the integration of passivating contacts between the metal electrodes and the c-Si substrate has been identified as the next step to further improve the photovoltaic conversion efficiency. Passivating contacts consisting in a highly-doped poly-crystalline silicon (poly-Si) layer on top of a thin layer of silicon oxide (SiOx) offer the most promising approach to bridge the gap between device efficiencies in R&D and those in production. However, their development has mainly proceeded through “trial and error” so far, resulting in a limited understanding of their underlying working principle. More specifically, the surface passivation provided by poly-Si contacts is a combination of different mechanisms, among which the limiting one is still unclear due to: i) the interplay between these different mechanisms and ii) the challenge of characterizing thin-film stacks with features to the nanometric scale. Moreover, p-type poly-Si contacts, which are of prime interest since they could provide an alternative to the conventional contact at the rear side of mainstream p-type c-Si solar cells, have so far demonstrated lower passivation properties than their n-type counterparts, the fundamental reason for this difference remaining unclear. Within the SLICE project, a dedicated methodology based on lifetime spectroscopy scpecially adapted to the c-Si surface will be applied to identify electrically active defects limiting the lifetime of charge carriers at the interface between poly-Si contacts and the c-Si.  The investigation of different passivating thin-film stacks of iterative complexity will enable to relate their properties to their fabrication process. The insights gained from this original characterization of interfacial defects will support the fabrication of better passivating poly-Si contacts and ultimately solar cells with higher efficiency.