Atomic Scale Understanding of Poly-Si/SiO2/c-Si Passivated Contacts: Passivation Degradation Due to Metallization

We report on the application of analytical microscopy to identify material bottlenecks for silicon solar cell efficiency on an atomic scale, including contacts, interfaces, and passivating layer morphologies. With high lifetime bulk n-Cz wafers available on a mass production scale, the push for higher efficiency (> 20%) is focused on passivation and reduced recombination at the metal contacts. These stringent passivation requirements should be retained during the subsequent device processing. Our device structures involve n-Cz silicon wafers with passivated contacts (poly-Si/SiO₂/n-Cz), and Al₂O₃/SiN_x front surface passivation layers, designed for incorporation into IBC solar cells. Using analytical microscopy, we study failure modes from the macroscopic scale (blisters in the passivation layers, metal adhesion problems) thru the microscopic (micropyramids, microblisters, microcracks) down to the nanoscale (nanopinholes, precipitates, blister edges, grain boundary decoration by dopants, dopant distribution) and atomic scale (dopant aggregation on surfaces and interfaces, atomic bonding valence and character). Metallization degrades our passivated contacts by promoting blistering along the poly-Si/SiO₂ interface, which is shown in detail by dissecting blisters and mapping them from a micron-to atomic scale using aberration corrected scanning transmission electron microscopy. A fundamental materials understanding focused on the effects of device processing, especially metallization, on retaining high-efficiency passivated Si devices is therefore gained over these series of presented results, and high resolution analytical microscopy emerges as a powerful tool in guiding high performance Si cell research.