Development and Characterization of NO-Plasma Oxide Layers for High-Temperature p-Type Passivating Contacts in Silicon Solar Cells.

Full-area passivating contacts based on SiO/poly-Si stacks are key for the new generation of industrial silicon solar cells substituting the passivated emitter and rear cell (PERC) technology. Demonstrating a potential efficiency increase of 1 to 2% compared to PERC, the utilization of n-type wafers with an n-type contact at the back and a p-type diffused boron emitter has become the industry standard in 2024. In this work, variations of this technology are explored, considering p-type passivating contacts on p-type Si wafers formed via a rapid thermal processing (RTP) step. These contacts could be useful in conjunction with n-type contacts for realizing solar cells with passivating contacts on both sides. Here, a particular focus is set on investigating the influence of the applied thermal treatment on the interfacial silicon oxide (SiO) layer. Thin SiO layers formed via ultraviolet (UV)-O exposure are compared with layers obtained through a plasma treatment with nitrous oxide (NO). This process is performed in the same plasma enhanced chemical vapor deposition (PECVD) chamber used to grow the Si-based passivating layer, resulting in a streamlined process flow. For both oxide types, the influence of the RTP thermal budget on passivation quality and contact resistivity is investigated. Whereas the UV-O oxide shows a pronounced degradation when using high thermal budget annealing ( > 860 °C), the NO-plasma oxide exhibits instead an excellent passivation quality under these conditions. Simultaneously, the contact resistivity achieved with the NO-plasma oxide layer is comparable to that yielded by UV-O-grown oxides. To unravel the mechanisms behind the improved performance obtained with the NO-plasma oxide at high thermal budget, characterization by high-resolution (scanning) transmission electron microscopy (HR-(S)TEM), X-ray reflectometry (XRR) and X-ray photoelectron spectroscopy (XPS) is conducted on layer stacks featuring both NO and UV-O oxides after RTP. A breakup of the UV-O oxide at high thermal budget is observed, whereas the NO oxide is found to maintain its structural integrity along the interface. Furthermore, chemical analysis reveals that the NO oxide is richer in oxygen and contains a higher amount of nitrogen compared to the UV-O oxide. These distinguishing characteristics can be directly linked to the enhanced stability exhibited by the NO oxide under higher annealing temperatures and extended dwell times.