Development\nand Characterization of N<sub>2</sub>O‑Plasma\nOxide Layers for High-Temperature p‑Type Passivating Contacts\nin Silicon Solar Cells

Full-area\npassivating contacts based on SiO<sub><i>x</i></sub>/poly-Si\nstacks are key for the new generation of industrial\nsilicon solar cells substituting the passivated emitter and rear cell\n(PERC) technology. Demonstrating a potential efficiency increase of\n1 to 2% compared to PERC, the utilization of n-type wafers with an\nn-type contact at the back and a p-type diffused boron emitter has\nbecome the industry standard in 2024. In this work, variations of\nthis technology are explored, considering p-type passivating contacts\non p-type Si wafers formed via a rapid thermal processing (RTP) step.\nThese contacts could be useful in conjunction with n-type contacts\nfor realizing solar cells with passivating contacts on both sides.\nHere, a particular focus is set on investigating the influence of\nthe applied thermal treatment on the interfacial silicon oxide (SiO<sub><i>x</i></sub>) layer. Thin SiO<sub><i>x</i></sub> layers formed via ultraviolet (UV)–O<sub>3</sub> exposure\nare compared with layers obtained through a plasma treatment with\nnitrous oxide (N<sub>2</sub>O). This process is performed in the same\nplasma enhanced chemical vapor deposition (PECVD) chamber used to\ngrow the Si-based passivating layer, resulting in a streamlined process\nflow. For both oxide types, the influence of the RTP thermal budget\non passivation quality and contact resistivity is investigated. Whereas\nthe UV–O<sub>3</sub> oxide shows a pronounced degradation when\nusing high thermal budget annealing (<i>T</i> > 860 °C),\nthe N<sub>2</sub>O–plasma oxide exhibits instead an excellent\npassivation quality under these conditions. Simultaneously, the contact\nresistivity achieved with the N<sub>2</sub>O-plasma oxide layer is\ncomparable to that yielded by UV–O<sub>3</sub>-grown oxides.\nTo unravel the mechanisms behind the improved performance obtained\nwith the N<sub>2</sub>O-plasma oxide at high thermal budget, characterization\nby high-resolution (scanning) transmission electron microscopy (HR-(S)TEM),\nX-ray reflectometry (XRR) and X-ray photoelectron spectroscopy (XPS)\nis conducted on layer stacks featuring both N<sub>2</sub>O and UV–O<sub>3</sub> oxides after RTP. A breakup of the UV–O<sub>3</sub> oxide at high thermal budget is observed, whereas the N<sub>2</sub>O oxide is found to maintain its structural integrity along the interface.\nFurthermore, chemical analysis reveals that the N<sub>2</sub>O oxide\nis richer in oxygen and contains a higher amount of nitrogen compared\nto the UV–O<sub>3</sub> oxide. These distinguishing characteristics\ncan be directly linked to the enhanced stability exhibited by the\nN<sub>2</sub>O oxide under higher annealing temperatures and extended\ndwell times.