Controlling Passivation Quality of Tunnel Oxide Layers via Thermal Oxidation with Gas Flow Control for High-Efficiency TOPCon Solar Cells

Tunnel Oxide Passivated Contact (TOPCon) solar cells have gained much attention due to their high open-circuit voltage (<i>V_oc</i>) resulting from the superior passivation performance of the tunnel oxide layer compared to conventional crystalline silicon solar cells such as PERC or PERT. The passivation quality of the tunnel oxide layer is determined by its thickness and uniformity, which significantly affect the <i>V&lt;span style="font-size:10.8333px"&gt;oc &lt;/span&gt;</i>value due to the trade-off between photo-induced charge collection and surface passivation. Chemical oxidation using nitric acid (HNO₃) introduces unintended nitrogen atoms into the tunnel oxide layer, deteriorating its passivation quality. In this study, we formed high-quality tunnel oxide layers using thermal oxidation by systematically adjusting the oxygen and nitrogen gas flow. By adjusting the gas flow ratio, the thickness uniformity of the tunnel oxide layer was improved from ±0.7 nm to ±0.2 nm on a 4-inch wafer scale, as analyzed by ellipsometry mapping. At an optimized thickness of 1.3 nm, the tunnel oxide layer exhibited a resistance of 10 mohm cm, an implied <i>V&lt;span style="font-size:10.8333px"&gt;oc&lt;/span&gt;</i> of 734 mV, and a lifetime of 2 ms. This study verifies that the method of forming tunnel oxide layers using gas flow control can produce high-quality, large-area ultrathin silicon oxide (SiO_x) layers in TOPCon solar cells.