Tunnel Oxide Passivated Contacts Formed by Ion Implantation for Applications in Silicon Solar Cells

Passivated contacts (poly-Si/SiO_x/c-Si) doped by shallow ion implantation are an appealing technology for high efficiency silicon solar cells, especially for interdigitated back contact (IBC) solar cells where a masked ion implantation facilitates their fabrication. This paper presents a study on tunnel oxide passivated contacts formed by low-energy ion implantation into amorphous silicon (a-Si) layers and examines the influence of the ion species (P, B, or BF₂), the ion implantation dose (5 × 10¹⁴cm^-2 to 1 × 10¹⁶cm^-2), and the subsequent high-temperature anneal (800 °C or 900 °C) on the passivation quality and junction characteristics using double-sided contacted silicon solar cells. Excellent passivation quality is achieved for n-type passivated contacts by P implantations into either intrinsic (undoped) or in-situ B-doped a-Si layers with implied open-circuit voltages (iV_oc) of 725 and 720 mV, respectively. For p-type passivated contacts, BF₂ implantations into intrinsic a-Si yield well passivated contacts and allow for iV_oc of 690 mV, whereas implanted B gives poor passivation with iV_oc of only 640 mV. While solar cells featuring in-situ B-doped selective hole contacts and selective electron contacts with P implanted into intrinsic a-Si layers achieved V_oc of 690 mV and fill factor (FF) of 79.1%, selective hole contacts realized by BF₂ implantation into intrinsic a-Si suffer from drastically reduced FF which is caused by a non-Ohmic Schottky contact. Finally, implanting P into in-situ B-doped a-Si layers for the purpose of overcompensation (counterdoping) allowed for solar cells with V_oc of 680 mV and FF of 80.4%, providing a simplified and promising fabrication process for IBC solar cells featuring passivated contacts.