Rapid thermal processed high-mobility IO:H films and their application to c-Si solar cells featuring TOPCon passivating contacts

Abstract

High-mobility hydrogenated indium oxide (IO:H) thin films were developed, and the influence of post-deposition treatment on their opto-electronic properties was investigated. The IO:H films were deposited by magnetron sputtering and subsequently annealed using Rapid Thermal Processing (RTP), involving short exposure to temperatures between 400 and 700 °C. After annealing, the electron mobility of the IO:H films reached 146 cm²/Vs, demonstrating the effectiveness of short high-temperature annealing compared with conventional longer low-temperature annealing processes.

Transparent conductive oxides (TCOs) can reduce electrical and optical losses in crystalline silicon (c-Si) solar cells with Tunnelling Oxide Passivating Contacts (TOPCon). Therefore, this study examines how IO:H deposition and post-deposition treatment affect the passivation quality of these contacts. Deposition of IO:H on symmetrically passivated c-Si wafers significantly degrades passivation due to particle bombardment, which breaks Si–Si bonds and introduces defect states. Part of this degradation can be recovered through post-deposition annealing. Reducing the sputtering power density proved essential for mitigating passivation damage; a maximum power density of 0.62 W/cm² minimizes degradation and enables substantial recovery during annealing. However, due to dopant in-diffusion within the contact structure, the annealing temperature must remain below 550 °C.

Cox–Strack measurements of symmetric TOPCon/IO:H contact stacks show relatively high contact resistivity, primarily caused by oxygen adsorption when IO:H is deposited at low power density. This leads to the formation of an electron barrier, limiting the field-effect-driven mobility of electrons traveling from the contact into the TCO. Reducing both the thickness and carrier concentration of the IO:H buffer layer is therefore important for lowering the contact resistivity. Literature further indicates that inserting a thin indium tin oxide (ITO) interlayer between IO:H and silver can significantly reduce contact resistivity and improve overall solar cell performance.