Quantum Transport across Amorphous-Crystalline Interfaces in Tunnel Oxide Passivated Contact Solar Cells: Direct versus Defect-Assisted Tunneling

Tunnel oxide passivated contact solar cells have evolved into one of the most promising silicon solar cell concepts of the past decade, achieving a record efficiency of 25%. We study the transport mechanisms of realistic tunnel oxide structures, as encountered in tunnel oxide passivating contact (TOPCon) solar cells. Tunneling transport is affected by various factors, including oxide layer thickness, hydrogen passivation, and oxygen vacancies. When the thickness of the tunnel oxide layer increases, a faster decline of conductivity is obtained computationally than that observed experimentally. Direct tunneling seems not to explain the transport characteristics of tunnel oxide contacts. Indeed, it can be shown that recombination of multiple oxygen defects in a-SiO_x can generate atomic silicon nanowires in the tunnel layer. Accordingly, new and energetically favorable transmission channels are generated, which dramatically increase the total current, and could provide an explanation for our experimental results. Our work proves that hydrogenated silicon oxide (SiO_x:H) facilitates high-quality passivation, and features good electrical conductivity, making it a promising hydrogenation material for TOPCon solar cells. By carefully selecting the experimental conditions for tuning the SiO_x:H layer, we anticipate the simultaneous achievement of high open-circuit voltage and low contact resistance.