Interface defect and charge carrier transport properties in substrate-dependent p-type tunnel oxide passivated contact layers

Abstract

Most tunnel oxide passivated contact (TOPCon) solar cells are based on a phosphorus-doped (n-type) Si wafer owing to its lifetime stability and defect tolerance compared to a boron-doped (p-type) Si wafer. A gallium-doped (p-type) Si wafer has been adopted due to its better stability under illumination and is now a mainstream for the passivated emitter and rear cell. To use the existing facilities and reduce the cost in the solar industries, it is important to understand the charge carrier transport properties and underlying mechanisms considering p-type TOPCon solar cells as potential candidates for industrial applications. Here, we investigate the correlation between charge transport properties and defect distributions on p⁺ poly-Si layers/SiO_x/two different p-type Si wafers, doped with boron and gallium, respectively. SiO_x and p⁺ poly-Si layers have been fabricated by in-situ low pressure chemical vapor deposition. Then, we perform carrier lifetime measurements using quasi-steady state photoconductance to quantify bulk and surface carrier lifetime. The electrochemical capacitance-voltage method is utilized to investigate the doping levels and profiles by measuring the active boron concentration. Furthermore, Kelvin probe force microscopy measurements are conducted to examine the local photovoltage and defect states on the film surfaces. We also perform current-voltage measurements to analyze the current behavior under both dark and light illumination conditions. Our findings uncover defects dependent on Si wafer types and variations in charge carrier dynamics within p+ poly-Si layers/SiO_x/p-type Si wafers. This study enhances our comprehension of charge carrier transport properties in p-type Si wafers, facilitating advancements in photovoltaic performance for p-TOPCon solar cells.