Coupled effect of pinhole and trap-assisted tunneling on carrier transport in TOPCon solar cells

Carrier transport across the ultrathin SiO x layer is a key factor limiting the performance of TOPCon solar cells. In realistic SiO x /poly-Si contacts, direct tunneling, trap-assisted tunneling, and pinhole-mediated transport may coexist, yet their coupled effects remain insufficiently understood. Here, we develop a two-dimensional axisymmetric drift-diffusion framework to investigate the synergistic effects of pinholes and oxide defect states on carrier transport and device performance in n-type TOPCon solar cells. Both fully penetrated and locally thinned pinholes are explicitly considered to represent different local oxide configurations. For fully penetrated pinholes, direct tunneling dominates in ultrathin oxides, while trap-assisted tunneling becomes increasingly important when the oxide is thick enough to suppress direct tunneling but still lacks sufficient pinhole-mediated transport. Increasing pinhole density or size enhances carrier extraction and improves the fill factor and power conversion efficiency, whereas excessive pinholes increase interface recombination and degrade the open-circuit voltage. For locally thinned pinholes, efficient carrier transport can still be achieved when the residual oxide is sufficiently thin or contains a sufficiently high density of defect states. In this case, the residual oxide thickness and the trap-assisted tunneling strength jointly determine the transport efficiency. Compared to fully penetrated pinholes, locally thinned pinholes are less prone to inducing severe recombination losses due to the presence of a residual oxide layer. Importantly, the results reveal an inherent non-uniqueness in interpreting experimental observations, where similar device characteristics may arise from different combinations of pinhole density and trap-assisted tunneling strength. These findings establish a unified physical picture of carrier transport in TOPCon solar cells and provide guidance for the coordinated optimization of tunnel-oxide thickness, pinhole characteristics, and oxide defect properties.