Toward Mitigating Surface Degradation of TOPCon Passivation: Understanding the Effects of Fast-Firing Temperature and Poly-Si Doping

We investigate the degradation of n- and p-type tunnel oxide passivating contact (TOPCon) under illumination and elevated temperatures to elucidate the degradation mechanism and identify mitigation strategies. Because the understanding regarding surface-related degradation (SRD) differs between n- and p-type TOPCon, we address different aspects for each layer type. Regarding p-type TOPCon, the focus is on the impact of the fast-firing peak temperature, whose correlation has already been investigated for n-type TOPCon but not for p-type TOPCon. It is found that the degradation extent of p-type TOPCon is inversely correlated with the peak temperature of the fast-firing step. This is consistent with previous reports on n-type TOPCon, making a common degradation mechanism for n- and p-type TOPCon likely. For n-type TOPCon, samples processed by plasma-enhanced chemical vapor deposition and low-pressure chemical vapor deposition (LPCVD) exhibit markedly different degradation extents. We attribute the lower degradation extent observed for LPCVD layers to its higher phosphorus concentration. We propose a unified mechanism for the degradation of both, n- and p-type TOPCon, that explains the various experimental data: hydrogen diffuses from a hydrogen containing dielectric layer (e.g., silicon nitride, SiN_x) through the highly doped poly-Si layer toward the Si/SiO₂ interface, where interface defects are formed. Based on this pathway, the degradation can be mitigated either by reducing the hydrogen content within the dielectric layers or by hindering the diffusion of hydrogen to the Si/SiO_x interface—for example, by increasing the poly-Si dopant concentration.