Unveiling thermal stress-induced trap-assisted tunneling in POLO junctions and its impact on TOPCon solar cells

In POLO junctions and TOPCon solar cells with oxide layers thicker than $1.7\mathrm{nm}$, pinhole-mediated transport has widely been regarded as the dominant charge transport mechanism. However, high-temperature annealing processes not only generate pinholes in the oxide layer but also introduce additional defects, which can significantly influence charge transport. This study theoretically investigates the impact of thermal stress-induced trap-assisted tunneling on charge transport mechanisms within POLO junctions and TOPCon solar cells. By quantitatively comparing simulated I–V characteristics with previously reported experimental data, we show that when TAT is regarded as the primary charge transport mechanism, the simulated dark I–V characteristics of POLO junctions with $1.8\mathrm{nm}$ oxide layers closely match experimental observations. This highlights the critical role of TAT as a significant charge transport pathway in POLO junctions with thick tunneling oxides. Moreover, in cases where pinhole density is relatively low, TAT can even become the dominant transport mechanism. Numerical simulations under illumination further demonstrate that incorporating TAT provides a reasonable explanation for the experimentally observed relatively high fill factor and power conversion efficiency in TOPCon solar cells with thick oxides. These findings emphasize the importance of TAT in understanding and optimizing the performance of POLO junctions and TOPCon solar cells.