Investigation and Quantitative Understanding of Front Field Passivation in Rear Junction Selective Double-Side TOPCon Solar Cells

This paper investigated the opportunity and challenge in fabrication of rear junction (RJ) selective double side tunnel oxide passivated contact (DS-TOPCon) cell, featuring full-area p-TOPCon on the back and a selective-area n-TOPCon on the front of n-type c-Si wafer. $2\mathrm{D}$ device simulation showed that this cell structure, which exploits the potential of TOPCon on both sides, possesses the potential to reach cell efficiency higher than 25.0 %, provided that the passivation on front field region between the metal grid can attain a J0 value of $< 5\text{fA}/\text{cm}^{2}$. It is shown in this work that $\text{Al}_{2}\mathrm{O}_{3}/\text{SiNx};\mathrm{H}$ passivation stack can achieve J0 of $\sim 4 \text{fA}/\text{cm}^{2}$ on the front field region. However, the J-V parameters of fabricated selective DS-TOPCon cell showed a significant cell efficiency degradation, presumably due to the formation of inversion layer, which is induced by a negative fixed charge in $\text{Al}_{2}\mathrm{O}_{0}$ layer. This causes minority carrier leakage via tunneling or shunting at the $\mathrm{p}^{+}$ inversion layer on the field region and $\mathrm{n}^{+}$ region under the metal-Si contact. A second passivation scheme, $\text{SiNx};\mathrm{H}$ layer, gave much higher J0 of $\sim 20\text{fA}/\text{cm}^{2}$ due to poor chemical passivation at the $\text{SiNx};\mathrm{H}/\text{Si}$ interface. Our recent experimental result showed that growth of 15 nm of thermal $\text{SiO}_{2}$ capped with $\text{SiNx};\mathrm{H}$ can achieve the J0 of $\sim 10\text{fA}/\text{cm}^{2}$ with potential of further improvement.