Analysis of passivation property using thin Al₂O₃ layer and simulation for realization of high-efficiency TOPCon cell

Conventional p–n junction solar cells exhibit good thermal stability but poor carrier selectivity. The main aim of this study was to achieve improved passivation along with carrier-selective contact for a solar cell having a tunnel oxide passivated contact (TOPCon) structure. To this end, we attempted to optimize the polysilicon (poly-Si) layer deposited on an ultrathin (~1.3 nm) SiO₂ tunnel oxide layer to achieve high carrier lifetime and low recombination. We observed the passivation properties under varied thicknesses of the poly-Si layer and the phosphine (PH₃) gas flow rate at three different annealing temperatures. Our experimental approach was able to achieve the best passivation properties by use of a moderately thin poly-Si layer. The poly-Si layer thickness was varied from 55 to 113 nm, and the PH₃ flow rate was varied from 10 to 80 sccm at three different annealing temperatures in the range of 800–950 °C. The 113-nm-thick poly-Si layer was able to yield an implied open-circuit voltage (i-V_OC) of 733 mV along with a very low recombination current density (J₀) of 5.2 fA/cm² at a PH₃ flow rate of 40 sccm and post-deposition annealing temperature of 900 °C. A higher annealing temperature resulted in damage to the substrate, which in turn led to poor passivation; hence, optimization of this temperature was necessary. The passivation properties were further improved via the deposition of a thin (10-nm thick) Al₂O₃ layer on one side (i.e., front side) of the symmetric cell structure. This deposition caused a decrease in J₀ to 2.5 fA/cm² and an increase in i-V_OC to 742 mV. These results are attributed to a decrease in the number of interfacial defects, promoted formation of a carrier collector layer, and good field-effect passivation. We performed a simulation study for the realization of a TOPCon solar cell and employed therein the experimentally determined parameters, which yielded highly promising cell results. Our proposed simple and cost-effective approach has high potential for future large-scale production of high-efficiency TOPCon solar cells.