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-VOC) 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-VOC 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.