Thermal annealing effects on tunnel oxide passivated hole contacts for high-efficiency crystalline silicon solar cells

Abstract

Tunnel oxide passivated contacts (TOPCon) embedding a thin oxide layer between polysilicon and base crystalline silicon have shown great potential in the development of solar cells with high conversion efficiency. In this study, we investigate the formation mechanism of hole-carrier selective contacts with TOPCon structure on n-type crystalline silicon wafers. We explore the thermal annealing effects on the passivation properties in terms of the stability of the thermally-formed silicon oxide layer and the deposition conditions of boron-doped polysilicon. To understand the underlying principle of the passivation properties, the active dopant in-diffusion profiles following the thermal annealing are investigated, combined with an analysis of the microscopic structure. Based on PC1D simulation, we find that shallow in-diffusion of boron across a robust tunnel oxide forms a p–n junction and improves the passivation properties. Our findings can provide a pathway to understanding and designing high-quality hole-selective contacts based on the TOPCon structure for the development of highly efficient crystalline silicon solar cells.

Design Rule of Electron- and Hole-Selective Contacts for Polycrystalline Silicon-Based Passivating Contact Solar Cells

“…In hole contacts, hole transport was known to rely on direct transport through the pinhole. 15,31,32,38 Thus, the substrate contributed as a lateral carrier transport path during the Hall effect measurements in this case, as well. However, the hole concentration was lower than the average B concentration obtained from SIMS for poly-Si, even after annealing at 1000 °C.…”

“…In addition, the average B concentration obtained from SIMS for the subsurface (blue square) increased from 1.32 × 10 19 to 1.22 × 10 20 cm –3 . Similar to the case of electron contacts, the change in the average B concentration obtained from SIMS was also due to B in-diffusion during annealing . However, the hole concentration was lower than the average B concentration obtained from SIMS for poly-Si, even after annealing at 1000 °C, unlike the case of electron contacts.…”

“…After annealing electron contacts, Kale et al observed that SiO x was locally thinned from 2.3 to 1.4 nm. After annealing hole contacts with SiO x thicknesses of 1.2 and 1.5 nm, Kim et al observed that an increase in annealing temperature resulted in a decrease in SiO x thickness. From Figure d,h, we observed that annealing both electron and hole contacts at 1000 °C led to SiO x rupture and pinhole formation.…”

“…18,32,38,41 In the case of hole contacts, hole transport must rely on direct transport through pinholes because hole tunneling is inefficient. 27,32,38 Holes have a tunneling probability lower than electrons. This is because the higher SiO x tunneling barrier height for holes (Φ B_Ed v in Figure 1a) compared to that for electrons (Φ B_Ed c in Figure 1a) and the relatively larger effective mass of holes compared to that of electrons contribute to the lower tunneling probability of holes.…”

“…SiO x can be thinned or ruptured to form pinholes, which facilitates direct carrier transport through the pinholes. 18,32,38,41 In the case of hole contacts, hole transport must rely on direct transport through pinholes because hole tunneling is inefficient. 27,32,38 Holes have a tunneling probability lower than electrons.…”