Hole-Collection Mechanism in Passivating Metal-Oxide Contacts on Si Solar Cells: Insights from Numerical Simulations

Silicon heterojunction solar cells enable high conversion efficiencies, thanks to their passivating contacts which consist of layered stacks of intrinsic and doped amorphous silicon. However, such contacts may reduce the photo current, when present on the illuminated side of the cell. This motivates the search for wider bandgap contacting materials, such as metal oxides. In this paper, we elucidate the precise impact of the material parameters of MoO_x on device characteristics, based on numerical simulations. The simulation results allow us to propose design principles for hole-collecting induced junctions. We find that if MoO_x has a sufficiently high electron affinity (5.7 eV), direct band-To-band tunneling is the dominant transport mechanism; whereas if it has a lower electron affinity (< 5.7 eV), trap-Assisted tunneling dominates, which might introduce additional series resistance. At even lower electron affinity, S-shaped J-V curves may appear for these solar cells, which are found to be due to an insufficient trap state density in the MoO_x film in contrast to the expectation of better performance at low trap density. These traps may assist carrier transport when present near the conduction band edge of the MoO_x film. Our simulations predict that performance optimization for the MoO_x film has to target either 1) a high electron affinity and a moderate doping density film or, 2) if the electron affinity is lower than the optimum value, a high defect density not exceeding the doping density inside the film.