Investigation of the thermal stability of MoO_x as hole-selective contacts for Si solar cells

The stoichiometry and work function of molybdenum oxide (MoO_x) are of crucial importance for its performance as hole selective contact for crystalline silicon solar cells. Hydrogenated amorphous silicon (a-Si:H) is typically used as an interface passivation layer in combination with MoO_x to reduce surface recombination. As the fabrication process of a solar cell typically contains subsequent high-temperature processes, the consideration of thermal stability of MoO_x with and without a-Si:H becomes critical. In this work, in situ x-ray spectroscopy (XPS)/ultraviolet photoelectron spectroscopy and Fourier transform infrared spectroscopy in the temperature range from 300 K to 900 K are used to investigate the thermal stability of MoO_x with and without a-Si:H. In addition, both the passivation and contact performance are studied by evaluating the surface saturation current density J_0s, carrier lifetime τ_eff, and contact resistivity ρ_c. The XPS results reveal that the as-evaporated MoO_x on top of both c-Si and a-Si:H is sub-stoichiometric, and the work function of both films is higher than 6 eV. While after in situ annealing, the evolution of MoO_x phase on top of a-Si:H shows a different behavior compared to it on c-Si which is attributed to H diffusion from a-Si:H after 600 K, whereas the work function shows a similar trend as a function of the annealing temperature. The J_0s of a p-type Si symmetrically passivated by MoO_x is found to be 187 fA/cm² and the ρ_c is ∼82.5 mΩ·cm² in the as-evaporated state. With a-Si interface passivation layer, J_0s is significantly lower at 5.39 fA/cm². The J_0s and the ρ_c increase after post-deposition annealing. The evolution of these functional properties can be attributed to the material properties.