Transmission Electron Microscopy and Electron Energy-Loss Spectroscopy Studies of Hole-Selective Molybdenum Oxide Contacts in Silicon Solar Cells

In this study, substochiometric hole-selective molybdenum oxide (MoO_x) contacts in crystalline silicon (c-Si) solar cells were investigated by a combination of transmission electron microscopy (TEM) and spatially resolved electron energy-loss spectroscopy (SR-EELS). It was observed that a ≈ 4 nm SiO_x interlayer grows at the MoO_x/c-Si interface during the evaporation of MoO_x over a c-Si substrate. SR-EELS analyses revealed the presence of a 1.5 nm diffused MoO_x/indium tin oxide (ITO) interface in both as-deposited and annealed samples. Moreover, the presence of a 1 nm thin layer with a lower oxidation state of Mo was detected at the SiO_x/MoO_x interface in an as-deposited state, which disappears upon annealing. Overall, it was evident that no hole-blocking interlayer is formed at the MoO_x/ITO interface during annealing and homogenization of the MoO_x layer takes place during the annealing process. Furthermore, device simulations revealed that efficient hole collection is dependent on MoO_x work function and that reduction in the work function of MoO_x results in loss of band bending and negatively impacts hole selectivity.