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) 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 ≈ 4 nm SiO interlayer grows at the MoO/c-Si interface during the evaporation of MoO over a c-Si substrate. SR-EELS analyses revealed the presence of a 1.5 nm diffused MoO/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/MoO interface in an as-deposited state, which disappears upon annealing. Overall, it was evident that no hole-blocking interlayer is formed at the MoO/ITO interface during annealing and homogenization of the MoO layer takes place during the annealing process. Furthermore, device simulations revealed that efficient hole collection is dependent on MoO work function and that reduction in the work function of MoO results in loss of band bending and negatively impacts hole selectivity.