Origin of the high work function and high conductivity of MoO3

The large work function of MoO3 of 6.6 eV is due to its closed shall character and the dipole layer created by planes of terminal O1 oxygen sites which lower the electrostatic potential of the inner Mo-O units. These O1 sites arise from the high stoichiometry of MoO3. The O vacancy is most stable at the 2-fold O2 site. It is a shallow donor and has a small formation energy in the O poor limit so that MoO3 easily becomes a degenerate semiconductor.