The mechanism of charge recombination in transition metal oxide-based interconnectors for tandem organic photovoltaic cells is investigated, where the interconnector is composed of an abrupt heterointerface between a Mg-doped 4,7-diphenyl-1,10-phenanthroline (Mg:BPhen) layer and a MoO₃ film. Based on the results of the interface energetics determined by ultraviolet photoelectron spectroscopy, as well as the corresponding device characteristics, it is revealed that the MoO₃ layer pronouncedly modifies the energy level alignment of the interconnector, which is beneficial for the charge recombination process at the interface between MoO₃ and the adjacent donor material for electrons and holes injected from stacked subcells. The incorporation of Mg:BPhen is essential for the conduction of the generated electrons from the bottom subcell into the conduction band of MoO₃.